استاندارد ieee
ANSI/IEEE Std 32-1972
(Reaffirmed 1990)
An American National Standard
IEEE Standard Requirements, Terminology,
and Test Procedure for Neutral Grounding
Devices
Sponsor
Surge Protective Devices Committee
of the
IEEE Power Engineering Society
Approved March 21, 1972
Reaffirmed June 16, 1978
Reaffirmed September 13, 1984
Reaffirmed September 28, 1990
IEEE Standards Board
Approved March 29, 1985
Reaffirmed February 27, 1991
American National Standards Institute
Abstract:
IEEE Std 32-1972 (Reaff 1990); IEEE Standard Requirements, Terminology, and Test Procedures for
Neutral Grounding Devices
. This standard applies to devices (other than surge arresters) used for the purpose of
controlling the ground current or the potentials to ground of alternating current systems. It defines usual and unusual
service conditions, and gives bases for rating such devices. Specifications are given for temperature limitations,
dielectric tests, routine tests, losses and impedance measurements, and construction items.
© Copyright 1972 by
The Institute of Electrical and Electronics Engineers, Inc
345 East 47th Street, New York, NY 10017, USA
No part of this publication may be reproduced in any form, in an electronic retrieval system or otherwise, without
prior written permission of the publisher.
Foreword
This standard is a revision of AIEE Standard 32, 1947, to which many changes and additions have been made.
This standard was developed by the Neutral Grounding Subcommittee of the Surge Protective Devices Committee of
the IEEE Power Engineering Society. The membership of the Sub-committee is as follows:
N. Stadtfeld, Jr
, Chair
G. G. Auer
L. B. Crann
G. S. Haralampu
P. F. Hargreaves
D. W. Jackson
I. B. Johnson
J. L. Koepfinger
F. R. Licini
J. A. McKinley
D. E. Orten
W. J. Vandergrift
G. G. Auer was Chairman of the Subcommittee during the bulk of the period of preparation of this issue. Others who
took part are as follows:
E. W. Boehne
G. D. Breuer
L. V. Devine
J. B. Hays
C. C. Jones
J. C. Russ
L. E. Sauer
When the Surge Protective Devices Committee of the IEEE Power Engineering Society reviewed and approved this
document, it had the following membership:
A. G. Yost
, Chair
I. B. Johnson
, Vice Chair
W. R. Ossman
, Secretary
D. L. Andrews
S. A. Annestrand
J. J. Archambault
G. G. Auer
R. D. Ball
G. A. Baril
F. G. Berg
E. W. Bochne
P. W. Bogner
G. D. Breuer
L. B. Crann
W. H. Eason
H. E. Foelker
G. L. Gaibrois
R. S. Gens
G. S. Haralampu
A. R. Hileman
D. W. Jackson
S. S. Kershaw, Jr
J. L. Koepfinger
F. R. Licini
H. Linck
D. J. Melvold
B. D. Miller
V. B. Nolan
J. D. M. Phelps
R. W. Powell
W. S. Price
R. E. Seabrook
J. W. Simpson
N. Stadtfeld, Jr
M. W. Stoddart
H. L. Thwaites
A. C. Westrom
iii
CLAUSE PAGE
1. General .................................................. .................................................. .................................................. ..........1
1.1 Scope .................................................. .................................................. .................................................. .... 1
1.2 Service Conditions .................................................. .................................................. ................................. 1
1.3 Operation at Altitudes in Excess of 3300 ft (1000 m) .................................................. ............................. 2
2. Basis for Rating............................................ .................................................. .................................................. ...3
2.1 Conditions .................................................. .................................................. .............................................. 3
2.2 Rated Current .................................................. .................................................. ......................................... 3
2.3 Rated Voltage .................................................. .................................................. ........................................ 4
2.4 Rated Frequency .................................................. .................................................. ................................... 5
2.5 Rated Time .................................................. .................................................. ............................................. 5
3. Insulation Classes and Dielectric Withstand Levels .................................................. .........................................5
3.1 Basic Impulse Insulation Levels and Insulation Classes .................................................. ........................ 5
3.2 Dielectric Withstand Test Levels .................................................. .................................................. ........... 7
3.3 Protective Devices........................................... .................................................. ......................................... 7
4. Temperature Limitations....................................... .................................................. ............................................7
4.1 Limits .................................................. .................................................. .................................................. ... 7
4.2 Steady-State Temperature Rises .................................................. .................................................. ........... 7
4.3 Rated-Time Temperature Rises .................................................. .................................................. ............ 7
5. Tests .................................................. .................................................. .................................................. ..............8
5.1 Routine Tests .................................................. .................................................. ......................................... 8
5.2 Dielectric Tests .................................................. .................................................. ................................... 12
5.3 Losses and Impedance .................................................. .................................................. ......................... 14
6. Construction .................................................. .................................................. .................................................. 14
6.1 Bushings, Insulators, and Oil .................................................. .................................................. ............... 14
6.2 Nameplates........................................ .................................................. .................................................. ... 14
6.3 Tanks and Enclosures........................................ .................................................. ..................................... 15
7. Reactors.......................................... .................................................. .................................................. ...............15
7.1 Insulation Levels .................................................. .................................................. .................................. 15
7.2 Dielectric Tests .................................................. .................................................. .................................... 15
8. Ground-Fault Neutralizers .................................................. .................................................. ............................16
8.1 Insulation Levels .................................................. .................................................. .................................. 16
8.2 Dielectric Tests .................................................. .................................................. .................................... 16
9. Grounding Transformers...................................... .................................................. ...........................................17
9.1 Insulation Levels .................................................. .................................................. .................................. 17
9.2 Dielectric Tests .................................................. .................................................. .................................... 17
iv
CLAUSE PAGE
10. Resistors .................................................. .................................................. .................................................. ......18
10.1 Resistor Element .................................................. .................................................. .................................. 18
10.2 Insulation Levels .................................................. .................................................. .................................. 19
10.3 Dielectric Tests .................................................. .................................................. .................................... 19
11. Capacitors........................................ .................................................. .................................................. ..............19
11.1 Insulation Levels .................................................. .................................................. .................................. 19
11.2 Dielectric Tests .................................................. .................................................. .................................... 20
12. Combination Devices .................................................. .................................................. ....................................20
12.1 Insulation Levels .................................................. .................................................. .................................. 20
12.2 Dielectric Tests .................................................. .................................................. .................................... 20
13. Definitions and Terminology .................................................. .................................................. ........................21
14. Test Code .................................................. .................................................. .................................................. ....26
14.1 Resistance Measurements .................................................. .................................................. .................... 26
14.2 Dielectric Tests .................................................. .................................................. .................................... 28
14.3 Impedance and Loss Measurements .................................................. .................................................. .... 33
14.4 Temperature-Rise Tests .................................................. .................................................. ....................... 34
14.5 Temperature-Rise Calculations .................................................. .................................................. ............ 38
Copyright © 1972 IEEE All Rights Reserved
1
IEEE Standard Requirements,
Terminology, and Test Procedure for
Neutral Grounding Devices
1. General
1.1 Scope
This standard applies to devices used for the purpose of controlling the ground current or the potentials to ground of
an alternating current system. These devices are: grounding transformers, ground-fault neutralizers, resistors, reactors,
capacitors, or combinations of these.
1.2 Service Conditions
1.2.1 Usual Temperature and Altitude Service Conditions.
Devices conforming to this standard shall be suitable for operation at their ratings provided that:
1) The temperature of the cooling air (ambient temperature) does not exceed 40
°C and the average temperature
of the cooling air for any 24-hour period does not exceed 30
°C.
NOTE — It is recommended that the average temperature of the cooling air be calculated by averaging 24 consecutive
hourly readings. When the outdoor air is the cooling medium, the average of the maximum and minimum daily
temperatures may be used. The value which is obtained in this manner is usually slightly higher than the true
daily average but not by more than
1/2°C.
2) The altitude does not exceed 3300 ft (1000 m).
1.2.2 Unusual Temperature and Altitude Service Conditions.
Devices conforming to this standard may be applied to higher ambient temperatures or at higher altitudes than
specified in 1.2.1, but their performance will be affected and special consideration should be given to these
applications.
2
Copyright © 1972 IEEE All Rights Reserved
IEEE Std 32-1972 IEEE STANDARD REQUIREMENTS, TERMINOLOGY, AND
1.2.3 Other Unusual Service Conditions.
Where unusual conditions other than those discussed in 1.2.2 exist, they should be brought to the attention of those
responsible for the design and application of the equipment. Examples of such conditions are:
1) Exposure to damaging fumes, radiation, or vapors; excessive abrasive or magnetic dust; explosive mixtures of
dust, vapors, or gases; steam, excessive moisture, dripping water, fog or spray; salt or acid; etc.
2) Abnormal vibration, shock, or tilting from earthquakes or other causes.
3) Unusual transportation or storage conditions.
4) Unusual space limitations.
5) Abnormal operating duty, frequency of operation, difficulty of maintenance, poor waveform, excessive
unbalance voltage, special insulation requirements, lack of normal lightning arrester protection, unusual
magnetic shielding problems, harmonics in excess of or other than those expressed in Note of Section 2.2.
NOTE — Unless an air-core reactor is fully shielded magnetically, consideration should be given to its location relative
to other apparatus and to metallic structures which may suffer overheating due to stray fields and mechanical
damage under fault conditions.
6) System having a ratio of reactance to resistance
X/R greater than 10. See Section 2.2.
1.3 Operation at Altitudes in Excess of 3300 ft (1000 m)
Standard devices may be applied in locations having an altitude in excess of 3300 ft (1000 m), but the dielectric
strength of air-insulated parts and the current-carrying capacity will be affected.
1.3.1 Insulation.
The dielectric strength of air-insulated parts of a given insulation class at or above 3300 ft (1000 m) should be
multiplied by the proper correction factor, as given in Table 1, to obtain the dielectric strength at the required altitude.
An altitude of 15 000 ft (4500 m) is considered a maximum for standard devices.
1.3.2 Operation at Rated Current.
Devices with standard temperature rise may be used at rated current at altitudes greater than 3300 ft (1000 m) provided
the average temperature of the cooling air does not exceed the values in Table 2 for the respective altitudes. Under
these conditions, standard temperature limits will not be exceeded.
Table 1— Dielectric Strength Correction Factors for
Standard Devices at Altitudes
Greater than 3300 ft (1000 m)
Altitude
meters feet
Factor for
Correction
1000
1200
1500
1800
2100
2400
2700
3000
3600
4200
4500
3300
4000
5000
6000
7000
8000
9000
9900
12 000
14 000
15 000
1.00
0.98
0.95
0.92
0.89
0.86
0.83
0.80
0.75
0.70
0.67
Copyright © 1972 IEEE All Rights Reserved
3
TEST PROCEDURE FOR NEUTRAL GROUNDING DEVICES IEEE Std 32-1972
2. Basis for Rating
2.1 Conditions
Ratings for devices are based on standard operating conditions and shall include the following:
1) Current
2) Voltage
3) Frequency
4) Basic Impulse Insulation Level (BIL) and Insulation Class
5) Circuit Voltage of System
6) Service (Indoor or Outdoor)
7) Time
2.2 Rated Current
Unless otherwise specified, the basis for this rating shall be the thermal current. This will be the current through the
neutral device during a ground-fault condition at the device location.
Implicit in the thermal current rating is an associated continuous current which, unless otherwise specified, shall bear
the following relationship to the thermal current rating:
Devices shall be able to withstand, without mechanical failure, forces associated with the crest of the offset current
wave, assuming subtransient reactance fault conditions. This current crest, for devices other than resistors, shall be
determined from the following equation:
(1)
where
I
c = Crest of the initial offset current
Continuous Duty Current in Percent of
Thermal Current Rating
Reactors, Ground-Fault
Neutralizers, and
Transformers Used For
Grounding
Rated Time of
Device Resistors
10 s 3 0
1 min 7 0
10 min 30 0
Extended Time 30 0
NOTE — Where there is a third harmonic component of current,
it shall not exceed 15 percent of the rated continuous
duty current.
I
c
= KIT
4
Copyright © 1972 IEEE All Rights Reserved
IEEE Std 32-1972 IEEE STANDARD REQUIREMENTS, TERMINOLOGY, AND
K
= Multiplier from Table 3
I
T = Thermal current rating
2.3 Rated Voltage (see definition in Section 13)
The rated voltage except for certain resistors (see 10.1.1) and grounding transformers (see 9.2.3), shall be taken as the
product of the rated thermal current and the impedance of the device at rated frequency and at 25
°C.
Table 2—
Table 3— Value of K for Eq 1
A device consisting of tapped sections or sections connected in series may have a rated voltage for each section
determined from the impedance and the rated thermal current of the section as above.
Maximum Allowable Average Temperature,
* Degrees Celsius
of Cooling Air for Carrying Rated Current
*For recommended calculation of average temperature, see Note in 1.2.1.
Methods of Cooling
Apparatus
3300 ft
(1000 m)
6600 ft
(2000 m)
9900 ft
(3000 m)
13 200 ft
(4000 m)
Oil-Immersed Self-Cooled
Dry-Type Self-Cooled
30 28 25 23
55
°C rise 30 27 24 21
80
°C rise 30 26 22 18
150
°C rise 30 22 15 7
Resistors 30 3
–14 –25
X/
R K X/R K X/R K
1000 3.390 28 3.215 13 3.026
250 3.379 24 3.186 12 3.000
100 3.342 20 3.148 11 2.970
65 3.312 18 3.121 10 or less 2.940
45 3.278 16 3.089
34 3.245 15 3.070
NOTE —
K = 1.2 { 1 + exp [ –p (R/X) ] }
When sequence impedances for the system are not specified, the ratio
X/R shall
be taken to be the ratio of ohms reactance to ohms resistance (dc) in the winding
of the device through which the current flows.
When the system to which the neutral device is connected has an
X/R ratio at that
point which is greater than 10, the ohmic values of sequence impedances should
be specified. When specified, the manufacturer shall combine them with the
X
and
R of the device to determine the value of K to be used in calculating Ic. The
1.2 factor is based on use of transient reactance in calculating thermal current.
2
Copyright © 1972 IEEE All Rights Reserved
5
TEST PROCEDURE FOR NEUTRAL GROUNDING DEVICES IEEE Std 32-1972
2.4 Rated Frequency (see definition in Section 13)
The rated frequency shall be the fundamental, except that for some devices such as neutral wave traps, the rating may
include additional harmonic frequencies which the device is designed to control.
2.5 Rated Time (see definition in Section 13)
Rated time shall be 10 seconds, 1 minute, 10 minutes, and extended time. Extended-time operation shall not exceed an
average of 90 days per year.
3. Insulation Classes and Dielectric Withstand Levels
3.1 Basic Impulse Insulation Levels and Insulation Classes (see definition in Section 13)
All apparatus covered by this standard shall be assigned BIL (basic impulse insulation levels) and/or insulation class
designations.
6
Copyright © 1972 IEEE All Rights Reserved
IEEE Std 32-1972 IEEE STANDARD REQUIREMENTS, TERMINOLOGY, AND
Table 4— Insulation Classes for Neutral Grounding Devices
Specific instructions for selecting the BIL and insulation class, and associated dielectric tests for a particular device,
are covered in the section of the standard which is set aside for the device in question.
Basic impulse insulation levels and insulation class designations used with grounding devices are given in Table 4.
The two ends of the winding of a neutral device may be assigned different insulation levels.
Insulation Class
System Insulation
Class, kV (See Note 1)
Fault Voltage Criteria
(See Note 2)
Column 1 Column 2 Column 3 Column 4
Class BIL kV kV
1.2
2.5
5.0
8.7
15.0
23.0
34.5
46.0
69.0
92.0
115.0
138.0
161.0
180.0
196.0
230.0
45
60
75
95
110
150
200
250
350
450
550
650
750
825
900
1050
1.2
2.5
5.0
8.7
8.7
15.0
25.0
34.5
46.0
69.0
69.0
92.0
92.0
115.0
115.0
138.0
1.2
2.5
5.0
8.7
8.7
8.7
8.7
15.0
15.0
15.0
15.0
15.0
15.0
15.0
15.0
15.0
NOTES:
1 — Where the insulation class for the terminal of a device is specified to be
the system insulation class, the nominal system insulation level shall
apply except that reduced BIL may be used where appropriate.
2 — When the fault voltage criterion applies, the maximum rms voltage that
may exist between the terminal and ground, under fault conditions, is
determined. If this fault voltage falls between the values in columns 3 and
4 corresponding to the system insulation class in column 1, the system
insulation class at the terminal in question shall be the value in column 3
which equals or is next higher than the maximum fault voltage. If the
fault voltage is less than the value in column 4, the system insulation
class at the terminal in question shall be the value in column 4. If the fault
voltage exceeds the value in column 3 the system insulation class at the
terminal in question shall be the value in column 1.
Reactors Section 7.
Ground-Fault Neutralizers Section 8.
Grounding Transformers Section 9.
Resistors Section 10.
Capacitors Section 11.
Combination Devices Section 12.
Copyright © 1972 IEEE All Rights Reserved
7
TEST PROCEDURE FOR NEUTRAL GROUNDING DEVICES IEEE Std 32-1972
3.2 Dielectric Withstand Test Levels
Applied-potential, induced-potential, and impulse test withstand levels are those given in Table 5. For duration and
frequency of test, see 14.2.2.
3.3 Protective Devices
Suitable protective devices shall be provided whenever necessary to hold transient overvoltages at the terminals of a
device to values within the limits set by the selected insulation levels.
4. Temperature Limitations
4.1 Limits
Steady-state and rated-time temperature rises for current-carrying parts, determined under standard operating
conditions, shall not exceed the limits established in Table 6.
Top oil temperature rises determined under standard operating conditions shall not exceed the limits established in
Table 8.
Parts, other than current-carrying parts, in contact with or adjacent to insulation or oil, shall not attain temperature rises
in excess of those allowed for current-carrying parts. Other parts shall not attain temperatures that will be injurious to
structures, or personnel, or produce excessive smoke or toxic fumes.
4.2 Steady-State Temperature Rises (see definition in Section 3)
The steady-state average winding temperature rise shall be determined by routine test (see Section 5.1).
The steady-state hot-spot winding temperature rise
THS shall be determined by design test.
4.3 Rated-Time Temperature Rises (see definition in Section 13)
4.3.1 Ten-Second and One-Minute Ratings.
The rated-time temperature rise of 10-second and 1-minute devices is an average winding rise (except resistors, for
which it is a hot-spot rise).
The rated-time temperature rise of 10-second and 1-minute devices shall be taken as the sum of the steady-state rise
determined as in 4.2 and the additional rise caused by the flow of rated thermal current (or, for certain resistors, the
application of rated voltage) for rated time, determined by calculation, using Eq 11 (for resistors, Eq 17).
4.3.2 Ten-Minute Ratings.
The rated-time temperature rise of 10-minute devices is an average winding rise (except resistors, for which it is a hotspot
rise).
The rated-time temperature rise of 10-minute devices shall be taken as the winding rise above ambient resulting from
the flow of rated thermal current (or, for certain resistors, the application of rated voltage) for 10 minutes starting with
an initial temperature rise equal to the steady-state rise of the device.
8
Copyright © 1972 IEEE All Rights Reserved
IEEE Std 32-1972 IEEE STANDARD REQUIREMENTS, TERMINOLOGY, AND
The rated-time temperature rise of 10-minute devices shall be determined by routine test (Section 5.1).
4.3.3 Extended-Time Ratings (see definition in Section 13)
The rated-time temperature rise of extended-time devices is an average winding rise (except resistors, for which it is a
hot-spot rise).
The rated-time temperature rise of extended-time devices shall be taken as the ultimate rise above ambient resulting
from the continued flow of rated thermal current (or, for certain resistors, the continued application of rated voltage).
The rated-time temperature rise of extended-time devices shall be determined by routine test (Section 5.1).
5. Tests
Unless otherwise specified, tests shall be made at the manufacturer's facilities.
5.1 Routine Tests (see definitions in Section 13)
The following routine tests shall be made on all devices where feasible, except as covered in the section of the standard
which is set aside for the device in question. The numbers shown do not necessarily indicate the sequence in which the
tests shall be made. All tests shall be made in accordance with Section 14., Test Code.
1) Resistance measurements of all windings on the rated voltage connection and taps or sections of each device
2) Voltage measurements on the rated voltage connection and on all tap connections
3) Polarity and phase relation tests on the rated voltage connection
4) Excitation loss at rated voltage on the rated voltage connection
5) Excitation current at rated voltage on the rated voltage connection
6) Impedance voltage and power at rated current on rated connection of each device and the tap connections of
one unit only of a given design
7) Temperature test or tests shall be made on a device only if no record of a temperature test, made in accordance
with these standards, on a duplicate or essentially duplicate design, is available
NOTE — This standard provides for using values calculated from tests at reduced currents.
8) Dielectric tests
Copyright © 1972 IEEE All Rights Reserved
9
TEST PROCEDURE FOR NEUTRAL GROUNDING DEVICES IEEE Std 32-1972
Table 5— Dielectric Test Voltage and Time to Test-Gap Flashover
Grounding Transformers, Ground-Fault Neutralizers & Reactors
*
*Dry-type reactors do not receive chopped-wave impulse test.
Resistors
†
†For resistors made in sections, see 10.3.2.
Capacitors
Impulse Tests Applied Potential Induced Potential
Chopped Wave Full Wave Applied
Potential
Applied
Potential
Induced
Insul- Potential
ation
Class
Oil Dry Minimum
Time
to
Flashover
Oil Dry
(Excluding
Reactors) Dry
Reactors
Oil Dry
(Excluding
Reactors) Dry
Reactors
Excluding
Reactors
Reactors
Oil Dry
Outdoor
Indoor
Outdoor
InOil
Dry door
kV
rms kV crest
Microseconds
kV crest kVrms kV rms
kV
crest kV rms kV rms kV rms kV rms
Col. 1 Column 2 Column 3 Column 4 Column 5 Column 6 Column 7 Column 8
1.2 54 10 1.5 1.0 45 10 45 10 4 10 Two
Times
Rated
Voltage
45 13 10 5 10 5 Two
2.5 69 20 1.5 1.0 60 20 60 15 10 19 60 25 19 7.5 15 10 Times
5.0 88 25 1.6 1.0 75 25 75 19 12 26 75 35 26 13.5 19 15 Rated
8.7 110 35 1.8 1.0 95 35 95 26 19 36 95 48 36 22 26 22 Voltage
15 130 50 2.0 1.25 110 50 110 34 31 50 110 67 50 36 34 28
18 145 - 2.25 125 - 125 40 40 - 125 - - - 40
25 175 75 3 150 75 150 50 50 70 150 93 70 60 50
34.5 230 105 3 200 105 200 70 70 95 200 127 95 80 70
46 290 140 3 250 140 250 95 95 250 95
60 345 - 3 300 - 300 120 120 300 120
69 400 215 3 350 215 350 140 140 350 140
92 520 3 450 450 185 450 185
115 630 3 550 550 230 550 230
138 750 3 650 650 275 650 275
161 865 3 750 750 325 750 325
180 950 3 825 825 360 825 360
196 1035 3 900 900 395 900 395
215 1120 3 975 975 430 975 430
230 1210 3
105
0 1050 460 1050 460
10
Copyright © 1972 IEEE All Rights Reserved
IEEE Std 32-1972 IEEE STANDARD REQUIREMENTS, TERMINOLOGY, AND
Table 6— Limiting Temperature Rises Above 30
° CAmbient for Current Carrying Parts—Neutral Devices*†
Column 1 Column 2 Column 3 Column 4 Column 5 Column 6 Column 7
55
°C
Oil-Immersed
55
°C
Dry-Type
80
°C
Dry-Type
150
°C
Dry-Type
Resistors
Temperature Rise,
°C
Temperature
Rise
°
C
Time
Factor
‡
seconds
Temperature
Rise
°
C
Time
Factor‡
seconds
Temperature
Rise
°
C
Time
Factor‡
seconds
Temperature
Rise
°
C
Time
Factor‡
seconds
Stainless
Steel
**
Cast
Grid #
Steady State
for Continuous
Current
Ratings
Steady State
(Hot-Spot)
Section 4.2
65 65 110 180 385 385
Steady State
(Average)
Section 4.2
55 55 80 150 - -
Rated Time
for Thermal
Current Ratings
(rated
voltage for
certain
resistors)
Extended-Time
(Average)
Section 4.3.3
75 75 110 200 610 385
Ten-Minute
(Average)
Section 4.3.2
125 125 185 275 610 460
Less than10
Minutes
(Average)
Section 4.3.1 120 22 500 120 103 000 160 143000 250 98 400 760 510
125 15 000 130 53 300 170 86 700 260 63 000
130 9 000 140 30 000 180 53 700 270 41 000
135 6 700 150 16700 190 33 300 280 26 600
140 4 300 160 9 330 200 21 000 290 18 000
145 3 000 170 5 330 210 13 700 300 12 000
150 2 100 180 3 330 220 9 170 310 8 340
155 1 500 190 2 000 230 6 170 320 5 670
160 1 030 200 1 270 240 4 170 330 4 000
165 733 210 767 250 2 830 340 2 800
170 517 220 500 260 1970 350 1 968
Copyright © 1972 IEEE All Rights Reserved
11
TEST PROCEDURE FOR NEUTRAL GROUNDING DEVICES IEEE Std 32-1972
180 270 230 33.3 280 967 360 1 434
190 143 240 257 300 500 380 750
200 77 250 150 320 277 400 406
210 40 260 103 340 157 420 243
220 25 270 73 360 94 440 140
230 15 280 50 380 61 460 84
240 8.8 300 26 400 34 480 51
250 5.3 320 14 420 22 500 32.4
260 3.3 340 7.7 440 14 520 20.5
NOTE — The values of both C and P shall he taken at the temperature corresponding to
q1 and standard ambient conditions.
*The values in this table are based on the thermal aging characteristics of the insulation. Devices built to these thermal limits will have normal insulation life.
Other factors may limit temperature rises in specific designs. For example:
1) The reduction in the mechanical strength and increase in elongation of copper at temperatures above 300
° C and aluminum above 350° C.
2) Gas evolution from insulation and oil adjacent to hot conductors.
3) Auto-ignition of insulation or oil.
†No limits have been established for capacitors.
‡The time factor of a device for use in determining the limit of temperature rise shall he calculated as follows:
f
q
1 = 1.182 q2 — q3 for 55° C oil-immersed devices
= 1.182
q2 for 55° C dry-type devices
= 1.373
q2 for 80° C dry-type devices
= 1.200
q2 for 150° C dry-type devices
where
q
1 = Steady-state hot-spot temperature rise at continuous current rating either above top oil temperature for oil-immersed equipment or above the ambient air temperature for dry-type equipment.
q
2 = Average winding rise over ambient for rated continuous current under standard operating conditions
q
3 = Top oil rise over ambient for rated continuous current under standard operating conditions
P
= Specific power (watts per unit mass of conductor material)
C
= Specific thermal capacitance (joules per degree Celsius unit mass) of conductor material and its associated insulation, as calculated in Eqs 13 and 14
M
= Multiplier from Table 7.
**The temperature rise limits for extended time, ten-minute or less rated resistors are hot-spot values.
Table 6— Limiting Temperature Rises Above 30
° CAmbient for Current Carrying Parts—Neutral Devices*† (Continued)
Column 1 Column 2 Column 3 Column 4 Column 5 Column 6 Column 7
55
°C
Oil-Immersed
55
°C
Dry-Type
80
°C
Dry-Type
150
°C
Dry-Type
Resistors
Temperature Rise,
°C
Temperature
Rise
°
C
Time
Factor
‡
seconds
Temperature
Rise
°
C
Time
Factor‡
seconds
Temperature
Rise
°
C
Time
Factor‡
seconds
Temperature
Rise
°
C
Time
Factor‡
seconds
Stainless
Steel
**
Cast
Grid #
C
q1Mseconds
P
= ----------------------------------
12
Copyright © 1972 IEEE All Rights Reserved
IEEE Std 32-1972 IEEE STANDARD REQUIREMENTS, TERMINOLOGY, AND
Table 7— Time Factor Multiplier M for Use in Equation in Table 6
Table 8— Limits of Top Oil Temperature Rise
5.2 Dielectric Tests (see definition in Section 13)
Dielectric tests shall consist of impulse tests (when required), followed by low-frequency applied-potential tests and
induced-potential tests.
Oil-Immersed Dry-Type
55
°C Rise 55°C Rise 80°C Rise 150°C Rise
Steady-State
Average
Winding Rise
Multiplier
M
Steady-State
Average
Winding Rise
Multiplier
M
Steady-State
Average
Winding Rise
Multiplier
M
Steady-State
Average
Winding Rise
Multiplier
M
55 1.0 55 1.0 80 1.0 150 1.0
54 0.13 54 0.15 79 0.18 149 0.21
53 0.074 53 0.088 78 0.098 148 0.12
52 0.053 52 0.066 77 0.069 147 0.085
51 0.043 51 0.052 76 0.056 146 0.067
50 0.037 50 0.045 75 0.047 145 0.056
48 0.031 49 0.040 74 0.041 144 0.049
46 0.026 48 0.036 73 0.037 142 0.040
44 0.024 46 0.031 72 0.034 140 0.034
40 0.022 44 0.027 70 0.030 138 0.031
35 0.021 42 0.025 68 0.027 136 0.028
<30 0.020 40 0.024 66 0.025 134 0.026
35 0.022 64 0.024 132 0.025
30 0.021 62 0.023 130 0.024
<25 0.020 60 0.022 128 0.023
55 0.021 124 0.022
<50 0.020 120 0.021
<115 0.020
Conservator or
Inert Gas
Cushion
No
Conservator or
Inert Gas
Cushion
Steady-State 55
°C 50°C
Extended-Time 55
°C 50°C
Short-Time 60
°C 55°C
Copyright © 1972 IEEE All Rights Reserved
13
TEST PROCEDURE FOR NEUTRAL GROUNDING DEVICES IEEE Std 32-1972
5.2.1 Impulse Tests (see definition in Section 13).
The impulse test wave is a 1.2
´ 50 microsecond wave, as described in Section 14.
1)
Reduced Full-Wave Test. For this test, the applied voltage wave shall have a crest value of 50 to 70 percent
of the full-wave value given in Table 5.
2)
Chopped-Wave Test. For this test, the applied voltage wave shall have a crest voltage and time to flashover in
accordance with Table 5. The chopped wave shall be obtained by flashover of an external rod gap.
3)
Full-Wave Test. For this test, the applied voltage wave shall have a crest value in accordance with Table 5.
The impulse test on a line terminal consists of one application of a reduced voltage full wave, two applications of a
chopped wave, followed by one application of a full wave. Either, but not both, positive or negative waves may be used.
Waves of negative polarity for oil-immersed devices and of positive polarity for dry-type or compound-filled devices
are recommended. If, in testing oil-immersed devices, the atmospheric conditions at the time of test are such that the
bushings will not withstand the specified polarity wave, then a wave of the opposite polarity may be used.
The impulse test on the neutral of a device, when brought out through a bushing, will consist of one reduced full wave
and two full waves having a crest voltage for the insulation class of the neutral, provided the size of the neutral bushing
pemits. This test on the neutral will be made only when specifically called for.
If the insulation class of the highest voltage neutral bushing for which provision is made is different from the insulation
class of the neutral end of the winding, the impulse test voltage will be for whichever insulation class is lower.
When applied directly to the neutral, the test waves shall be 1.2
´ 50 microsecond waves. When induced in the neutral
by the application of 1.2
´ 50 microsecond waves to other terminals, the wave shape shall be that resulting from the
characteristics of the winding. If the neutral ends of windings are specified for alternate or future connection as line
terminals, standard tests shall be applied to the neutral as outlined above.
The foregoing paragraphs also apply to the terminals of the series devices to be located in the neutral. If the series
windings have taps, tests should be made with maximum turns in the windings.
For impulse tests on devices of very low impedance, see Section 14.
When devices covered by this standard are to be assembled in a common tank and inter-connected, the complete
assembly shall be tested as a unit at 100 percent of the lowest test voltage required by any of the components.
5.2.2 Applied-Potential Test (See definition in Section 13).
For devices designed so that either terminal of a winding can be used as the line terminal, the applied-potential test
shall be made by applying between each winding and all other windings connected to ground a low-frequency voltage
from an external source in accordance with Table 5.
For devices with reduced insulation at the neutral, the applied-potential test shall be made by applying between the
winding and all other windings connected to ground a low-frequency voltage from an external source, in accordance
with Table 5 based upon the insulation class of the neutral end.
All other dielectric circuits and metal parts shall be grounded during the test.
The duration of the applied-potential test shall be one minute.
5.2.3 Induced-Potential Test (see definition in Section 13).
The induced-potential test for devices which receive the full standard applied-potential test shall be made by applying
between the terminals of one winding a voltage of twice the normal voltage developed in the winding, unless this will
14
Copyright © 1972 IEEE All Rights Reserved
IEEE Std 32-1972 IEEE STANDARD REQUIREMENTS, TERMINOLOGY, AND
produce a voltage between the terminals of any winding higher than the low-frequency test voltage specified in Table
5. In this case, the induced voltage developed between terminals of any winding shall be limited to the specified lowfrequency
test voltage for that winding.
If test conditions are such that the induced-potential test, if made as described in the previous paragraph, would not
produce the voltage required therein between adjacent three-phase line terminals, the test shall be modified to produce
this voltage.
For devices with reduced insulation at the neutral, when neither the applied-potential nor the induced-potential tests,
if made as described above, would produce between line terminals and ground (not necessarily between line terminal
and neutral) the low-frequency test voltage specified in Table 5, corresponding to the insulation class of the line end,
then the induced-potential test shall be modified to produce this voltage.
The induced-potential test shall be applied for 7200 cycles if the frequency is 120 Hz or more. The duration shall be
60 seconds for frequencies less than 120 Hz.
5.3 Losses and Impedance
Excitation losses and exciting current shall be determined from the rated voltage and frequency on a sine-wave basis
unless a different form is inherent in the operation of the apparatus.
Load losses shall be determined for rated voltage, or current, or both, and frequency, and shall be corrected to a
reference temperature of 75
°C.
6. Construction
6.1 Bushings, Insulators, and Oil
Bushings shall comply with IEEE Std 21-1964, Requirements and Test Code for Outdoor Apparatus Bushings (ANSI
C76.1-1964 (R 1970)).
Insulator units shall comply with IEEE Std 324-1971, Definitions and Requirements for High-Voltage Air Switches,
Insulators, and Bus Supports (ANSI C37.30-1970 ).
The dielectric strength of insulating oils when shipped shall be 30 kV minimum as measured by the standard methods
of testing electrical insulating oils. See American National Standard Method of Testing Electrical Insulating Oils,
C59.2-1970.
6.2 Nameplates
Nameplates of devices designed primarily as grounding devices shall have as a minimum amount of information the
following:
1) Name of manufacturer
2) Serial number
3) Name of device
4) Type designation (if any)
5) Impedance (except resistors)
6) Number of phases
7) Temperature coefficient of resistance and resistance at 25
°C (for resistors only)
Copyright © 1972 IEEE All Rights Reserved
15
TEST PROCEDURE FOR NEUTRAL GROUNDING DEVICES IEEE Std 32-1972
8) Rated current
NOTES:
1 — Initial value of current for neutral grounding resistors rated on constant voltage.
2 — When a neutral grounding device has more than one current-time rating, each rating shall appear on the nameplate.
9) Rated frequency
10) Rated time
11) Rated voltage
12) BIL of “line”
13) Indoor or outdoor service
14) Weight
15) Volume of oil
16) Instruction book number or equivalent
The nameplate shall be affixed to each device by the manufacturer. Unless otherwise specified, it shall be made of
corrosion-resistant materials. On devices with one or more taps not brought out of the case, a diagram shall be
included.
6.3 Tanks and Enclosures
Tanks where applicable shall be designed to withstand without permanent deformation a pressure 25 percent greater
than the maximum operating pressures resulting from the system of insulation preservation or ventilation used. For
tanks, the maximum operating pressures (positive and negative) for which the tank is designed shall be indicated on the
rating plate.
Bolted or welded main cover construction shall be considered as alternate standards.
A pigment paint shall be used when surfaces are painted.
Tanks shall be designed for vacuum filling in the field on all current-limiting devices with 350 BIL or higher and all
rated 2000 kVA equivalent or higher of any insulation level.
7. Reactors
7.1 Insulation Levels
The line end and ground end insulation levels shall be selected from Table 4 on the basis of fault voltage criteria,
Columns 3 and 4.
7.2 Dielectric Tests
Dielectric test withstand levels shall be those listed in Table 5 for applicable insulation class, as follows:
Impulse chopped-wave test
(oil-type only) Column 2
Impulse full-wave test Column 3
Applied-potential test Column 4
Induced-potential test Column 5
16
Copyright © 1972 IEEE All Rights Reserved
IEEE Std 32-1972 IEEE STANDARD REQUIREMENTS, TERMINOLOGY, AND
7.2.1 Impulse Tests.
Reactors shall be designed to withstand impulse tests, but the impulse tests are required only when specified.
7.2.2 Applied-Potential Tests.
Applied-potential tests are required. They shall be made at the test voltage corresponding to the insulation class
assigned to the ground end if it is lower than the insulation class assigned to the line end. The external ground
connection is removed for this test. If the ground end bushing is not capable of withstanding this test voltage. it is to
be disconnected, and, if necessary, special insulation is to be provided for test purposes.
7.2.3 Induced-Potential Tests.
Induced-potential tests are required. The ground end shall be connected to ground for this test. When taps are provided,
the test shall be made on the tap which produces the highest voltage per turn in the winding and also on the connections
having the greatest number of turns, but the voltage to ground at the line end shall not exceed the specified test voltage.
For dry-type reactors a voltage of suitable frequency having a crest value equal to the crest value of the voltage listed
in Column 5, Table 5, shall be applied between the terminals of each winding.
For oil-immersed reactors, each line end and each ground end of the winding shall be tested by applying to the terminal
one 50-percent, followed by two 100-percent full waves of the value given in Column 5, Table 5, for the voltage class
in Column 1 corresponding to the voltage class of the winding being tested.
8. Ground-Fault Neutralizers
8.1 Insulation Levels
The line end and ground end insulation levels shall be selected from Table 4 on the basis of fault voltage criteria.
Columns 3 and 4.
8.2 Dielectric Tests
Dielectric test withstand levels shall be those listed in Table 5 for applicable insulation class, as follows:
8.2.1 Impulse Tests.
Ground-fault neutralizers shall be designed to withstand impulse tests, but the impulse tests are required only when
specified.
Impulse chopped-wave test Column 2
Impulse full-wave test Column 3
Applied-potential test Column 4
Induced-potential test Column 5
Copyright © 1972 IEEE All Rights Reserved
17
TEST PROCEDURE FOR NEUTRAL GROUNDING DEVICES IEEE Std 32-1972
8.2.2 Applied-Potential Tests.
Applied-potential tests are required. They shall be made at the test voltage corresponding to the insulation class
assigned to the ground end if it is lower than the insulation class assigned to the line end. The external ground
connection is removed for this test. If the ground end bushing is not capable of withstanding this test voltage, it is to
be disconnected, and, if necessary, special insulation is to be provided for test purposes.
8.2.3 Induced-Potential Tests.
Induced-potential tests are required. The ground end shall be connected to ground for this test. When taps are provided,
the test shall be made on the tap which produces the highest voltage per turn in the winding and also on the connections
having the greatest number of turns, but the voltage to ground at the line end shall not exceed the specified test voltage.
9. Grounding Transformers
9.1 Insulation Levels
The line end and ground end insulation levels shall be selected from Table 4 on the following basis:
9.2 Dielectric Tests
Dielectric test withstand levels shall be those listed in Table 5 for the applicable insulation class, as follows:
9.2.1 Impulse Tests.
Grounding transformers shall be designed to withstand impulse tests, but the impulse tests are required only when
specified.
Line End Ground End
Disconnectable
from Ground
System Insulation
Class Columns
1 and 2
System Insulation Class
Columns
1 and 2
Permanently
Grounded
System Insulation
Class Columns
1 and 2
Fault Voltage
Criteria Columns
3 and 4
Impulse chopped-wave test Column 2
Impulse full-wave test Column 3
Applied-potential test Column 4
Induced-potential test Column 5
18
Copyright © 1972 IEEE All Rights Reserved
IEEE Std 32-1972 IEEE STANDARD REQUIREMENTS, TERMINOLOGY, AND
9.2.2 Applied-Potential Tests.
Applied-potential tests are required. They shall be made at the test voltage corresponding to the insulation class
assigned to the ground end if it is lower than the insulation class assigned to the line end. The external ground
connection is removed for this test. If the ground end bushing is not capable of withstanding this test voltage, it is to
be disconnected and, if necessary, special insulation is to be provided for test purposes.
9.2.3 Induced-Potential Tests.
Induced-potential tests are required. The ground end shall be connected to ground for this test. When taps are provided,
the test shall be made on the tap which produces the highest voltage per turn in the winding and also on the connections
having the greatest number of turns, but the voltage to ground at the line end shall not exceed the specified test voltage.
The rated voltage of a grounding transformer is the maximum line-to-line voltage at which it is designed to operate
continuously.
10. Resistors
10.1 Resistor Element
A resistor element is the conducting unit which functions to limit the current flow to a predetermined value.
The element material shall possess a balanced combination of properties, uniformity of resistance, and mechanical
stability over the intended operating temperature range, without any injurious effects to the elements and its associated
insulation.
10.1.1 Rated Voltage (see definition in Section 13).
Since the active material used in resistors has an appreciable temperature coefficient, the resistance is materially
changed during the time of operation causing the voltage to increase or the current to decrease. When the product of
the fault current and resistance at 25
°C exceeds 80 percent of the line-to-neutral voltage of the circuit, the resistor shall
be rated for constant voltage and the rated voltage shall be taken equal to the line-to-neutral voltage.
10.1.2 Temperature Coefficient of Resistance.
1) The conductor element resistance changes to some extent with temperature. The change may be calculated
from the temperature coefficient of resistivity.
(2)
(3)
R
1 and R2 are resistances in ohms at temperatures q1, and q2 in degrees Celsius, respectively, and a is the
temperature coefficient of resistance.
2) Where special temperature coefficient is required, such data are to be brought to the attention of those
responsible for the design of an unusual service condition.
a
R
2 – R1
R
1(q2 – q1) = ----------------------------
R
2 = R1[1 + a(q2 – q1)]
Copyright © 1972 IEEE All Rights Reserved
19
TEST PROCEDURE FOR NEUTRAL GROUNDING DEVICES IEEE Std 32-1972
10.1.3 Conductor Connections.
All conductor terminations shall be bolted, welded, or brazed. Low-melting alloys used to join connectors which
would be adversely affected by the resistor operating temperatures shall not be used. All conductor terminations must
be mechanically secure to provide continuous electrical continuity.
10.1.4 Resistance Test.
Overall resistance shall be measured to determine that the resistance is within the design value. Unless the application
requires close resistance tolerance, the dc resistance shall not vary more than
± 10 percent from the guaranteed value.
10.2 Insulation Levels
The line end and ground end insulation levels shall be selected from Table 4 on the basis of fault voltage criteria,
Columns 3 and 4.
10.3 Dielectric Tests
Dielectric test withstand levels shall be those listed in Table 5, Column 6.
10.3.1 Impulse Tests.
Impulse tests are not required for resistors.
10.3.2 Applied-Potential Tests.
Applied-potential tests are required. They shall be made by applying between terminals and ground for the complete
device, or between terminals of each unit and its own individual frame, the specified voltage from a suitable external
source. When specifications do not require that such a resistor be completely assembled at the factory, it shall be
permissible for the manufacturer to waive the applied voltage test of the complete device, substituting the appliedpotential
test of each section, supplemented by insulation d a which will show that the complete resistor will meet the
insulation requirements of service and would pass the applied-potential test when assembled.
In many cases resistors are made in sections insulated from each other and from ground by standard apparatus
insulators whose insulation value has been established. Each section may consist of one or more frames or unit
assemblies of resistance material supported on a suitable framework. In such cases each frame or unit assembly shall
receive an applied-potential test. The voltage applied from the terminals of each assembly to its own frame shall be
twice the rated voltage of the section of which the frame is a part plus 1000 V when rated 600 V or less, or 2.25 times
the rated value plus 2000 V when rated over 600 V.
10.3.3 Induced-Potential Tests.
Induced-potential tests (except as may be incidental to any required temperature test) are not required for resistors.
11. Capacitors
11.1 Insulation Levels
The line end and ground end insulation levels shall be selected from Table 4 on the basis of fault voltage criteria,
Columns 3 and 4.
20
Copyright © 1972 IEEE All Rights Reserved
IEEE Std 32-1972 IEEE STANDARD REQUIREMENTS, TERMINOLOGY, AND
11.2 Dielectric Tests
Dielectric test withstand levels shall be those listed in Table 5, Columns 7 and 8.
11.2.1 Impulse Tests.
Impulse tests are not required for capacitors.
11.2.2 Applied-Potential Tests.
Applied-potential tests (terminal-to-case) are required for all neutral grounding capacitors, except those having one
terminal permanently grounded inside the case (the case being solidly grounded under operating conditions).
Capacitors designed for operation with one terminal permanently grounded outside of the case may have the ground
connection removed for this test.
11.2.3 Induced-Potential Tests.
Induced-potential tests (terminal-to-terminal) are required on all neutral grounding capacitors. The induced test
voltages shall be two times the rated voltage of the capacitor. If a capacitor has two or more voltage ratings, the test
voltage shall be based on the highest voltage rating.
12. Combination Devices
Combination devices are any combination of devices of Sections 7. through 11., including distribution transformers,
with resistance, inductance, or capacitance, or combination of two or more of these elements connected in their lowvoltage
winding or the delta of such low-voltage windings.
12.1 Insulation Levels
The line end and ground end insulation levels shall be selected from Table 4 on the following basis:
12.2 Dielectric Tests
Dielectric test withstand levels shall be those listed in Table 5 for the applicable insulation class, as follows:
Line End Ground End
Disconnectable
from Ground
System Insulation
Class Columns
1 and 2
System Insulation
Class Columns
1 and 2
Permanently
Grounded
System Insulation
Class Columns
1 and 2
Fault Voltage
Criteria Columns
3 and 4
Copyright © 1972 IEEE All Rights Reserved
21
TEST PROCEDURE FOR NEUTRAL GROUNDING DEVICES IEEE Std 32-1972
12.2.1 Impulse Tests.
Combination devices shall be designed to withstand impulse tests, but the impulse tests are required only when
specified. Components not requiring impulse tests shall be disconnected or suitably protected.
12.2.2 Applied-Potential Tests.
Applied-potential tests are required. They shall be made by applying between terminals and ground for the complete
device, or between terminals of each unit and its own individual frame, the specified voltage from a suitable external
source. When specifications do not require that such a device be completely assembled at the factory, it shall be
permissible for the manufacturer to waive the applied-potential test of the complete device, substituting the appliedpotential
test of each section, supplemented by insulation data which will show that the complete device will meet the
insulation requirements of service and would pass the applied-potential test when assembled.
12.2.3 Induced-Potential Tests.
Induced-potential tests shall be made as required for each component part but not on the whole.
13. Definitions and Terminology
acceptance test:
A test to demonstrate the degree of compliance of a device with purchaser's requirements.
ambient temperature:
The temperature of the medium such as air, water, or earth into which the heat of the
equipment is dissipated.
NOTES:
1 — For self-ventilated equipment, the ambient temperature is the average temperature of the air in the immediate neighborhood
of the equipment.
2 — For air- or gas-cooled equipment with forced ventilation or secondary water cooling, the ambient temperature is taken as that
of the ingoing air or cooling gas.
3 — For self-ventilated enclosed (including oil-immersed) equipment considered as a complete unit, the ambient temperature is
the average temperature of the air outside of the enclosure in the immediate neighborhood of the equipment.
applied-potential test:
A dielectric test in which the test voltage is a low-frequency alternating voltage from an
external source applied between conducting parts, and between conducting parts and ground.
askarel:
A synthetic nonflammable insulating liquid, that, when decomposed by an electric arc, evolves only
nonflammable gaseous mixtures.
basic impulse insulation level (BIL):
A reference impulse insulation strength expressed in terms of the crest value of
the withstand voltage of a standard full impulse voltage wave.
chopped wave:
A voltage impulse that is terminated intentionally by sparkover of a gap.
compound-filled (for a grounding device):
Having the windings encased in an insulating fluid that becomes solid or
remains slightly plastic, at normal operating temperatures.
Impulse chopped-wave test Column 2
Impulse full-wave test Column 3
Applied-potential test Columns 4, 6, and 7
Induced-potential test Columns 5 and 8
22
Copyright © 1972 IEEE All Rights Reserved
IEEE Std 32-1972 IEEE STANDARD REQUIREMENTS, TERMINOLOGY, AND
conservator system or expansion tank system:
A method of oil preservation in which the oil in the main tank is
sealed from the atmosphere, over the temperature range specified, by means of an auxiliary tank partly filled with oil
and connected to the completely filled main tank.
critical impulse flashover voltage:
The crest value of the impulse wave that, under specified conditions, causes
flashover through the surrounding medium on 50 percent of the applications.
design test:
A test made by the manufacturer to obtain data for design application.
dielectric tests:
Tests that consist of the application of a voltage, higher than the rated voltage, for a specified time to
prove compliance with the required voltage class of the device.
disconnectable device:
A grounding device that can be disconnected from ground by the operation of a disconnecting
switch, circuit breaker, or other switching device.
dry-type (for a grounding device):
Having the windings immersed in an insulating gas.
extended-time rating (of a grounding device):
A rated time in which the period of time is greater than the time
required for the temperature rise to become constant but is limited to a specified average number of days operation per
year.
gas-oil sealed system:
A method of oil preservation in which the interior of the tank is sealed from the atmosphere
over the temperature range specified, by means of an auxiliary tank or tanks, to form a gas-oil seal operating on the
manometer principle.
ground:
A conducting connection, whether intentional or accidental, by which an electric circuit or equipment is
connected to the earth, or to some conducting body, of a relatively large extent, that serves in place of the earth. It is
used for establishing and maintaining the potential of the earth (or of the conducting body) or approximately that
potential, on conductors connected to it, and for conducting ground current to and from the earth (or the conducting
body).
ground current:
The current flowing in the earth or in a grounding connection.
grounded:
Connected to earth or to some extended conducting body that serves instead of the earth, whether the
connection is intentional or accidental.
grounded circuit:
A circuit in which one conductor or point (usually the neutral conductor or neutral point of
transformer or generator windings) is intentionally grounded, either solidly or through a grounding device.
grounded concentric wiring system:
A grounded wiring system in which the external (outer) conductor is solidly
grounded and completely surrounds the internal (inner) conductor throughout its length. The external conductor is
usually uninsulated.
grounded conductor:
A conductor that is intentionally grounded, either solidly or through a current-limiting device.
grounded parts:
Parts that are intentionally connected to ground.
grounded system:
A system of conductors in which at least one conductor or point (usually the neutral conductor or
neutral point of transformer or generator windings) is intentionally grounded, either solidly or through a currentlimiting
device.
ground end (of a neutral grounding device):
The end or terminal that is grounded directly or through another device.
ground-fault neutralizer:
A grounding device that provides an inductive component of current in a ground fault that
is substantially equal to and therefore neutralizes the rated-frequency capacitive component of the ground-fault
current, thus rendering the system resonant grounded.
ground-fault neutralizer grounded:
Reactance grounded through such values of reactance that, during a fault
between one of the conductors and earth, the rated-frequency current flowing in the grounding reactances and the
rated-frequency capacitance current flowing between the unfaulted conductors and earth shall be substantially equal.
In the fault, these two components of fault current will be substantially 180 degrees out of phase.
Copyright © 1972 IEEE All Rights Reserved
23
TEST PROCEDURE FOR NEUTRAL GROUNDING DEVICES IEEE Std 32-1972
NOTE — When a system is ground-fault neutralizer grounded, it is expected that the quadrature component of the rated-frequency
single-phase-to-ground fault current will be so small that an arc fault in air will be self-extinguishing.
grounding device:
An impedance device used to connect conductors of an electric system to ground for the purpose
of controlling the ground current or voltage to ground.
NOTE — The grounding device may consist of a grounding transformer or a neutral grounding device, or a combination of these.
Protective devices, such as lightning arresters, may also be included as an integral part of the device.
grounding transformer:
A transformer intended primarily to provide a neutral point for grounding purposes.
NOTE — It may be provided with a delta winding in which resistors or reactors may be connected.
impedance grounded:
Grounded through impedance.
NOTE — The components of the impedance need not be at the same location.
impedance voltage:
Comprises an effective resistance component corresponding to the impedance losses, and a
reactance component corresponding to the flux linkages of the winding.
impulse flashover voltage:
The crest voltage of an impulse causing a complete disruptive discharge through the air
between electrodes of a test specimen.
impulse tests:
Dielectric tests in which the voltage applied is an impulse voltage of specified wave shape. The wave
shape of an impulse test wave is the graph of the wave as a function of time or distance.
NOTE — It is customary in practice to express the wave shape by a combination of two numbers, the first part of which represents
the wave front and the second the time between the beginning of the impulse and the instant at which one-half crest value
is reached on the wave tail, both values being expressed in microseconds, such as a 1.2
´ 50 microsecond wave.
impulse withstand voltage:
The crest value of an applied impulse voltage which does not cause a flashover, puncture,
or disruptive discharge on the test specimen.
induced-potential test:
A dielectric test in which the test voltage is an alternating voltage of suitable frequency,
applied or induced between the terminals.
inert gas-pressure system:
A method of oil preservation in which the interior of the tank is sealed from the
atmosphere, over the temperature range specified, by means of a positive pressure of inert gas maintained from a
separate inert-gas source and reducing-valve system.
inhibited oil:
Mineral transformer oil to which a synthetic oxidation inhibitor has been added.
insulation class (of a grounding device):
A number that defines the insulation levels of the device.
insulating materials:
classification. See Table 9.
line end (of a neutral grounding device):
The end or terminal of the device that is connected to the line circuit
directly or through another device.
24
Copyright © 1972 IEEE All Rights Reserved
IEEE Std 32-1972 IEEE STANDARD REQUIREMENTS, TERMINOLOGY, AND
Table 9— Classification of Insulating Materials
NOTES:
1 — Insulation is considered to be “impregnated” when a suitable substance provides a bond between components of the structure
and also a degree of filling and surface covering sufficient to give adequate performance under the extremes of temperature,
surface contamination (moisture, dirt, etc), and mechanical stress expected in service. The impregnant must not flow or
deteriorate enough at operating temperature so as to seriously affect performance in service.
2 — The electrical and mechanical properties of the insulation must not be impaired by the prolonged application of the limiting
insulation temperature permitted for the specific insulation class. The word “impaired” is here used in the sense of causing any
change which could disqualify the insulating material for continuously performing its intended function, whether creepage
spacing, mechanical support, or dielectric barrier action.
3 — In the above definitions, the words “accepted test” are intended to refer to recognized test procedures established for the
thermal evaluation of materials by themselves or in simple combinations. Experience or test data, used in classifying
insulating materials, are distinct from the experience based on test data derived for the use of materials in complete insulation
systems. The thermal endurance of complete systems may be determined by test procedures specified by the cognizant
technical committees. A material that is classified as suitable for a given temperature may be found suitable for a different
temperature, either higher or lower. by an insulation system test procedure. For example, it has been found that some materials
suitable for operation at one temperature in air may be suitable for a higher temperature when used in a system operated in an
inert-gas atmosphere.
4 — It is important to recognize that other characteristics, in addition to thermal endurance, such as mechanical strength, moisture
resistance, and corona endurance, are required in varying degrees in different applications for the successful use of insulating
materials.
5 — The rate of thermal degradation of materials is influenced by environmental and other factors. The thermal characteristics of
a material may be radically altered by exposure to nuclear radiation and by various processing techniques. Because of these
many factors, realistic temperature classifications for insulating materials should be based on information obtained from field
experience and from thermal aging tests. They cannot be based on composition alone.
losses (of a grounding device):
The I2R loss in the windings, core loss, dielectric loss, loss due to stray magnetic
fluxes in windings and other metallic parts of the device, and, in cases involving parallel windings, losses due to
circulating currents.
NOTES:
1 — The losses as here defined do not include any losses produced by the device in adjacent apparatus or materials not a part of
the device.
2 — Losses will normally be considered at the rated thermal current but in some cases may be required at other current ratings, if
more than one rating is specified, or at no load, as for grounding transformers. The losses may be given at 25
°C or 75°C.
Class 105
°C
(Class A)
Materials or combinations of materials such as cotton, silk, and
paper when suitably impregnated or coated or when immersed in
a dielectric liquid such as oil. Other materials or combinations of
materials may be included in this class if by experience or
accepted tests they can be shown to be capable of operation at
105
°C.
Class 130
°C
(Class B)
Materials or combinations of materials such as mica, glass, fiber,
asbestos, etc, with suitable bonding substances. Other materials
or combinations of materials, not necessarily inorganic, may be
included in this class if by experience or accepted tests they can
be shown to be capable of operation at 130
°C.
Class 220
°C Materials or combinations of materials which by experience or
accepted tests can be shown to be capable of operation at 220
°C.
Class Over
220
°C
(Class C)
Insulation that consists entirely of mica, porcelain, glass, quartz,
and similar inorganic materials. Other materials or combinations
of materials may be included in this class if by experience or
accepted tests they can be shown to be capable of operation at
temperatures over 220
°C.
Copyright © 1972 IEEE All Rights Reserved
25
TEST PROCEDURE FOR NEUTRAL GROUNDING DEVICES IEEE Std 32-1972
mechanical limit:
The rated maximum instantaneous value of current, in amperes, that the device will withstand
without mechanical failure.
minimum impulse flashover voltage:
The crest value of the lowest voltage impulse at a given wave shape and
polarity that causes flash-over.
neutral grounding device:
A grounding device used to connect the neutral point of a system of electric conductors to
earth.
NOTE — The device may consist of a resistance, inductance, or capacitance element, or a combination of them.
neutral-grounding capacitor:
A neutral grounding device, the principal element of which is capacitance.
neutral grounding impedor:
A neutral grounding device comprising an assembly of at least two of the elements,
resistance, inductance, or capacitance.
neutral grounding reactor:
A neutral grounding device, the principal element of which is inductive reactance.
neutral grounding resistor:
A neutral grounding device, the principal element of which is resistance.
neutral grounding wave trap:
A neutral grounding device comprising a combination of inductance and capacitance
designed to offer a very high impedance to a specified frequency or frequencies.
NOTE — The inductances used in neutral grounding wave traps should meet the same requirements as neutral grounding reactors.
oil:
Includes synthetic insulating liquids as well as mineral transformer oil.
oil-immersed (for a grounding device):
Means that the windings are immersed in an insulating oil.
permanently grounded device:
A grounding device designed to be permanently connected to ground, either solidly
or through current transformers and/or another grounding device.
rated continuous current:
The current expressed in amperes, root mean square, that the device can carry
continuously under specified service conditions without exceeding the allowable temperature rise.
rated current (of a neutral device) (current rating):
The rated thermal current of a neutral device.
rated frequency (of a grounding device):
The frequency of the alternating current for which a device is designed.
rated thermal current:
The rms neutral current in amperes which the device is rated to carry under standard operating
conditions for rated time without exceeding temperature limits.
rated time (time rating):
The time during which the device will carry its rated thermal current (or, for certain
resistors, withstand its rated voltage) under standard operating conditions, without exceeding the limitations
established by these standards.
rated-time temperature rise (for a grounding device):
The maximum temperature rise above ambient attained by
the winding of a device as the result of the flow of rated thermal current (or, for certain resistors, the maintenance of
rated voltage across the terminals) under standard operating conditions, for rated time and with a starting temperature
equal to the steady-state temperature. It may be expressed as an average or a hot-spot winding rise.
rated voltage:
The rms voltage, at rated frequency, which may be impressed between the terminals of the device under
standard operating conditions for rated time (continuously for grounding transformers) without exceeding the
limitations established by these standards.
reactance grounded:
Grounded through impedance, the principal element of which is reactance.
NOTE — The reactance may be inserted either directly, in the connection to ground, or indirectly by increasing the reactance of the
ground return circuit. The latter may be done by intentionally increasing the zero-sequence reactance of apparatus
connected to ground, or by omitting some of the possible connections from apparatus neutrals to ground.
resistance grounded:
Grounded through impedance, the principal element of which is resistance.
NOTE — The resistance may be inserted either directly, in the connection to the ground, or indirectly, as, for example, in the
secondary of a transformer, the primary of which is connected between neutral and ground, or in series with the deltaconnected
secondary of a wye-delta grounding transformer.
26
Copyright © 1972 IEEE All Rights Reserved
IEEE Std 32-1972 IEEE STANDARD REQUIREMENTS, TERMINOLOGY, AND
resistance method of temperature determination:
The determination of the temperature by comparison of the
resistance of a winding at the temperature to be determined, with the resistance at known temperature.
routine test:
A test made for quality control by the manufacturer on every device, on representative samples, or on
parts or materials as required to verify during production that the product meets the design specifications.
sealed tank system:
A method of oil preservation in which the interior of the tank is sealed from the atmosphere and
in which the gas plus the oil volume remains constant over the temperature range.
short-time rating (of a grounding device):
A rated time of ten minutes or less.
solidly grounded:
Grounded through an adequate ground connection in which no impedance has been inserted
intentionally.
NOTE — Adequate as used here means suitable for the purpose intended.
starting temperature (for a grounding device):
The winding temperature at the start of the flow of thermal current.
steady-state temperature rise (for a grounding device):
The maximum temperature rise above ambient which will
be attained by the winding of a device as the result of the flow of rated continuous current under standard operating
conditions. It may be expressed as an average or a hot-spot winding rise.
tap:
An available connection which permits changing the active portion of the device in the circuit.
thermal current rating:
For resistors whose rating is based on constant voltage, the initial rms symmetrical value of
the current that will flow when rated voltage is applied.
thermometer method of temperature determination:
The determination of the temperature by mercury, alcohol,
resistance, or thermocouple thermometers, any of these instruments being applied to the hottest accessible part of the
device.
ungrounded:
Without an intentional connection to ground except through potential indicating of measuring devices or
other very-high-impedance devices.
uninhibited oil:
Mineral transformer oil to which no synthetic oxidation inhibitor has been added.
voltage to ground:
The voltage between any live conductor of a circuit and the earth.
NOTE — Where safety considerations are involved, the voltage to ground that may occur in an ungrounded circuit is usually the
highest voltage normally existing between the conductors of the circuit, but in special circumstances higher voltages
may occur.
14. Test Code
The usual program of testing a neutral grounding device includes some or all of the following tests:
1) Resistance measurements
2) Dielectric tests
3) Impedance and loss measurements
4) Temperature-rise tests
14.1 Resistance Measurements
14.1.1 Necessity for Resistance Measurements.
Resistance measurements are of fundamental importance for two purposes:
1) For the calculation of the conductor
I2 R loss
2) For the calculation of winding temperatures at the end of a temperature test
Copyright © 1972 IEEE All Rights Reserved
27
TEST PROCEDURE FOR NEUTRAL GROUNDING DEVICES IEEE Std 32-1972
14.1.2 Determination of Cold Temperature.
The cold temperature of the winding shall be determined as accurately as possible when measuring the cold resistance.
The following precautions shall be observed'.
14.1.2.1 General.
Cold resistance measurements shall not be taken on a device when it is located in drafts or when it is located in a room
in which the temperature is fluctuating rapidly.
14.1.2.2 Windings Out of Oil.
The temperature of the windings shall be recorded as the average of several thermocouples or thermometers inserted
between the winding sections, with extreme care used to see that their junctions or bulbs are as nearly as possible in
actual contact with the windings. It should not be assumed that the windings are at the same temperature as the
surrounding air.
14.1.2.3 Windings Immersed in Oil.
The temperature of the windings shall be assumed to be the same as the temperature of the oil, provided the device has
been under oil with no current in its winding from 3 to 8 hours before the cold resistance is measured, depending upon
the size of the device.
14.1.3 Drop-of-Potential Method.
The drop-of-potential method is generally more convenient than the bridge method for measurements made in the
field.
In all cases, greater accuracy may be obtained by the use of potentiometers for the measurement of both current and
voltage although the setup may be rather cumbersome.
Figure 1— Connections for the Drop-of-Potential Method of Resistance Measurement
Measurement is made with direct current, and simultaneous readings of current and voltage are taken using the
connections of Fig. 1. The required resistance is calculated from the readings in accordance with Ohm's law.
In order to minimize errors of observation, the measuring instruments insofar as possible shall have such ranges as will
give reasonably large deflections.
28
Copyright © 1972 IEEE All Rights Reserved
IEEE Std 32-1972 IEEE STANDARD REQUIREMENTS, TERMINOLOGY, AND
The voltmeter leads shall be connected as closely as possible to the terminals of the winding to be measured. This is
to avoid including in the reading the resistances of current-carrying leads and their contacts and of extra lengths of
leads.
To avoid dangerous induced voltages, a rheostat should be used to reduce the current to less than
1/2 percent of rated
winding current before opening the circuit. Also, to protect the voltmeter from injury by off-scale deflections, it should
be disconnected before switching the current on or off.
If the drop of potential is less than 1 V, a potentiometer or millivoltmeter shall be used.
Readings shall not be taken until after the current and voltage have reached steady-state values.
Readings shall be taken with not less than four values of a current when deflecting instruments are used. The average
of the resistances calculated from these measurements shall be considered to be the resistance of the circuit.
The current used shall not exceed 15 percent of the rated continuous current of the winding whose resistance is to be
measured. Larger values may cause inaccuracy by heating the winding and thereby changing its temperature and
resistance.
14.1.4 Bridge Methods.
Bridge methods are generally preferred because of their accuracy and convenience, since they may be employed for
the measurement of resistances up to 10 000 ohms. The rheostat should always be turned to minimum current before
opening the circuit.
Bridge methods are especially recommended for all measurements that are to be used in connection with temperaturerise
determination.
14.2 Dielectric Tests
14.2.1 Test Procedure.
Unless otherwise specified, dielectric tests shall be made in accordance with IEEE Std 4-1968, Techniques for
Dielectric Tests (ANSI C68.1-1968).
NOTE — Where a sphere gap is used to measure voltage, IEEE Std 4-1968 provides for reduction of the series resistance when the
test frequency exceeds 1 kHz. The resistance should be in an inverse ratio to the frequency, which for current-limiting
reactors means the sphere gap resistance must be short-circuited.
14.2.1.1 Factory Dielectric Tests.
The purpose of dielectric tests in the factory is to check the insulation and workmanship, and, when required, to
demonstrate that the device has been designed to withstand the insulation tests required by the purchase specifications.
Impulse tests, when required, shall precede the low-frequency tests.
Dielectric tests should preferably be made at the temperature assumed under normal operation or at the temperature
attained under the conditions of commercial test.
14.2.1.2 Insulation Resistance.
The insulation resistance of machinery is of doubtful significance as compared with the dielectric strength. It is subject
to wide variation with temperature, humidity, and cleanliness of the parts. When the insulation resistance falls below
prescribed values, it can, in most cases of good design and where no defect exists, be brought up to the required
Copyright © 1972 IEEE All Rights Reserved
29
TEST PROCEDURE FOR NEUTRAL GROUNDING DEVICES IEEE Std 32-1972
standard by cleaning and drying the machine. The insulation resistance, therefore, may afford a useful indication as to
whether the machine is in suitable condition for application of the dielectric test.
The insulation-resistance test shall be made with all circuits of equal voltage above ground connected together.
Circuits or groups of circuits of different voltage above ground shall be tested separately.
14.2.1.3 Periodic Dielectric Tests in the Field.
It is recognized that dielectric tests impose a severe stress on the insulation, and if applied frequently will hasten
breakdown or may cause breakdown, the stress imposed, of course, being the more severe the higher the value of the
applied voltage. Hence, practice in this matter has varied widely among operating companies, and the advisability of
periodic testing may be questionable.
It is recommended that field tests of insulation should not be in excess of 75 percent of the factory test voltage; that for
old apparatus rebuilt in the field, tests should not be in excess of 75 percent of the factory test voltage; and that periodic
insulation tests in the field should not be in excess of 65 percent of the factory test voltage. These recommendations
relate to dielectric tests applied between windings and ground and to tests between turns.
Under some conditions, devices may be subjected to periodic insulation test using dc voltage. In such cases, the test dc
voltage should not exceed the original factory test rms alternating voltage; for example, the factory test was 26 kV rms,
then the routine test dc voltage should not exceed 26 kV.
Periodic dc tests should not be applied to devices of higher than 34.5 kV rating.
14.2.1.4 Tests on Bushings.
When tests are required on bushings separately from the devices, the tests shall be made in accordance with IEEE Std
21-1964 , Requirements and Test Code for Outdoor Apparatus Bushings (ANSI C76.1-1964 (R1970)).
14.2.2 Applied-Potential Tests.
The terminal ends and taps brought out of the case of the winding under test should all be joined together and to the
line terminal of the testing transformer. The duration of these tests shall be 1 minute and of a value given in Table 5.
All other terminals and parts (including magnetic shield and tank) should be connected to ground and to the other
terminal of the testing transformer.
The ground connections between the apparatus being tested and the testing transformer must be a substantial metallic
circuit. All connections must make a good mechanical joint without forming sharp corners or points.
Small bare wire may be used in connecting the respective taps and line terminal together, but care must be taken to
keep the wire on the high-voltage side well away from the ground.
The high-voltage lead from the testing transformer should preferably be at least
1/8 inch (3 mm) in overall diameter.
Neutral Capacitor Terminal-to-Case
. The terminal-to-case applied-potential test shall be made by applying between
terminal and case, with the case connected to ground, the specified alternating voltage.
Neutral Resistor
. Tests shall be made by applying between terminals and ground, for the complete device, or between
terminals of each unit and its own frame, for individual frames, the specified alternating voltage.
No appreciable resistance should be placed between the testing transformer and the device under test. It is permissible,
however, to use reactive coils at or near the terminals of the testing transformer.
30
Copyright © 1972 IEEE All Rights Reserved
IEEE Std 32-1972 IEEE STANDARD REQUIREMENTS, TERMINOLOGY, AND
A sphere gap set at a voltage 10 percent or more in excess of the specified test voltage shall be connected during the
applied-potential test.
For devices to be tested at 50 kV or less, it is permissible to depend on the ratio of the testing transformer to indicate
the proper test voltage and also to omit the sphere gap, if the kVA equivalent parts are less than 100 for grounding
transformers and ground-fault neutralizers or 500 for current-limiting reactors and other devices.
14.2.3 Induced-Potential Tests
14.2.3.1 Grounding Transformers and Ground-Fault Neutralizers.
As this test over-excites the device, the frequency of the applied potential should be high enough to prevent the exciting
current of the device under test from exceeding about 30 percent of its rated-load current. Ordinarily, this requirement
necessitates the use of a frequency of 120 Hz or more, when testing 60-Hz units.
When frequencies higher than 120 Hz are used, the severity of the test is abnormally increased, and for this reason the
duration of the test should be reduced in accordance with the table given below.
To avoid switching surges, the voltage should be started at one-quarter or less of the full value and be brought up
gradually to full value, within a period of 15 seconds.
After being held for the duration of time specified in the table above, it should be reduced gradually to one-quarter of
the maximum value within a period of 5 seconds, at which time the circuit may be opened.
14.2.3.2 Dry-Type Current-Limiting Reactors.
Because of the low impedance of current-limiting reactors, inducing voltage between turns will have to be done with
a high frequency, well above the power frequency range.
This can be obtained by repeatedly charging a capacitor and discharging it into the reactor winding. The number of
discharges should be sufficient to produce 7200 cycles of high frequency having crest values outlined in 7.2.3 except
that testing time shall not exceed 60 seconds. If the available equipment cannot induce sufficient voltage in the reactor
coil directly, then turns of insulated cable may be placed around the mid-section of the reactor as a primary of a Tesla
coil and the capacitor discharged into it. Where a sphere gap is used to check, the current-limiting resistance normally
in series with the spheres should be short-circuited. Detection shall be by noise, smoke, or spark discharge in the
windings.
14.2.3.3 Oil-Immersed Current-Limiting Reactors.
For oil-immersed reactors, each line terminal of each winding shall be induced-potential tested as outlined in 7.2.3.
Frequency in Hz Duration in seconds
120 and less 60
180 40
240 30
360 20
400 18
Copyright © 1972 IEEE All Rights Reserved
31
TEST PROCEDURE FOR NEUTRAL GROUNDING DEVICES IEEE Std 32-1972
14.2.3.4 Neutral Capacitors.
The terminal-to-terminal induced-potential test shall be made by applying between the terminals of each unit the
specified alternating voltage.
14.2.3.5 Neutral Resistors.
No induced test required.
14.2.4 Standard Impulse Tests.
The standard impulse test consists of applying, in the following order, one reduced full wave, two chopped waves, and
one full wave.
14.2.4.1 Reduced Full-Wave Test.
For this test, the applied voltage wave shall have a crest value between 50 and 70 percent of the full wave in accordance
with 3.2. Crest voltages near the lower limit are preferable.
14.2.4.2 Chopped-Wave Test.
For this test, the applied voltage wave shall be chopped by a suitable air gap. It shall have a crest voltage and time to
flashover in accordance with 3.2.
To avoid recovery of insulation strength if failure has occurred during a previous impulse, the time interval between
application of the last chopped wave and the final full wave should be minimized, and preferably should not exceed 5
minutes.
14.2.4.3 Full-Wave Test.
For this test, the voltage wave shall have a crest value in accordance with 3.2, and no flashover of the bushing or test
gap shall occur.
To avoid flashover of the bushing during adverse conditions of humidity and air density, the bushing flashover may be
increased by appropriate means.
In general, the tests shall be applied to each terminal one at a time.
All impulses applied to a device shall be recorded by a cathode-ray oscillograph if their crest voltage exceeds 40
percent of the crest of the full wave in accordance with 3.2.
When reports require oscillograms, those of the first reduced full wave, the last two chopped waves, and the last full
wave of voltages shall represent a record of the successful application of the impulse test to the device.
14.2.4.4 Connections for Impulse Tests.
One terminal of the winding under test shall be grounded directly or through a small resistance if current
measurements are to be made. Other terminals in the same winding not being tested or terminals in the windings of
other phases may be protected by grounding or other appropriate means.
In some cases the inductance of the winding is so low that the desired impulse voltage magnitude and the duration to
the 50-percent point on the tail of the wave cannot be obtained with available test equipment. Such low-inductance
windings may be tested by inserting a resistor of not more than 500
W in the grounded end of the winding.
32
Copyright © 1972 IEEE All Rights Reserved
IEEE Std 32-1972 IEEE STANDARD REQUIREMENTS, TERMINOLOGY, AND
The secondaries of current transformers, either on bushings or permanently connected to the equipment being tested,
shall be short-circuited and grounded. Any magnetic shielding or metallic housing shall be grounded for all impulse
tests.
14.2.4.5 Detection of Failure.
Because of the nature of impulse test failures, one of the most important matters is the detection of failure. There are
a number of indications of insulation failure. Some of these are:
1) Noise; presence of smoke or bubbles; failure of the gap or bushing to flash over, although the oscillogram
indicates a chopped wave.
2) Any difference between the reduced full wave and the final full wave detected by superimposing the two
oscillograms or any difference between the two chopped waves from each other or from the full wave up to
the time of flashover similarly detected. Such deviations may, however, be caused by conditions in the
impulse test circuit external to the device and should be fully investigated.
3) Measurement of the current in the grounded end of the winding tested. The current is measured by means of
a cathode-ray oscillograph connected to a suitable shunt inserted between the normally grounded end of the
winding and the grounded tank. Any deviation of current-wave shape obtained during the reduced full wave
and full-wave tests indicates changes in impedance arising from insulation breakdown within the device, or
changes in the impulse circuit external to the device, and the cause should be investigated.
14.2.4.6 Wave to be Used for Impulse Tests.
A nominal 1.2
´ 50 microsecond wave shall be used for impulse tests.
Either, but not both, positive or negative waves may be used.
Waves of negative polarity for oil-immersed apparatus and of positive polarity for dry-type or compound-filled
apparatus are recommended and shall be used unless otherwise specified.
If in testing oil-immersed apparatus the atmospheric conditions at the time of test are such that the bushings will not
withstand the specified polarity wave, then a wave of the opposite polarity may be used.
The time to crest on the front from virtual time zero to actual crest shall not exceed 2.5 microseconds, except for
windings of large impulse capacitance (low-voltage high-apparent-power and some high-voltage high-apparent-power
windings). To demonstrate that the large capacitance of the winding causes the long front, the impulse generator series
resistance may be reduced, which should cause imposed oscillations. Only the inherent generator and lead inductances
should be in the circuit.
For convenience in measurement, the time to crest may be considered as 1.67 times the actual time between points on
the front of the wave at 30 percent and 90 percent of the crest value.
The time on the tail to the point of half-crest voltage of the applied wave shall be 50 microseconds from the virtual time
zero.
The virtual time zero can be determined by locating points on the front of the wave at which the voltage is, respectively,
30 and 90 percent of the crest value and then drawing a straight line through these points. The inter-section of this line
with the time axis (zero-voltage line) is the virtual time zero.
If there are oscillations on the front of the wave, the 30 and 90 percent points should be determined from the average,
smooth wave front sketched in through the oscillations.
The magnitude of the oscillations preferably should not exceed 10 percent of the applied voltage. (With superimposed
oscillations of high magnitude, evaluation of wave crest is difficult, while if generator characteristics are such as to
Copyright © 1972 IEEE All Rights Reserved
33
TEST PROCEDURE FOR NEUTRAL GROUNDING DEVICES IEEE Std 32-1972
give a completely smooth wave it may be difficult to detect failures of small portions of the winding insulation by
means of the cathode-ray oscillograph. If the impulse generator is sufficiently flexible, a good compromise is the use
of generator constants such that the device impedance largely determines the length of the tail of the applied wave.)
14.3 Impedance and Loss Measurements
It is not practical to measure the resistive and reactive components of the impedance voltage separately, but after the
total impedance loss and impedance voltage are measured, the components may be separated by calculation.
Resistance and reactance components of the impedance voltage are determined by the use of the following equations:
Figure 2— Connections for Impedance Voltage and Impedance Loss Tests
(4)
(5)
where
E
= Potential impressed on winding
E
r = Component of E that is in phase with I
E
x = Component of E that is in quadrature with I
P
= Power measured in impedance test of winding carrying current
I
= Current in winding on which voltage is impressed
The
I2 R component of the impedance loss increases with the temperature, the stray-loss component diminishes with
the temperature, and, therefore, when it is desired to convert the impedance losses from one temperature to another, as
for instance when calculating efficiency which calls for 75
°C losses, the two components of the impedance loss are
converted separately. Thus,
(6)
(7)
where
T
= 234.5 for copper
T
= 225 for aluminum
P
r¢ and Ps¢ are desired resistance and stray losses, respectively, at the specified temperature q¢; and Pr and Ps, are
measured resistance and stray losses at temperature .
E
r
P
I
= ---
E
x E
2 Er
2
= –
P
x
¢
Px
T
q¢ +
T
+ q
= --------------
P
s
¢
Ps
T
+ q
T
q¢ +
= --------------
34
Copyright © 1972 IEEE All Rights Reserved
IEEE Std 32-1972 IEEE STANDARD REQUIREMENTS, TERMINOLOGY, AND
14.3.1 Wattmeter Method.
Voltage at rated frequency is applied and adjusted to circulate rated current in the winding (see Fig 2).
With current and frequency adjusted to the rated values as nearly as possible, simultaneous readings should be taken
on the ammeter, voltmeter, wattmeter, and frequency meter. Because of the extremely low power factor, correction
must be made for phase angle and losses of meters and metering transformers. Test current and potential leads should
leave the winding terminals in a radial plane for a distance not less than one coil diameter when measuring coils in air.
In the case of reactors of the current-limiting type, when reactor windings are being measured outside of enclosures or
shielded tanks, the measurement may be made at lower than rated current to minimize the effect of short-circuited
loops of nearby magnetic materials that are not part of the reactor and would have a disproportionately larger current
induced in them at rated current.
The temperature of the winding shall be taken immediately before and after the impedance measurements in a manner
similar to that described in 14.1. The average shall be taken as the true temperature.
The
I2 R loss of the winding is calculated from the resistance measurements (corrected to the temperature at which the
impedance test was made) and the currents which were used in the impedance measurement. These
I2 R losses
subtracted from the impedance losses give the stray losses of the winding. When reactor windings are enclosed in
shielded housings or tanks, part or all of which are magnetic material, part of the stray loss must be considered with the
winding
I2 R when correcting losses from measured temperature to other temperatures. Since this varies with
proportions of design and type of shield, it will have to be approximated for each design but can be checked by
measurement of loss at the start and finish of the temperature run.
Per-unit values of the resistance, reactance, and impedance voltage are obtained by dividing
E, Er, Ex in Eqs 4 and 5
by the rated voltage. Percentage values are obtained by multiplying per-unit values by 100.
Temperature correction shall be made as in Eqs 6 and 7. The stray-loss component of the impedance watts is obtained
by subtracting from the latter the
I2 R losses of the reactor.
14.3.2 Bridge Method.
Bridges are frequently used and are generally more accurate than the wattmeter method.
14.4 Temperature-Rise Tests
14.4.1 Loading for Temperature-Rise Tests.
Neutral grounding devices shall be tested under loading conditions that will give losses approximating, as nearly as
possible, those obtained at rated frequency with rated current in the device. Units having taps for more than one rating
shall be tested on the connection providing the highest losses. For devices rated less than 10 minutes, the loading shall
be adjusted to obtain a steady-state rise approximating that listed in Table 6 for the class of insulation involved. The
data so obtained shall be used to determine the degree of compliance with the short-time temperature requirements by
calculation using the method of Section 14.5.1.
14.4.2 Determination of Average Measured Winding Temperature by the Hot-Resistance Method.
The average measured temperature of the winding conductor may be determined by either of the following equations:
q
(8) R
R
o
= -----
(T + qo) – T
Copyright © 1972 IEEE All Rights Reserved
35
TEST PROCEDURE FOR NEUTRAL GROUNDING DEVICES IEEE Std 32-1972
(9)
where
T
= 234.5 for copper
T
= 225 for aluminum
q
= Temperature in degrees Celsius corresponding to hot resistance R
q
= Temperature in degrees Celsius corresponding to cold resistance Ro
R
= Hot resistance (see Section 14.4.5)
R
o = Cold resistance determined in accordance with the rules in this standard
The induction time for the measuring current to become stable should be noted during the cold-resistance
measurements, in order to assure that sufficient time elapses for the induction effect to disappear before hot-resistance
readings are taken.
When tests are made at an altitude not exceeding 3300 ft (1000 m) above sea level, no altitude correction shall be
applied to the temperature rise.
When a device which is tested at an altitude less than 3300 ft (1000 m) is to be operated at an altitude in excess of that
altitude, it shall be assumed that the observed temperature rise will increase in accordance with the following relation:
Increase in Temperature Rise
or
where
A is the altitude in meters, B is the altitude in feet, and F is an empirical factor equal to 0.004 for oil-immersed
self-cooled and 0.005 for dry-type self-cooled.
The observed rise in the foregoing equation is:
Top oil rise, or average oil rise, over the ambient temperature for oil-immersed self-cooled devices
Winding rise over the ambient temperature for dry-type self-cooled devices
The winding rise for oil-immersed devices over ambient at altitude
A is the observed winding rise over ambient plus
the calculated increase in temperature rise.
Devices shall be completely assembled and if oil-immersed they shall be filled to the proper level.
If the devices are equipped with thermal indicators, bushing-type current transformers, etc, such apparatus shall be
assembled with the device.
The temperature-rise test shall be made in a room which is essentially free from drafts.
The temperature of the surrounding air (ambient temperature) shall be determined by at least three thermocouples, or
thermometers, spaced uniformly around the devices under test. They should be located about one-half the height of the
device, and at a distance of about 3 to 6 ft (1 to 2 m) from the device. They should be protected from drafts and
abnormal heating. For dry-type reactors, they should be located one coil diameter from the coil at the level of the
bottom of the coil.
q
R
– Ro
R
o
---------------
T qo + ( ) qo = +
Observed Rise
(
A – 1000)
100
=
´ --------------------------(F)
Observed Rise
(
B – 3300)
330
=
´ --------------------------(F)
36
Copyright © 1972 IEEE All Rights Reserved
IEEE Std 32-1972 IEEE STANDARD REQUIREMENTS, TERMINOLOGY, AND
To reduce to a minimum the errors due to time lag between the temperature of the devices and the variations in the
ambient temperature, the thermocouples, or thermometers, shall be placed in suitable containers which shall have such
proportions as will require not less than 2 hours for the indicated temperature within the container to change 6.3
°C if
suddenly placed in air which has a temperature 10
°C higher, or lower, than the previous steady-state indicated
temperature within the container.
The temperature rise of metal parts (other than the winding conductor) in contact with, or adjacent to, insulation, and
of other metal parts, shall be determined by thermocouple or by thermometer.
Provision shall be made to measure the surface temperature of iron or alloy parts surrounding or adjacent to the outlet
leads or terminals carrying large currents. Readings shall be taken at intervals or immediately after shutdown.
The determination of the temperature rise of metal parts within the case, other than winding conductors, is a design test
and shall be made when so specified unless a record of this test made on a duplicate, or essentially duplicate, unit can
be furnished. This test will not be made unless definitely specified because provision for the proper placement of the
thermocouples and leads must frequently be made during the design of the devices. Comparison with other devices
having metal parts of similar design and arrangement, but not necessarily having the same rating, will in many cases
be adequate.
A thermocouple is the preferred method of measuring surface temperature. When used for this purpose on tanks or
enclosures, the thermocouples should be soldered to a thin metal plate or foil approximately 1 in (25 mm) square. The
plate is to be placed and held firmly and snugly against the surface. In either case, the thermocouple should be
thoroughly insulated thermally from the surrounding medium.
It is permissible to shorten the time required for the test by the use of initial over-loads, restricted cooling, or any other
suitable method.
The temperature rise of the winding shall be determined by the resistance method, or by thermometer when so
specified.
The ultimate temperature rise is considered to be reached when the temperature rise becomes constant; that is, when
the temperature rise does not vary more than 2.5 percent during a period of 3 consecutive hours.
14.4.3 Temperature-Rise Tests — Oil-Immersed Devices.
The top oil temperature shall be measured by a thermocouple or alcohol thermometer immersed approximately 2 in
(51 mm) below the top oil surface.
The average temperature of the oil shall be determined when it is to be used.
The average oil temperature is equal to the top oil temperature minus one-half the difference in temperature of the
moving oil at the top and the bottom of the cooling means, as determined by suitable measurements.
For devices with external cooling means, this temperature difference may be closely approximated by careful
determination of the temperature on the external surfaces of the oil inlet and oil outlet of the cooling means by the use
of thermocouples.
The ambient temperature shall be taken as that of the surrounding air which should be not less than 10
°C or more than
40
°C.
No corrections for variations of ambient temperature within this range shall be applied.
Temperature tests may be made with ambient temperature outside the range specified, if suitable and agreed upon
correction factors are available.
Copyright © 1972 IEEE All Rights Reserved
37
TEST PROCEDURE FOR NEUTRAL GROUNDING DEVICES IEEE Std 32-1972
14.4.4 Temperature-Rise Tests — Dry-Type Devices.
When the ambient air temperature is other than 30
°C, a correction shall be applied to the temperature rise of the
winding by multiplying it by the correction factor
K which is given by the ratio
(10)
where
T
= 234.5 for copper
T
= 225 for aluminum
q
= Ambient air temperature, °C
When the temperature-rise tests by thermometer are required, it is important that the coil thermometers be properly
placed in the air ducts in such a manner as to indicate the winding temperature and yet not restrict the ventilation. This
may be accomplished by means of grooved sticks of dry wood or some other kind of insulating material slightly larger
than the thermometer bulbs.
When the thermometers are used for measuring temperature of apparatus other than oil-immersed, the bulbs shall be
covered by felt pads cemented to the equipment. When pads interfere with ventilation, as in ventilating ducts between
coils, grooved wooden sticks may be used.
Dimensions of felt pads for use with large apparatus should be approximately 1.5 in by 2 in by 0.125 in (40 mm by 50
mm by 3 mm).
When the temperature rise has become constant, the test voltage and current should be removed and the blowers, if
used, shut off. Immediately thereafter, the coil thermometers and any other temperature-indicating devices should be
read continually in rotation until the temperature begins to fall. If any of the thermometer temperatures are higher than
those observed during the run, the highest temperature should be recorded as the final thermometer temperature.
The temperature rise of the device above the specified ambient temperature, when tested or calculated in accordance
with the rating, shall not exceed values given in Table 6.
For times of one minute or less the temperature rise shall be determined in accordance with Section 4.3.1.
For a time of 10 minutes and greater, Sections 4.3.2 and 4.3.3, the temperature rise is the limiting observable
temperature rise using the methods of measurement as follows:
Resistance method for grounding transformers, ground-fault neutralizers, and reactors
Thermometer method and radiation thermometer method for resistors
When the rated voltage of a resistor is equal to the line-to-neutral voltage of the circuit, the specified temperature limits
are based upon the application of line-to-neutral voltage at rated frequency to the resistor for a time equal to the rated
time, the current being allowed to decrease during the rated time.
NOTE — This assumes there will be sufficient additional impedance in the circuit to keep the current at the start of the test from
exceeding the mechanical current rating when the product of the rated current and the resistance at the starting
temperature is less than 83
1/3 percent of the line-to-neutral voltage.
K
T
+ 30
T
+ q
= ---------------
38
Copyright © 1972 IEEE All Rights Reserved
IEEE Std 32-1972 IEEE STANDARD REQUIREMENTS, TERMINOLOGY, AND
14.4.5 Correction Back to Shutdown
14.4.5.1 Empirical Method.
The empirical method utilizes correction factors which represent average results from usual commercial designs. This
method is not to be used for forced-oil-cooled devices, nor for those designs which deviate considerably from usual
commercial proportions. In such cases, the cooling curve method should be used (Section 14.4.5.2).
Take one hot-resistance reading on each winding, record the time after shutdown, and determine the corresponding
temperature rise.
When the conductor loss of oil-immersed equipment does not exceed 7 W/lb (15 W/kg) for copper or 12 W/lb (26 W/kg)
for aluminum, an arbitrary correction of 1
°C per minute may be used.
The conductor loss in watts per pound in a winding shall be taken as the sum of the calculated
I2R and eddy-current
loss at a temperature equal to the rated temperature rise plus 20
°C divided by the weight of the active conductor in
pounds.
14.4.5.2 Cooling Curve Method.
Take a series of at least four, preferably more, readings on one phase of each winding, and record the time after
shutdown for each reading.
The readings should be time spaced to assure accurate extrapolation back to shutdown.
The overall reading time should exceed 4 minutes and may extend considerably beyond.
The first reading on each winding should be taken as quickly as possible after shutdown, but not before the measuring
current has become stable, and must be taken within 4 minutes.
Plot the resistance time data on suitable coordinate paper, and extrapolate the curve back to instant of shutdown.
The resistance value so obtained shall be used to calculate the average winding temperature at instant of shutdown.
The resistance time curve obtained on one phase may be used to determine the correction back to shutdown for the
other phases provided the first reading on each of the other windings has been taken within 4 minutes after shutdown.
If necessary the temperature test may be resumed so that the first readings on any windings may be completed within
the required 4 minutes.
14.5 Temperature-Rise Calculations
14.5.1 Thermal Short-Time Capability Calculations for Reactors, Ground-Fault Neutralizers, and
Transformers Used for Grounding.
The increase in winding temperature
qf during short-time conditions shall be estimated on the basis of all heat stored
in the conductor material and its associated turn insulation.
The thermal capability of the conductor material shall be taken as the average of the values at the starting and finishing
temperatures.
All temperatures are in degrees Celsius.
The increase in winding temperature
qf which will occur during a specified short time t shall be calculated by the
following equation:
Copyright © 1972 IEEE All Rights Reserved
39
TEST PROCEDURE FOR NEUTRAL GROUNDING DEVICES IEEE Std 32-1972
(11)
where
m
= at
(12)
t
= Time, in seconds
T
= 234.5 for copper
T
= 225 for aluminum
q
f = Increase in winding temperature during time t (not to be greater than the difference between starting
temperature
qs and the limiting temperature during short-circuit conditions given in Table 7)
q
s = Starting temperature—reference ambient temperature (30°C) plus steady-state hottest-spot temperature
rise above reference ambient temperature at continuous current rating, using
1) measured hottest-spot temperature rise, if tested
2) standard hottest-spot limiting temperature rise, if not temperature tested
W
s = Short-circuit resistance loss at starting temperature s, watts per kilogram or pound of conductor material
W
r =Short-circuit resistance loss at 75°C, watts per kilogram or pound of conductor material
e
= Per-unit eddy-current loss, based on resistance loss at the starting temperature
e
=
e
75 = Per-unit eddy-current loss, based on resistance loss at 75°C
C
av = Average specific thermal capacity in Joules per degree Celsius, per kilogram or pound of conductor
material and its associated turn insulation over the range of increase in winding temperature.
NOTE — Equation 11 is an approximate formula and its use should be restricted to values of m = 0.6 or less. The exact equation is:
Î
= 2.7183 = base of the natural logarithms
The thermal capacity
Cx at any temperature qx below 500°C may be closely estimated from the following empirical
equations:
(13)
(14)
q
f (T + qs) (1 + e)m 0.6m2 = [ + ]
a
W
s
C
av
--------
1
T
+ qs
--------------
W
x
C
av
(T + 75) = = -----------------------------
(
e75) T + 75
T
+ qs
---------------
2
q
f T qs + ( ) e2 m + e e2m = [ ( – 1)–1]
Metric
Cx = 384 + 0.099 qx + 243
A
i
A
c
-----
per kilogram of copper
English
Cx = 174 + 0.045 qx + 110
A
i
A
c
-----
per pound of copper
Metric
Cx = 893 + 0.441 qx + 794
A
i
A
c
-----
per kilogram of aluminum
English
Cx 405 0.200 qx 360
A
i
A
c
= + + -----
per pound of aluminum
40
Copyright © 1972 IEEE All Rights Reserved
IEEE Std 32-1972 IEEE STANDARD REQUIREMENTS, TERMINOLOGY, AND
where
A
i = Cross-sectional area of insulation
A
c = Cross-sectional area of conductor
Table 10— Respective Metric and English System Nomenclature for Eqs 15 and 16
14.5.2 Thermal Capability Calculation for Neutral Resistors
14.5.2.1 Respective Metric and English System Equations for Temperature Rise and Current
Density, When Current is Constant.
The eddy-current loss may usually be ignored due to the high-resistance materials used in neutral resistors.
The temperature rise when the current is held constant, and all heat is assumed to be stored in the active material, shall
be computed by the following equations, where all quantities have been defined in Table 10.
(15)
Symbol and Identity Metric English
q
Final temperature rise °C °F
q
1 Initial temperature rise °C °F
q
0 Initial temperature °C = q1 + 30 °F = q1 + 86
a
0 Temperature coefficient of
resistance, change in
resistance per degree, at
initial temperature,
°C °F
d
Density of material
*
C
*For cast iron, over the range of temperature covered by this standard, C shall be taken as
0.130.
Effective integrated
specific heat
J
0 Initial current density
r
0
Resistivity at initial
temperature
t
Time s s
log
10
-
1= antilog10x = 10x
g
cm
3
---------
lb
in
3
------
Cal
g C
° ×
-------------
Btu
lb F
° ×
--------------
A
cm
2 ---------
A
in
2 ------
W
cm
3
---------
W
in
3
------
Metric
q
1
a
o
----- = log
1–
10
0.104
aorotJo
2
C
d ----------------------------------
è ø
ç ÷
æ ö
– 1 +
q1
Copyright © 1972 IEEE All Rights Reserved
41
TEST PROCEDURE FOR NEUTRAL GROUNDING DEVICES IEEE Std 32-1972
For design purposes, it is more convenient to insert the desired temperature rise and derive the current density which
will produce the desired temperature rise. Thus
(16)
NOTE — Equations 15 and 16 apply only when the temperature coefficient of resistance a
0 is substantially constant over the
temperature range used, and must not be used for materials for which the coefficient varies greatly.
14.5.2.2 Respective Metric and English System Equations for Temperature Rise and Current
Density, When Voltage is Constant.
For some resistors (see 10.1.1) temperature rise is computed on the basis that constant voltage is maintained between
the terminals, the current being allowed to decrease as the resistance increases with temperature. The temperature rise,
with all heat stored in the active material and with constant voltage, shall be computed by the following equations
where all quantities have been defined in Table 10.
(17)
For design purposes, it is more convenient to insert the desired temperature rise and derive the current density that will
produce the temperature rise with the voltage maintained. Thus,
(18)
NOTE — Equation 18 applies only when the temperature coefficient of resistance ao is substantially constant over the temperature
range used. and must not be used for materials for which the coefficient varies greatly.
English
q
1
a
o
----- log
–1
10
a
o
rotJ2
o
2430
Cd --------------------
è ø
ç ÷
æ ö
= – 1 +
q1
J
o
9.62
Cd
a
orot
= -----------------log
10[1 + ao(q – q1)]
English
J
o
2430
Cd
a
orot
------------------- log
10 1 ao q q= [ + ( – 1)]
Metric
q
1
a
o
----- – 1 1
0.478
J2
o
roaot
C
d + + ----------------------------------
è ø
ç ÷
æ ö
= +
q1
English
q
1
a
o
----- – 1 1
1.898 10
–3J0
2
´ roaot
C
d + + ----------------------------------------------------
è ø
ç ÷
æ ö
q
= + 1
Metric
J
o
4.18
Cd
r
ot
-----------------
q – q1 ao
(q
– q1)2
2
= + ----------------------
English
J
o
105
Cd
r
ot
----------------
q – q1 ao
(q
– q1)2
2
= + ----------------------
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