ASTM D2477-96
(Test Method)Standard Test Method for Dielectric Breakdown Voltage and Dielectric Strength of Insulating Gases at Commercial Power Frequencies
Standard Test Method for Dielectric Breakdown Voltage and Dielectric Strength of Insulating Gases at Commercial Power Frequencies
SCOPE
1.1 This test method covers the determination of the dielectric breakdown voltage and dielectric strength of insulating gases used in transformers, circuit breakers, cables, and similar apparatus as an insulating medium. The test method is applicable only to gases with boiling points below room temperature at atmospheric pressure.
1.2 This standard may involve hazardous materials, operations, and equipment. This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility of the user of this standard to establish appropriate safety and health practices and determine the applicability of regulatory limitations prior to use.
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Designation: D 2477 – 96
Standard Test Method for
Dielectric Breakdown Voltage and Dielectric Strength of
Insulating Gases at Commercial Power Frequencies
This standard is issued under the fixed designation D 2477; the number immediately following the designation indicates the year of
original adoption or, in the case of revision, the year of last revision. A number in parentheses indicates the year of last reapproval. A
superscript epsilon (e) indicates an editorial change since the last revision or reapproval.
1. Scope significance of the dielectric strength test is given in the
Appendix.
1.1 This test method covers the determination of the dielec-
tric breakdown voltage and dielectric strength of insulating
5. Apparatus
gases used in transformers, circuit breakers, cables, and similar
5.1 Electrical Apparatus:
apparatus as an insulating medium. The test method is appli-
5.1.1 Transformer—The desired test voltage may be most
cable only to gases with boiling points below room temperature
readily obtained by a step-up transformer energized from a
at atmospheric pressure.
variable low-voltage commercial power frequency source. The
1.2 This standard does not purport to address all of the
transformer and controlling element shall be of such size and
safety concerns, if any, associated with its use. It is the
design that, with the test specimen in the circuit, the crest factor
responsibility of the user of this standard to establish appro-
(ratio of maximum to mean effective) of the 60-Hz test voltage
priate safety and health practices and determine the applica-
does not differ by more than6 5 % from that of a sinusoidal
bility of regulatory limitations prior to use.
wave over the upper half of the range of test voltage. The crest
2. Referenced Documents factor may be checked by means of an oscilloscope, a sphere
gap, or a peak-reading voltmeter in conjunction with an rms
2.1 ASTM Standards:
voltmeter. Where the waveform cannot be determined conve-
D 2864 Terminology Relating to Electrical Insulating Liq-
2 niently, a transformer having a rating of not less than ⁄2 kVA at
uids and Gases
the usual breakdown voltage shall be used. Transformers of
2.2 IEEE Standard:
larger kVA capacity may be used, but in no case should the
No. 4 Measurement of Test Voltage in Dielectric Tests
power frequency short circuit current in the specimen circuit be
3. Terminology outside the range of 1 to 10 mA/kV of applied voltage. This
limitation of current may be accomplished by using a suitable
3.1 Definitions—See Terminology D 2864 for definitions.
external series resistor or by employing a transformer with
4. Significance and Use
sufficient inherent reactance.
5.1.2 Circuit-Interrupting Equipment—The test transformer
4.1 The dielectric breakdown voltage and dielectric strength
primary circuit shall be protected by an automatic circuit-
of an insulating gas in a uniform field depends primarily on the
breaking device capable of opening (as nearly instantaneously
molecular structure of the gas. As different gases are mixed
as possible) on the current produced by the breakdown of the
either by plan or by contamination, any change in dielectric
test specimen; a circuit breaker that opens within 5 cycles may
breakdown voltage and dielectric strength will depend on both
be used if the short-circuit current as described in 5.1.1 does
the nature and proportion of the individual gases. This test
not exceed 200 mA. A prolonged flow of current at the time of
method uses plane and spherical electrodes which provide a
breakdown causes contamination of the gases and damage of
nearly uniform field (see Appendix) in the area of electrical
the electrodes, thereby affecting the subsequent test results, and
discharge. It is suitable for determining the dielectric break-
increasing the electrode and test cell maintenance and time of
down voltage and dielectric strength of different gases and
testing.
mixtures thereof for research and application evaluations and
5.1.3 Voltage-Control Equipment—The rate of voltage rise
also as a field test. A more complete discussion of the
shall be ⁄2 kV/s 6 20 %. Voltage control may be secured by a
motor-driven variable-ratio-autotransformer. Preference is
This test method is under the jurisdiction of ASTM Committee D-27 on
given to equipment having an approximately straight-line
Electrical Insulating Liquids and Gasesand is the direct responsibility of Subcom-
voltage-time curve over the desired operating range. Motor
mittee D27.05on Electrical Tests.
Current edition approved Oct. 10, 1996. Published December 1996. Originally
drive is preferred to manual drive because of the ease of
published as D 2477 – 66 T. Last previous edition D 2477 – 84 (1990).
maintaining a reasonably uniform rate-of-voltage rise with this
Annual Book of ASTM Standards, Vol 10.03.
3 test method. The rate-of-voltage rise may be calculated from
Available from The Institute of Electrical and Electronic Engineers, Inc., P.O.
Box 1331, Piscataway, NJ 08855. measurements of the time required to raise the voltage between
Copyright © ASTM, 100 Barr Harbor Drive, West Conshohocken, PA 19428-2959, United States.
D 2477
two prescribed values. When motor-driven equipment is used, 5.3 Electrodes and Test Cell:
calibrate the speed control rheostat in terms of rate-of-voltage 5.3.1 The sphere and plane electrodes shall be mounted
rise for the test transformer used. vertically as shown in Fig. 1. The sphere shall be a precision
5.1.4 Voltmeter—Measure the voltage by a method that steel bearing ball 0.75 in. (19.1 mm) in diameter. The plane
fulfills the requirements of IEEE Standard No. 4, giving crest electrode shall be of brass 1.50 in. (38.1 mm) in diameter. The
and also (if available) rms values, preferably by means of: gap setting shall be 0.100 6 0.001 in. (2.54 6 0.025 mm). The
5.1.4.1 A voltmeter connected to the secondary of a separate tolerance of all dimensions is 62 %, unless otherwise stated.
potential transformer, or 5.3.2 The cell shall consist of a borosilicate glass cylinder
5.1.4.2 A voltmeter connected to a well-designed tertiary clamped by flanges to end plates which seal the cell and
coil in the test transformer, or support the electrodes. The lower plane electrode shall be
5.1.4.3 A voltmeter connected to the low-voltage side of the fixed. The sphere electrode, held in place by a magnet, shall be
test transformer. adjustable by means of a micrometer screw suitably mounted
5.1.5 Accuracy—The combined accuracy of the voltmeter through the top plate. The micrometer screw must be suitable
and voltage divider circuit is not to exceed 5 % at the rate of for setting the electrodes to within the specified tolerance. The
voltage rise specified in 5.1.3. bottom plate shall have a valved port for evacuation and
5.2 Evacuation and Filling Apparatus: admission of the sample. If considered more convenient, two
5.2.1 Vacuum Pump—The vacuum pump shall have suffi- ports, one in the top for evacuation and one in the bottom for
cient pumping capacity to be able to evacuate the test cell to a admission of the sample may be used. The dimensions are
pressure below 1 torr. shown in Fig. 1.
5.2.2 Vacuum and Pressure Gage—Either a mercury ma-
6. Sampling
nometer, or one or more gages, capable of measuring pressures
6.1 Obtain the gas sample from the gas cylinder or gas-filled
below 1 torr and also near atmospheric pressure. The manom-
equipment through a pressure-reducing regulator valve so that
eter, or vacuum and pressure gages, shall be calibrated in
millimetres of mercury (torr).
5.2.3 Connections—Vacuum-tight tubing and valves shall
Detailed drawings of this apparatus are available at a nominal cost from ASTM.
be used while evacuating and purging the test cell and filling it
Request ADJD2477.
with the gas sample. Standard laboratory borosilicate glass pipe and connecting flanges may be used.
FIG. 1 Test Cell
D 2477
the flow into the cell may be controlled. The sample and cell 8.3 The breakdown voltage of an insulating gas may be
must be at room temperature before the gas is admitted to the affected by irradiation of the gas in the electrode gap. Such
cell. irradiation can occur from random sources such as sunlight,
artificial lights of various types, and nearby radioactive mate-
7. Preparation of Cell
rials. It is recommended that the test cell be shielded from such
sources as much as possible during test runs. See X1.4 and
7.1 Clean the cell except for the electrodes by washing with
X1.5, for further discussion
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