IEC 60156:2018
(Main)Insulating liquids - Determination of the breakdown voltage at power frequency - Test method
Insulating liquids - Determination of the breakdown voltage at power frequency - Test method
IEC 60156:2018 specifies the method for determining the dielectric breakdown voltage of insulating liquids at power frequency. The test procedure is performed in a specified apparatus, where the oil sample is subjected to an increasing AC electrical field until breakdown occurs. The method applies to all types of insulating liquids of nominal viscosity up to 350 mm2/s at 40 °C. It is appropriate both for acceptance testing on unused liquids at the time of their delivery and for establishing the condition of samples taken in monitoring and maintenance of equipment. This third edition cancels and replaces the second edition published in 1995. This edition constitutes a technical revision and, mainly, confirms the content of the previous edition even if some advances are included. The test method has not been changed for practical reason due to the very large number of instrumentation disseminated around the world, even if the use of stirring is now recommended.
Isolants liquides - Détermination de la tension de claquage à fréquence industrielle - Méthode d’essai
L'IEC 60156:2018 spécifie la méthode de détermination de la tension de claquage diélectrique des isolants liquides à fréquence industrielle. La procédure d’essai est réalisée dans un appareil spécifié dans lequel l’échantillon d’huile est soumis à un champ électrique alternatif croissant jusqu’à l’obtention du claquage. La méthode est applicable à tous les types d’isolants liquides de viscosité nominale allant jusqu’à 350 mm2/s à 40 °C. Elle convient aussi bien pour les essais de réception de liquides neufs à la livraison que pour définir l’état des échantillons prélevés lors de la surveillance et de la maintenance des matériels. Cette troisième édition annule et remplace la deuxième édition parue en 1995. Cette édition constitue une révision technique et valide essentiellement le contenu de l’édition précédente même si elle comporte certaines améliorations. La méthode d’essai n’a pas été modifiée pour des raisons pratiques et du fait du très grand nombre de dispositifs de mesure utilisés au niveau international, même si l’emploi de l’agitateur est maintenant recommandé.
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IEC 60156 ®
Edition 3.0 2018-08
REDLINE VERSION
INTERNATIONAL
STANDARD
colour
inside
Insulating liquids – Determination of the breakdown voltage at power
frequency – Test method
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IEC 60156 ®
Edition 3.0 2018-08
REDLINE VERSION
INTERNATIONAL
STANDARD
colour
inside
Insulating liquids – Determination of the breakdown voltage at power
frequency – Test method
INTERNATIONAL
ELECTROTECHNICAL
COMMISSION
ICS 29.040 ISBN 978-2-8322-5987-0
– 2 – IEC 60156:2018 RLV © IEC 2018
CONTENTS
FOREWORD . 4
INTRODUCTION . 6
1 Scope . 7
2 Normative references. 7
3 Terms and definitions . 7
4 Electrical apparatus . 7
4.1 General . 7
4.2 Voltage regulator . 8
4.3 Step-up transformer . 8
4.4 Switching system . 8
4.5 Current-limiting resistors . 9
4.6 Measuring device system . 9
5 Test assembly . 9
5.1 General . 9
5.2 Test cell . 9
5.3 Electrodes. 12
5.4 Stirring device (optional) . 12
6 Preparation of electrodes . 13
7 Test assembly preparation . 13
8 Sampling . 14
Condition of the sample .
9 Test procedure . 14
9.1 Sample preparation . 14
9.2 Filling of the cell . 15
10 Application of the voltage . 15
11 Report . 15
12 Test data dispersion and reproducibility . 16
12.1 Test data dispersion . 16
12.2 Reproducibility . 17
Annex A (informative) Improved test method . 18
A.1 Test procedure for improved test method . 18
A.2 Report . 19
Annex B (informative) Special test methods for low volume samples . 20
B.1 Low volume sample test . 20
Annex C (informative) Representative material for a performance test . 22
Bibliography . 23
Figure − Example of suitable cell and spherical electrodes .
Figure − Example of suitable cell and partially spherical electrodes .
Figure 1 – Examples of test cells with spherical electrodes 12,5 mm to 13,0 mm
diameter . 12
Figure 2 – Examples of test cells with partially spherical electrodes with 25 mm radius
and diameter of 36 mm . 12
Figure 3 – Graphical representation of coefficient of variation (standard deviation/mean
ratio) versus mean breakdown voltage . 17
Figure A.1 – Example of a sequence of breakdown shots for determination of the
breakdown voltage . 19
Figure B.1 – Example of low volume test cell, fixed electrode distance of 2 mm with
2 ml active volume under dielectric stress . 20
Figure B.2 – Example of low volume test cell, fixed electrode distance of 2,5 mm
(150 ml to 200 ml) . 21
– 4 – IEC 60156:2018 RLV © IEC 2018
INTERNATIONAL ELECTROTECHNICAL COMMISSION
____________
INSULATING LIQUIDS – DETERMINATION OF THE BREAKDOWN
VOLTAGE AT POWER FREQUENCY – TEST METHOD
FOREWORD
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This redline version of the official IEC Standard allows the user to identify the changes
made to the previous edition. A vertical bar appears in the margin wherever a change
has been made. Additions are in green text, deletions are in strikethrough red text.
International Standard IEC 60156 has been prepared by IEC technical committee TC 10:
Fluids for electrotechnical applications.
This third edition cancels and replaces the second edition published in 1995. This edition
constitutes a technical revision and, mainly, confirms the content of the previous edition even
if some advances are included. The test method has not been changed for practical reason
due to the very large number of instrumentation disseminated around the world, although the
use of stirring is now recommended.
The text of this International Standard is based on the following documents:
FDIS Report on voting
10/1061/FDIS 10/1065/RVD
Full information on the voting for the approval of this International Standard can be found in
the report on voting indicated in the above table.
This document has been drafted in accordance with the ISO/IEC Directives, Part 2.
The committee has decided that the contents of this document will remain unchanged until the
stability date indicated on the IEC website under "http://webstore.iec.ch" in the data related to
the specific document. At this date, the document will be
• reconfirmed,
• withdrawn,
• replaced by a revised edition, or
• amended.
IMPORTANT – The “colour inside” logo on the cover page of this publication indicates
that it contains colours which are considered to be useful for the correct understanding
of its contents. Users should therefore print this publication using a colour printer.
– 6 – IEC 60156:2018 RLV © IEC 2018
INTRODUCTION
As normally applied, breakdown voltage of insulating liquids is not a basic material property
but an empirical test procedure intended to indicate the presence of contaminants such as
water and solid suspended matter and the advisability of carrying out a drying and filtration
treatment.
The AC breakdown voltage value of insulating liquids strongly depends on the particular set of
conditions used in its measurement. Therefore, standardized testing procedures and
equipment are essential for the unambiguous interpretation of test results.
The method described in this document applies to either acceptance tests on new deliveries
of insulating liquids, or testing of treated liquids prior to or during filling into electrical
equipment, or to the monitoring and maintenance of oil-filled apparatus in service. It specifies
rigorous sample-handling procedures and temperature control that should be adhered to when
certified results are required. For routine tests, especially in the field, less stringent
procedures may be practicable and it is the responsibility of the user to determine their effect
on the results.
Annex A (informative) describes, for comparison, an alternative test method which could be
introduced in the future. Annex B (informative) describes special test methods, using cells
which may include low volume samples. Annex C (informative) describes a reference material
for a performance test and check according to IEC 60060-3[1] .
—————————
Numbers in square brackets refer to the Bibliography.
INSULATING LIQUIDS – DETERMINATION OF THE BREAKDOWN
VOLTAGE AT POWER FREQUENCY – TEST METHOD
1 Scope
This document specifies the method for determining the dielectric breakdown voltage of
insulating liquids at power frequency. The test portion, contained procedure is performed in a
specified apparatus, where the oil sample is subjected to an increasing AC electrical field by
means of a constant rate of voltage rise until breakdown occurs. The method applies to all
2 −1
types of insulating liquids of nominal viscosity up to 350 mm /s at 40 °C. It is appropriate
both for acceptance testing on unused liquids at the time of their delivery and for establishing
the condition of samples taken in monitoring and maintenance of equipment.
2 Normative references
The following documents are referred to in the text in such a way that some or all of their
content constitutes requirements of this document. For dated references, only the edition
cited applies. For undated references, the latest edition of the referenced document (including
any amendments) applies.
IEC 60052: 1960, Recommendations for voltage measurement by means of sphere-gaps (one
sphere earthed)
IEC 60060, High-voltage test techniques
IEC 60475:1974, Method of sampling insulating liquids dielectrics
3 Terms and definitions
No terms and definitions are listed in this document.
ISO and IEC maintain terminological databases for use in standardization at the following
addresses:
• IEC Electropedia: available at http://www.electropedia.org/
• ISO Online browsing platform: available at http://www.iso.org/obp
4 Electrical apparatus
4.1 General
The electrical apparatus consists of the following units:
1) voltage regulator,
2) step-up transformer,
3) switching system,
4) energy limiting devices.
4) current-limiting resistors,
5) measuring device.
Two or more of these units may be integrated in any equipment system.
– 8 – IEC 60156:2018 RLV © IEC 2018
4.2 Voltage regulator
Uniform increase of voltage with time by manual means is difficult and, for this reason,
automatic control is essential.
Voltage control may be achieved by one of the following methods:
a) Variable ratio auto-transformer
b) Electronic regulator
c) Generator-field regulation
d) Induction regulator
e) Resistive type voltage divider
The test voltage shall be increased with an automatic control of the required uniform voltage
rate of rise. The device should not introduce harmonics disturbances (< 3%) and the AC
source should be free from harmonics.
4.3 Step-up transformer
The test voltage is obtained by using a step-up or resonant transformer supplied from an AC
source using ( 48 Hz to 62 Hz) (sinusoidal waveform) voltage source whose value is gradually
increased. The voltage source value is constantly increased. The controls of the variable low-
voltage source shall be capable of varying the test voltage smoothly, uniformly and without
overshoots or transients. Incremental increases (produced, for example, by a variable auto-
transformer or an amplifier) shall not exceed 2 % of the expected breakdown voltage.
The voltage applied to the electrodes of the liquid-filled cell shall have an approximately
sinusoidal waveform, such that the peak factor is within the following limits: 1,41 ± 0,07.
The centre-point of the secondary winding of the transformer should be connected to earth.
4.4 Switching system
3.4.1 Basic requirements
The circuit shall be opened automatically if an established a sustained arc between the
electrodes occurs and the voltage between the electrodes collapses to a voltage less than
500 V. The primary circuit of the step-up transformer shall be fitted with a circuit-breaker
operated by the current sensing device, resulting from the breakdown of the sample and shall
break the voltage within 10 ms. The circuit may be opened manually if a transient spark
(audible or visible) occurs between the electrodes.
NOTE The sensitivity of the current or voltage sensing element depends on the energy-limiting
device employed and only approximate guidance can be given. Normally, triggering of cut-off
by a current of 4 mA maintained for 5 ms is acceptable, while fast energy-limiting (see 3.4.2)
triggering by a transient current of 1 A maintained for 1 µs has been found satisfactory.
A cut-off time of < 100 µs, as given in the previous edition of this document, is needed to
perform multiple breakdowns on silicone liquids.
3.4.2 Special requirements for silicone liquids
Silicone liquids can give rise to solid decomposition products through the action of electric
discharges, which may cause gross errors in the observed results. In such cases, all feasible
steps shall be taken to minimize the energy available for dissipation in the breakdown
discharge.
Whilst current limiting as above, combined with isolation of the step-up transformer primary
within 10 ms, is adequate for hydrocarbons. More satisfactory performance for silicone liquids
is obtained by short circuiting of the primary circuit of the transformer by a low-impedance or
by use of a low-voltage device for detection of breakdown acting within a few microseconds.
This device may be of either analogue (for example, modulating amplifier) or switching (for
example, thyristor) type. By the use of this device, the output voltage of the step-up
transformer shall be reduced to zero within 1 ms of detection of breakdown, and shall not
thereafter increase again until the next step of the test sequence is commenced.
4.5 Current-limiting resistors
To protect the equipment and to avoid excessive decomposition of the liquid at the instant of
breakdown of liquids such as silicone or ester liquids, a resistance limiting the breakdown
current may shall be inserted in series with the test cell.
The short-circuit current of the transformer and associated circuits shall be within the range of
10 mA to 25 mA for all voltages higher than 15 kV. This may be achieved by a combination of
resistors in either or both the primary and secondary circuits of the high-voltage transformer.
4.6 Measuring device system
For the purposes of this document, the magnitude of the test voltage is defined as its peak
value divided by 2 .
This voltage may be measured by means of a peak-voltmeter or by means of another type of
voltmeter connected to the input or output side of the testing transformer, or to a special
winding provided thereon; the instrument then used shall be calibrated against a standard up
to the full voltage which it is desired to measure.
The output voltage of the step-up transformer may be measured by means of a measuring
system consisting of a voltage divider or a measuring winding of the step-up transformer
coupled with a peak-voltmeter. The measuring system shall be calibrated up to the upper
scale voltage to be measured. A method of calibration which has been found satisfactory is
the use of a transfer standard. This is an auxiliary measuring device which is connected in
place of the test cell between the high-voltage terminals to which it presents the same an
impedance as the filled test cell similar to the one of the sample liquid. The auxiliary device is
separately calibrated against a primary standard, for example, a sphere gap in accordance
with IEC 52 (see also IEC 60) [2],[3].
5 Test assembly
5.1 General
The breakdown voltage test is performed following the method described herewith as a
routine test.
5.2 Test cell
The volume of the cell shall be between 350 ml and 600 ml.
The cell shall be made from electrically insulating materials, that are not hygroscopic. The cell
shall be transparent and chemically inert, resistant to the insulating liquid and to the cleaning
agents that may shall be used. A glass cell is the preferred option.
The cell shall be provided with a cover and shall be designed to permit easy removal of the
electrodes for cleaning and maintenance. To improve homogenization of the test liquid, a
rounded bottom shape of the cell is recommended. Containers and covers shall be cleaned by
washing with a suitable solvent or clean insulating liquid to remove residues of an earlier
– 10 – IEC 60156:2018 RLV © IEC 2018
sample. After cleaning, containers shall be immediately capped and kept closed until used
again. Electrodes shall be stored in clean insulating liquids.
NOTE It is preferable, in the case of esters, to use similar liquid to store the electrodes.
Examples of suitable cell designs are given in Figures 1 and 2.
Figure 1 − Example of suitable cell and spherical electrodes
Figure 2 − Example of suitable cell and partially spherical electrodes
– 12 – IEC 60156:2018 RLV © IEC 2018
Dimensions in millimetres
2,5
IEC
NOTE The stirring device can be mounted on the top (right side figure) or on the bottom (left side figure). The
stirring device position and Vernier shifter are reported only as reference.
Figure 1 – Examples of test cells with spherical electrodes
12,5 mm to 13,0 mm diameter
Dimensions in millimetres
R25
2,5
IEC
NOTE The stirring device can be mounted on the top (right side figure) or on the bottom (left side figure). The
stirring device position and Vernier shifter are reported only as reference.
Figure 2 – Examples of test cells with partially spherical electrodes
with 25 mm radius and diameter of 36 mm
5.3 Electrodes
The electrodes shall be made either of brass, bronze or austenitic stainless steel. They shall
be polished and, in shape, either spherical (12,5 mm to 13,0 mm diameter) as shown in
Figure 1 or in partially spherical shape (25 mm ± 0,25 mm radius) as shown in Figure 2. The
axis of the electrode system shall be horizontal and shall be at least 40 mm below the surface
of the test liquid. No part of the electrode shall be closer than 12 mm to the cell wall or stirrer.
Any part of the cell or stirrer shall not influence the electric field between the electrodes. The
gap between the electrodes shall be 2,50 mm ± 0,05 mm.
The electrodes shall be examined frequently for pitting or other damage and shall be
maintained or replaced as soon as such damage is observed.
NOTE The electrodes can be replaced or refurbished typically after 5 000 single breakdowns. The surface of the
electrodes can be polished with a maximum grain diameter of 10 µm. The limit of the arithmetical mean deviation of
the roughness profile of the electrodes can be Ra ≤ 0,5 µm, according to ISO 4287[4].
5.4 Stirring device (optional)
The test may be conducted with or without stirring. Differences between tests with or without
stirring have not been found statistically significant. A stirrer, however, may be convenient
especially with apparatus capable of automatic operation.
Stirring may be achieved by means of a two-bladed impeller of effective diameter 20 mm to 25
mm, axial depth 5 mm to 10 mm, rotating at a speed of 250 r.p.m to 300 r.p.m. The impeller
ø36
shall not entrain air bubbles and preferably rotate in such a direction that the resulting liquid
flow is directed downward. It shall be constructed so that it is easily cleaned.
Stirring by means of a magnetic bar (20 mm to 25 mm in length and 5 mm to 10 mm in
diameter) is an acceptable alternative when there is no risk of removing magnetic particles.
The dimensions of the stirring device shall conform to the clearance requirements in 4.2.
The use of an automatic stirring device is recommended, to be used at all times throughout
the test.
The stirrer shall be mounted in the test cell in order to maximize the homogenization of the
liquid. It shall be designed so that it is easily cleaned. Stirring shall be achieved by means of
a two-bladed or appropriate stirrer of effective diameter 25 mm to 35 mm, axial depth 5 mm to
10 mm, rotating at a speed of 200 r/min to 300 r/min. The stirrer shall not produce air bubbles.
It shall be fully immersed in the liquid sample. Examples of stirring systems mounted in test
cells are reported in Figures 1 and 2.
NOTE 1 To avoid bubbles between the electrodes the stirrer can rotate preferably in such a direction that bubbles
can be removed [5].
NOTE 2 The stirring device can be mounted on the top or on the bottom. In Figures 1 and 2, the stirring device
position is reported only as reference.
NOTE 3 A magnetic stirring device can be also used.
6 Preparation of electrodes
New electrodes, pitted electrodes, electrodes which have not been properly stored for a
considerable time shall be cleaned by and fulfil the requirements of 5.3. Preparation of the
electrodes shall be according to the following procedure:
– clean all surfaces with a suitable volatile solvent and allow the solvent to evaporate;
– polish with fine abrasive powder (for example, jeweller’s rouge) or abrasive paper or cloth,
for example crocus cloth (see 5.3);
– after polishing, clean with petroleum spirit (reagent quality: boiling range of 60°C about
40 °C to 80 °C) followed by acetone (reagent quality);
– assemble the electrodes in the cell, fill with a clean, unused insulating liquid of the type to
be tested;
– before the first breakdown test, raise the voltage until breakdown 24 times.
This procedure shall be repeated after each cleaning or change of electrodes.
7 Test assembly preparation
It is recommended that a separate test cell assembly be reserved for each different insulating
liquid types.
Test assemblies shall be stored in a dry place, covered and filled with dry insulating liquid of
the type in regular use in the cell.
On change of the type of liquid under test, remove all residues of the previous liquid with an
appropriate solvent, rinse the assembly with a clean, dry liquid of the same type as that the
one to be tested, drain and refill.
– 14 – IEC 60156:2018 RLV © IEC 2018
8 Sampling
7.1 Sample containers
Sample size should be approximately three times the capacity of the test cell.
Appropriate sample containers shall comply with IEC 475. An amber glass bottle is the
preferred container. Clear glass bottles may be used but they shall be shielded from direct
light until ready to be tested. Plastic containers which are not attacked by the liquid to be
tested may be used, but these shall not be used more than once. For sealing, screw caps with
polyolefine or polytetrafluoroethylene insert are preferred.
Containers and caps shall be cleaned by washing with a suitable solvent to remove residues
of an earlier sample. Containers shall next be rinsed with acetone, traces of which shall be
removed by blowing with warm air.
After cleaning, containers shall be immediately capped and kept sealed until used.
7.2 Sampling technique
Sampling of new and used insulating liquids shall be carried out in full compliance with
procedures detailed in IEC 475.
When sampling, containers should be almost filled with sample, leaving about 3 % of the
container volume as free air space.
Breakdown voltage is extremely sensitive to the slightest contamination of the sample by
water and particulate matters. Special reference is made to precautions necessary to avoid
contamination of the sample and the need for trained personnel and experienced supervision.
Unless otherwise required, the sample is taken where the liquid is likely to be most
contaminated, usually at the lowest point of the container holding it.
Sampling shall be carried out in accordance with IEC 60475.
NOTE Breakdown voltage is extremely sensitive to the slightest contamination of the sample by water and
particulate matter. Special precautions can be implemented to avoid contamination of the sample and the need for
trained personnel and experienced supervision. Unless otherwise required, the sample is taken where the liquid is
likely to be most contaminated, usually at the lowest point of the container holding it.
8 Condition of the sample
The test is carried out, unless otherwise specified, on the sample as received without drying
or degassing.
At the time of test, the temperatures of the test liquid and ambient air shall not differ by more
than 5 °C and for referee tests the liquid temperature shall be 20 °C ± 5 °C.
9 Test procedure
9.1 Sample preparation
Immediately before filling the test cell, the sample container is gently agitated and turned over
several times in such a way as to ensure, as far as possible, a homogeneous distribution of
the impurities contained in the liquid without causing the formation of air bubbles.
Unnecessary exposure to the ambient air of the sample shall be avoided.
A possible method is an automatic rotation of the sample container horizontally for 1 min with
a recommended speed of 30 r/min.
Equilibrate the sample to room temperature. Unnecessary exposure to the ambient air of the
sample shall be avoided.
9.2 Filling of the cell
Immediately before commencing the test, drain the test cell and rinse the walls, electrodes
and other component parts, with the test sample liquid. Drain and slowly fill with the test
sample liquid avoiding the formation of air bubbles.
Measure and record the temperature of the liquid.
Position the cell in the test equipment and start the stirrer if used.
10 Application of the voltage
At the time of test, the temperatures shall be maintained at room temperature (20 °C ± 5 °C).
Adjust the electrode gap distance to 2,5 mm ± 0,05 mm with a vernier or other system and
start the stirrer. The stirrer, if used, shall run continuously throughout the test.
Metallic gauges can damage the surface of the electrodes; hence, they have to be avoided.
The first application of voltage is started approximately 5 min after completion of filling and
checking that no air bubbles are visible in the electrode gap. Apply voltage to the electrodes
and uniformly increase voltage from zero at the rate of 2,0 kV/s ± 0,2 kV/s until breakdown
occurs.
Apply voltage to the electrodes and uniformly increase voltage from zero at the rate of
−1 −1
2,0 kV s ± 0,2 kV s until breakdown occurs. The breakdown voltage is the maximum
voltage reached at the time the circuit is opened either automatically (established arc) or
manually (visible or audible discharge detected).
Record the value in kilovolts.
Carry out six breakdowns on the same cell filling allowing a pause of at least 2 min after each
breakdown before re-application of voltage. Check that no gas bubbles are present within the
electrode gap. If a stirrer is used, it shall run continuously throughout the test.
Calculate the mean value of the six breakdowns in kilovolts, standard deviation and related
coefficient of variation (ratio between standard deviation and mean breakdown voltage).
For insulating liquids having a nominal viscosity higher than 15 mm /s (40°C), the resting time
before application of the voltage shall be increased in the range of 15 min to 30 min. In
addition, the resting time between two consecutive shots shall also be increased accordingly.
11 Report
Report the mean value, in kilovolts, of the six breakdowns as the test result.
The report shall also include:
– sample identification, possibly including the type of insulating liquids;
– value of each individual breakdown in kilovolts;
– 16 – IEC 60156:2018 RLV © IEC 2018
– mean breakdown value;
– type of electrodes used;
– temperature of the liquid (in the test cell);
– coefficient of variation (%) (optional);
– frequency of the test voltage (optional);
– the use of a stirrer (if any);
– stirring arrangement (optional).
In the case where the individual breakdown voltage is above the maximum equipment voltage
capability, the result shall be reported as greater than the maximum voltage capability
(example: > 80 kV).
12 Test data dispersion and reproducibility
The scatter of individual breakdown voltages has been found to be very dependent on the
value of the result. The graphical representation of figure 3 indicates the values of standard
deviation/mean ratio which have been found in a large body of test data in several
laboratories using transformer oil.
The full line in the graph shows the distribution of the median value of SD/mean as a function
of the value of the mean. The dotted lines indicate the expected 95 % range of values of
SD/mean as a function of the value of the mean.
12.1 Test data dispersion
The graphical representation of Figure 3 indicates the values of the coefficient of variation
and its mean value which have been found in a large body of test data in several laboratories
using transformer liquids. The solid line in the graph shows the distribution of the coefficient
of variation as a function of the mean breakdown value. The dotted lines indicate the
expected 2,5 % (0,025) to 97,5 % (0,975) range of values of standard deviation (SD)/mean as
a function of the value of the mean.
Typical coefficients of variation reported in Figure 3 are for information only and do not
represent an acceptance criteria for the obtained results.
97,5 %
%
50 %
2,5 %
0 20 40 60 80 100
Mean breakdown voltage (kV)
IEC
Figure 3 – Graphical representation of coefficient
of variation (standard deviation/mean ratio) versus mean breakdown voltage
12.2 Reproducibility
Experience has shown that the reproducibility of individual dielectric breakdown values is in
the range of ±30 %.
Coefficient of variation (%)
– 18 – IEC 60156:2018 RLV © IEC 2018
Annex A
(informative)
Improved test method
A.1 Test procedure for improved test method
Annex A describes an improved test method, believed to be able to reduce the scatter of the
results of breakdown voltage, which may be used [5],[6],[7]. The results obtained using both
methods around the world during the following years will assist in a future choice when this
document is revised.
Use the same instrument and prepare the test according to Clauses 4 to 9. Instead of the
procedure described in Clause 10, follow the procedure described hereafter (Figure A.1):
NOTE The software of the device can be aligned with the procedure described in Annex A.
1) The first application of voltage is started at least 5 min after completion of filling and after
checking that the liquid under test is free from air bubbles.
2) Apply voltage to the electrodes uniformly and increase the voltage from zero at the rate of
2 kV/s ± 0,2 kV/s until 10 kV is reached.
3) Maintain the 10 kV level for 10 s, then continue with a rate of voltage rise of 2 kV/s ±
0,2 kV/s until a breakdown occurs.
4) The breakdown voltage shall be recorded at the maximum voltage reached.
5) Carry out 10 breakdowns on the same filling, allowing a pause of at least 1 min after each
breakdown before re-application of the test voltage. Record each single breakdown.
Calculate the test results as the average and coefficient of variation (ratio between
standard deviation and mean breakdown voltage) of the remaining six results after
disregarding the two highest and two lowest results.
6) When the coefficient of variation of the test result (mean breakdown voltage) exceeds the
upper limit (Figure 3), the test procedure should proceed for the other 10 breakdowns,
repeating the procedure from 2) to 6) with the same sample liquid. Record also the results
of these additional breakdowns. Calculate the test results as the average and coefficient
of variation of the remaining 12 results after disregarding the four highest and four lowest
results.
For insulating liquids having a nominal viscosity higher than 15 mm /s (40°C), the resting time
before application of the voltage shall be increased in the range of 15 min to 30 min. In
addition, the resting time between two consecutive shots shall also be increased accordingly.
Shot 1
Shot 8
Shot 5
Shot 4
Shot 2
Shot 10
Shot 3
≈ 3 s
Shot 6
Shot 9
Shot 7
Pre-energizing
10 s
10 Initial resting
time > 3 min
1 min
0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19
Time (minutes)
5 Sec
IEC
In the average calculation, the results of four outliers (two highest and two lowest values) have to be discarded (in
this example, shots 1 and 8 are the highest and shots 7 and 9 are the lowest).
Figure A.1 – Example of a sequence of breakdown shots for determination
of the breakdown voltage
A.2 Report
See Clause 11.
2 kV/s
2 kV/s
kV
Resting time
– 20 – IEC 60156:2018 RLV © IEC 2018
Annex B
(informative)
Special test methods for low volume samples
B.1 Low volume sample test
The special test method reported in this annex is suggested for use with low sample volumes.
A limited body of data has shown that the results obtained are comparable to the results
obtained from the method described in the main body of this document. Examples of the
reduced volume test cell are shown in Figures B.1 and B.2.
A fast test on-site may require small portable testers, able to measure the breakdown voltage
of insulating liquids (in either direct current or alternating current). An example of such
instruments is a Cockcroft-Walton generator, which utilizes a small electrode gap cell and
measuring instrumentation. The cell in such an instrument also requires very small quantities
of test liquid.
NOTE The results obtained with such portable instruments cannot be used for diagnostic purposes. Results can
differ significantly unless comparability has been established.
IEC
Key
1 partially spherical electrodes, rounded disk electrode, 50 mm diameter, 2 mm gap
2 oil filled cup, test cell HV insulation
3 cover
4 electrode distance control
5 sample inlet
6 sample outlet
Figure B.1 – Example of low volume test cell, fixed electrode distance of 2 mm with
2 ml active volume under dielectric stress
ø50
Dimensions in millimetres
R15
2,5 Section A-A
IEC
Figure B.2 – Example of low volume test cell, fixed electrode distance of 2,5 mm
(150 ml to 200 ml)
ø36
– 22 – IEC 60156:2018 RLV © IEC 2018
Annex C
(informative)
Representative material for a performance test
The reference analysis may be used as a performance check to prove that the test system is
fit for use according to IEC 60060-3.
The representative material shall be unused, filtered and degassed mineral, silicone or ester
liquids. The minimum quality requirement of the liquid shall be according to IEC relevant
standards.
If the test result does not reach the required > 70 kV value, check the functionality of the
equipment, or prepare a fresh representative material sample and carry out a new
performance check.
Bibliography
[1] IEC 60060-3, High-voltage test techniques – Part 3: Definitions and requirements for
on-site testing
[2] IEC 60052:2002, Voltage measurement by means o
...
IEC 60156 ®
Edition 3.0 2018-08
INTERNATIONAL
STANDARD
NORME
INTERNATIONALE
Insulating liquids – Determination of the breakdown voltage at power
frequency – Test method
Isolants liquides – Détermination de la tension de claquage à fréquence
industrielle – Méthode d’essai
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IEC 60156 ®
Edition 3.0 2018-08
INTERNATIONAL
STANDARD
NORME
INTERNATIONALE
Insulating liquids – Determination of the breakdown voltage at power
frequency – Test method
Isolants liquides – Détermination de la tension de claquage à fréquence
industrielle – Méthode d’essai
INTERNATIONAL
ELECTROTECHNICAL
COMMISSION
COMMISSION
ELECTROTECHNIQUE
INTERNATIONALE
ICS 29.040 ISBN 978-2-8322-5959-7
– 2 – IEC 60156:2018 © IEC 2018
CONTENTS
FOREWORD . 4
INTRODUCTION . 6
1 Scope . 7
2 Normative references. 7
3 Terms and definitions . 7
4 Electrical apparatus . 7
4.1 General . 7
4.2 Voltage regulator . 7
4.3 Step-up transformer . 8
4.4 Switching system . 8
4.5 Current-limiting resistors . 8
4.6 Measuring system . 8
5 Test assembly . 8
5.1 General . 8
5.2 Test cell . 9
5.3 Electrodes. 9
5.4 Stirring device . 10
6 Preparation of electrodes . 10
7 Test assembly preparation . 10
8 Sampling . 11
9 Test procedure . 11
9.1 Sample preparation . 11
9.2 Filling of the cell . 11
10 Application of the voltage . 11
11 Report . 12
12 Test data dispersion and reproducibility . 12
12.1 Test data dispersion . 12
12.2 Reproducibility . 13
Annex A (informative) Improved test method . 14
A.1 Test procedure for improved test method . 14
A.2 Report . 15
Annex B (informative) Special test methods for low volume samples . 16
B.1 Low volume sample test . 16
Annex C (informative) Representative material for a performance test . 18
Bibliography . 19
Figure 1 – Examples of test cells with spherical electrodes 12,5 mm to 13,0 mm
diameter . 9
Figure 2 – Examples of test cells with partially spherical electrodes with 25 mm radius
and diameter of 36 mm . 9
Figure 3 – Graphical representation of coefficient of variation versus mean breakdown
voltage . 13
Figure A.1 – Example of a sequence of breakdown shots for determination of the
breakdown voltage . 15
Figure B.1 – Example of low volume test cell, fixed electrode distance of 2 mm with
2 ml active volume under dielectric stress . 16
Figure B.2 – Example of low volume test cell, fixed electrode distance of 2,5 mm
(150 ml to 200 ml) . 17
– 4 – IEC 60156:2018 © IEC 2018
INTERNATIONAL ELECTROTECHNICAL COMMISSION
____________
INSULATING LIQUIDS – DETERMINATION OF THE BREAKDOWN
VOLTAGE AT POWER FREQUENCY – TEST METHOD
FOREWORD
1) The International Electrotechnical Commission (IEC) is a worldwide organization for standardization comprising
all national electrotechnical committees (IEC National Committees). The object of IEC is to promote
international co-operation on all questions concerning standardization in the electrical and electronic fields. To
this end and in addition to other activities, IEC publishes International Standards, Technical Specifications,
Technical Reports, Publicly Available Specifications (PAS) and Guides (hereafter referred to as “IEC
Publication(s)”). Their preparation is entrusted to technical committees; any IEC National Committee interested
in the subject dealt with may participate in this preparatory work. International, governmental and non-
governmental organizations liaising with the IEC also participate in this preparation. IEC collaborates closely
with the International Organization for Standardization (ISO) in accordance with conditions determined by
agreement between the two organizations.
2) The formal decisions or agreements of IEC on technical matters express, as nearly as possible, an international
consensus of opinion on the relevant subjects since each technical committee has representation from all
interested IEC National Committees.
3) IEC Publications have the form of recommendations for international use and are accepted by IEC National
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4) In order to promote international uniformity, IEC National Committees undertake to apply IEC Publications
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between any IEC Publication and the corresponding national or regional publication shall be clearly indicated in
the latter.
5) IEC itself does not provide any attestation of conformity. Independent certification bodies provide conformity
assessment services and, in some areas, access to IEC marks of conformity. IEC is not responsible for any
services carried out by independent certification bodies.
6) All users should ensure that they have the latest edition of this publication.
7) No liability shall attach to IEC or its directors, employees, servants or agents including individual experts and
members of its technical committees and IEC National Committees for any personal injury, property damage or
other damage of any nature whatsoever, whether direct or indirect, or for costs (including legal fees) and
expenses arising out of the publication, use of, or reliance upon, this IEC Publication or any other IEC
Publications.
8) Attention is drawn to the Normative references cited in this publication. Use of the referenced publications is
indispensable for the correct application of this publication.
9) Attention is drawn to the possibility that some of the elements of this IEC Publication may be the subject of
patent rights. IEC shall not be held responsible for identifying any or all such patent rights.
International Standard IEC 60156 has been prepared by IEC technical committee TC 10:
Fluids for electrotechnical applications.
This third edition cancels and replaces the second edition published in 1995. This edition
constitutes a technical revision and, mainly, confirms the content of the previous edition even
if some advances are included. The test method has not been changed for practical reason
due to the very large number of instrumentation disseminated around the world, although the
use of stirring is now recommended.
The text of this International Standard is based on the following documents:
FDIS Report on voting
10/1061/FDIS 10/1065/RVD
Full information on the voting for the approval of this International Standard can be found in
the report on voting indicated in the above table.
This document has been drafted in accordance with the ISO/IEC Directives, Part 2.
The committee has decided that the contents of this document will remain unchanged until the
stability date indicated on the IEC website under "http://webstore.iec.ch" in the data related to
the specific document. At this date, the document will be
• reconfirmed,
• withdrawn,
• replaced by a revised edition, or
• amended.
– 6 – IEC 60156:2018 © IEC 2018
INTRODUCTION
As normally applied, breakdown voltage of insulating liquids is not a basic material property
but an empirical test procedure intended to indicate the presence of contaminants such as
water and solid suspended matter and the advisability of carrying out a drying and filtration
treatment.
The AC breakdown voltage value of insulating liquids strongly depends on the particular set of
conditions used in its measurement. Therefore, standardized testing procedures and
equipment are essential for the unambiguous interpretation of test results.
The method described in this document applies to either acceptance tests on new deliveries
of insulating liquids, or testing of treated liquids prior to or during filling into electrical
equipment, or to the monitoring and maintenance of oil-filled apparatus in service. It specifies
rigorous sample-handling procedures and temperature control that should be adhered to when
certified results are required. For routine tests, especially in the field, less stringent
procedures may be practicable and it is the responsibility of the user to determine their effect
on the results.
Annex A (informative) describes, for comparison, an alternative test method which could be
introduced in the future. Annex B (informative) describes special test methods, using cells
which may include low volume samples. Annex C (informative) describes a reference material
for a performance test and check according to IEC 60060-3[1] .
—————————
Numbers in square brackets refer to the Bibliography.
INSULATING LIQUIDS – DETERMINATION OF THE BREAKDOWN
VOLTAGE AT POWER FREQUENCY – TEST METHOD
1 Scope
This document specifies the method for determining the dielectric breakdown voltage of
insulating liquids at power frequency. The test procedure is performed in a specified
apparatus, where the oil sample is subjected to an increasing AC electrical field until
breakdown occurs. The method applies to all types of insulating liquids of nominal viscosity
up to 350 mm /s at 40 °C. It is appropriate both for acceptance testing on unused liquids at
the time of their delivery and for establishing the condition of samples taken in monitoring and
maintenance of equipment.
2 Normative references
The following documents are referred to in the text in such a way that some or all of their
content constitutes requirements of this document. For dated references, only the edition
cited applies. For undated references, the latest edition of the referenced document (including
any amendments) applies.
IEC 60475, Method of sampling insulating liquids
3 Terms and definitions
No terms and definitions are listed in this document.
ISO and IEC maintain terminological databases for use in standardization at the following
addresses:
• IEC Electropedia: available at http://www.electropedia.org/
• ISO Online browsing platform: available at http://www.iso.org/obp
4 Electrical apparatus
4.1 General
The electrical apparatus consists of the following units:
1) voltage regulator,
2) step-up transformer,
3) switching system,
4) current-limiting resistors,
5) measuring device.
Two or more of these units may be integrated in any equipment system.
4.2 Voltage regulator
The test voltage shall be increased with an automatic control of the required uniform voltage
rate of rise. The device should not introduce harmonics disturbances (< 3%) and the AC
source should be free from harmonics.
– 8 – IEC 60156:2018 © IEC 2018
4.3 Step-up transformer
The test voltage is obtained by using a step-up or resonant transformer supplied from an AC
source using 48 Hz to 62 Hz (sinusoidal waveform). The voltage source value is constantly
increased. The controls of the variable low-voltage source shall be capable of varying the test
voltage smoothly, uniformly and without overshoots or transients. Incremental increases
(produced, for example, by a variable auto-transformer or an amplifier) shall not exceed 2 %
of the expected breakdown voltage.
The centre-point of the secondary winding of the transformer should be connected to earth.
4.4 Switching system
The circuit shall be opened automatically if a sustained arc between the electrodes occurs
and the voltage between the electrodes collapses to a voltage less than 500 V. The primary
circuit of the step-up transformer shall be fitted with a circuit-breaker operated by the current
sensing device, resulting from the breakdown of the sample and shall break the voltage within
10 ms.
The sensitivity of the current or voltage sensing element depends on the energy-limiting
device employed and only approximate guidance can be given.
A cut-off time of < 100 µs, as given in the previous edition of this document, is needed to
perform multiple breakdowns on silicone liquids.
4.5 Current-limiting resistors
To protect the equipment and to avoid excessive decomposition at the instant of breakdown of
liquids such as silicone or ester liquids, a resistance limiting the breakdown current shall be
inserted in series with the test cell.
The short-circuit current of the transformer and associated circuits shall be within the range of
10 mA to 25 mA for all voltages higher than 15 kV. This may be achieved by a combination of
resistors in either or both the primary and secondary circuits of the high-voltage transformer.
4.6 Measuring system
For the purposes of this document, the magnitude of the test voltage is defined as its peak
value divided by 2 .
The output voltage of the step-up transformer may be measured by means of a measuring
system consisting of a voltage divider or a measuring winding of the step-up transformer
coupled with a peak-voltmeter. The measuring system shall be calibrated up to the upper
scale voltage to be measured. A method of calibration which has been found satisfactory is
the use of a transfer standard. This is an auxiliary measuring device which is connected in
place of the test cell between the high-voltage terminals to which it presents an impedance
similar to the one of the sample liquid. The auxiliary device is separately calibrated against a
primary standard [2],[3].
5 Test assembly
5.1 General
The breakdown voltage test is performed following the method described herewith as a
routine test.
5.2 Test cell
The volume of the cell shall be between 350 ml and 600 ml.
The cell shall be made from electrically insulating materials, that are not hygroscopic. The cell
shall be transparent and chemically inert, resistant to the insulating liquid and to the cleaning
agent that shall be used. A glass cell is the preferred option.
The cell shall be provided with a cover and shall be designed to permit easy removal of the
electrodes for cleaning and maintenance. To improve homogenization of the test liquid, a
rounded bottom shape of the cell is recommended. Containers and covers shall be cleaned by
washing with a suitable solvent or clean insulating liquid to remove residues of an earlier
sample. After cleaning, containers shall be immediately capped and kept closed until used
again. Electrodes shall be stored in clean insulating liquids.
NOTE It is preferable, in the case of esters, to use similar liquid to store the electrodes.
Examples of suitable cell designs are given in Figures 1 and 2.
Dimensions in millimetres
2,5
IEC
NOTE The stirring device can be mounted on the top (right side figure) or on the bottom (left side figure). The
stirring device position and Vernier shifter are reported only as reference.
Figure 1 – Examples of test cells with spherical electrodes
12,5 mm to 13,0 mm diameter
Dimensions in millimetres
R25
2,5
IEC
NOTE The stirring device can be mounted on the top (right side figure) or on the bottom (left side figure). The
stirring device position and Vernier shifter are reported only as reference.
Figure 2 – Examples of test cells with partially spherical electrodes
with 25 mm radius and diameter of 36 mm
5.3 Electrodes
The electrodes shall be made either of brass, bronze or austenitic stainless steel. They shall
be polished and, in shape, either spherical (12,5 mm to 13,0 mm diameter) as shown in
Figure 1 or in partially spherical shape (25 mm ± 0,25 mm radius) as shown in Figure 2. The
axis of the electrode system shall be horizontal and shall be at least 40 mm below the surface
ø36
– 10 – IEC 60156:2018 © IEC 2018
of the test liquid. Any part of the cell or stirrer shall not influence the electric field between the
electrodes. The gap between the electrodes shall be 2,50 mm ± 0,05 mm.
The electrodes shall be examined frequently for pitting or other damage and shall be
maintained or replaced as soon as such damage is observed.
NOTE The electrodes can be replaced or refurbished typically after 5 000 single breakdowns. The surface of the
electrodes can be polished with a maximum grain diameter of 10 µm. The limit of the arithmetical mean deviation of
the roughness profile of the electrodes can be Ra ≤ 0,5 µm, according to ISO 4287[4].
5.4 Stirring device
The use of an automatic stirring device is recommended, to be used at all times throughout
the test.
The stirrer shall be mounted in the test cell in order to maximize the homogenization of the
liquid. It shall be designed so that it is easily cleaned. Stirring shall be achieved by means of
a two-bladed or appropriate stirrer of effective diameter 25 mm to 35 mm, axial depth 5 mm to
10 mm, rotating at a speed of 200 r/min to 300 r/min. The stirrer shall not produce air bubbles.
It shall be fully immersed in the liquid sample. Examples of stirring systems mounted in test
cells are reported in Figures 1 and 2.
NOTE 1 To avoid bubbles between the electrodes the stirrer can rotate preferably in such a direction that bubbles
can be removed [5].
NOTE 2 The stirring device can be mounted on the top or on the bottom. In Figures 1 and 2, the stirring device
position is reported only as reference.
NOTE 3 A magnetic stirring device can be also used.
6 Preparation of electrodes
New electrodes shall be cleaned and fulfil the requirements of 5.3. Preparation of the
electrodes shall be according to the following procedure:
– clean all surfaces with a suitable volatile solvent and allow the solvent to evaporate;
– polish with fine abrasive powder (for example, jeweller’s rouge) or abrasive paper or cloth,
for example crocus cloth (see 5.3);
– after polishing, clean with petroleum spirit (reagent quality: boiling range of about 40 °C to
80 °C) followed by acetone (reagent quality);
– assemble the electrodes in the cell, fill with a clean, unused insulating liquid of the type to
be tested;
– before the first breakdown test, raise the voltage until breakdown 24 times.
This procedure shall be repeated after each cleaning or change of electrodes.
7 Test assembly preparation
It is recommended that a separate test cell assembly be reserved for different insulating liquid
types.
Test assemblies shall be stored in a dry place, covered and filled with dry insulating liquid of
the type in regular use in the cell.
On change of the type of liquid under test, remove all residues of the previous liquid with an
appropriate solvent, rinse the assembly with a clean, dry liquid of the same type as the one to
be tested, drain and refill.
8 Sampling
Sampling shall be carried out in accordance with IEC 60475.
NOTE Breakdown voltage is extremely sensitive to the slightest contamination of the sample by water and
particulate matter. Special precautions can be implemented to avoid contamination of the sample and the need for
trained personnel and experienced supervision. Unless otherwise required, the sample is taken where the liquid is
likely to be most contaminated, usually at the lowest point of the container holding it.
The test is carried out, unless otherwise specified, on the sample as received without drying
or degassing.
9 Test procedure
9.1 Sample preparation
Immediately before filling the test cell, the sample container is gently agitated and turned over
several times in such a way as to ensure, as far as possible, a homogeneous distribution of
the impurities contained in the liquid without causing the formation of air bubbles.
A possible method is an automatic rotation of the sample container horizontally for 1 min with
a recommended speed of 30 r/min.
Equilibrate the sample to room temperature. Unnecessary exposure to the ambient air of the
sample shall be avoided.
9.2 Filling of the cell
Immediately before commencing the test, drain the test cell and rinse the walls, electrodes
and other component parts, with the test liquid. Drain and slowly fill with the test liquid
avoiding the formation of air bubbles.
Measure and record the temperature of the liquid.
10 Application of the voltage
At the time of test, the temperatures shall be maintained at room temperature (20 °C ± 5 °C).
Adjust the electrode gap distance to 2,5 mm ± 0,05 mm with a vernier or other system and
start the stirrer. The stirrer, if used, shall run continuously throughout the test.
Metallic gauges can damage the surface of the electrodes; hence, they have to be avoided.
The first application of voltage is started approximately 5 min after completion of filling and
checking that no air bubbles are visible in the electrode gap. Apply voltage to the electrodes
and uniformly increase voltage from zero at the rate of 2,0 kV/s ± 0,2 kV/s until breakdown
occurs.
The breakdown voltage is the maximum voltage reached at the time the circuit is opened
either automatically (established arc) or manually (visible or audible discharge detected).
Record the value in kilovolts.
Carry out six breakdowns on the same cell filling allowing a pause of at least 2 min after each
breakdown before re-application of voltage. Check that no gas bubbles are present within the
electrode gap.
– 12 – IEC 60156:2018 © IEC 2018
Calculate the mean value of the six breakdowns, standard deviation and related coefficient of
variation (ratio between standard deviation and mean breakdown voltage).
For insulating liquids having a nominal viscosity higher than 15 mm /s (40°C), the resting time
before application of the voltage shall be increased in the range of 15 min to 30 min. In
addition, the resting time between two consecutive shots shall also be increased accordingly.
11 Report
The report shall include:
– sample identification, possibly including the type of insulating liquids;
– value of each individual breakdown in kilovolts;
– mean breakdown value;
– type of electrodes used;
– temperature of the liquid (in the test cell);
– coefficient of variation (%) (optional);
– frequency of the test voltage (optional);
– stirring arrangement (optional).
In the case where the individual breakdown voltage is above the maximum equipment voltage
capability, the result shall be reported as greater than the maximum voltage capability
(example: > 80 kV).
12 Test data dispersion and reproducibility
12.1 Test data dispersion
The graphical representation of Figure 3 indicates the values of the coefficient of variation
and its mean value which have been found in a large body of test data in several laboratories
using transformer liquids. The solid line in the graph shows the distribution of the coefficient
of variation as a function of the mean breakdown value. The dotted lines indicate the
expected 2,5 % (0,025) to 97,5 % (0,975) range of values of standard deviation (SD)/mean as
a function of the value of the mean.
Typical coefficients of variation reported in Figure 3 are for information only and do not
represent an acceptance criteria for the obtained results.
97,5 %
%
50 %
2,5 %
0 20 40 60 80 100
Mean breakdown voltage (kV)
IEC
Figure 3 – Graphical representation of coefficient of variation versus
mean breakdown voltage
12.2 Reproducibility
Experience has shown that the reproducibility of individual dielectric breakdown values is in
the range of ±30 %.
Coefficient of variation (%)
– 14 – IEC 60156:2018 © IEC 2018
Annex A
(informative)
Improved test method
A.1 Test procedure for improved test method
Annex A describes an improved test method, believed to be able to reduce the scatter of the
results of breakdown voltage, which may be used [5],[6],[7]. The results obtained using both
methods around the world during the following years will assist in a future choice when this
document is revised.
Use the same instrument and prepare the test according to Clauses 4 to 9. Instead of the
procedure described in Clause 10, follow the procedure described hereafter (Figure A.1):
NOTE The software of the device can be aligned with the procedure described in Annex A.
1) The first application of voltage is started at least 5 min after completion of filling and after
checking that the liquid under test is free from air bubbles.
2) Apply voltage to the electrodes uniformly and increase the voltage from zero at the rate of
2 kV/s ± 0,2 kV/s until 10 kV is reached.
3) Maintain the 10 kV level for 10 s, then continue with a rate of voltage rise of 2 kV/s ±
0,2 kV/s until a breakdown occurs.
4) The breakdown voltage shall be recorded at the maximum voltage reached.
5) Carry out 10 breakdowns on the same filling, allowing a pause of at least 1 min after each
breakdown before re-application of the test voltage. Record each single breakdown.
Calculate the test results as the average and coefficient of variation (ratio between
standard deviation and mean breakdown voltage) of the remaining six results after
disregarding the two highest and two lowest results.
6) When the coefficient of variation of the test result (mean breakdown voltage) exceeds the
upper limit (Figure 3), the test procedure should proceed for the other 10 breakdowns,
repeating the procedure from 2) to 6) with the same sample liquid. Record also the results
of these additional breakdowns. Calculate the test results as the average and coefficient
of variation of the remaining 12 results after disregarding the four highest and four lowest
results.
For insulating liquids having a nominal viscosity higher than 15 mm /s (40°C), the resting time
before application of the voltage shall be increased in the range of 15 min to 30 min. In
addition, the resting time between two consecutive shots shall also be increased accordingly.
Shot 1
Shot 8
Shot 5
Shot 4
Shot 2
Shot 10
Shot 3
≈ 3 s
Shot 6
Shot 9
Shot 7
Pre-energizing
10 s
10 Initial resting
time > 3 min
1 min
0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19
Time (minutes)
5 Sec
IEC
In the average calculation, the results of four outliers (two highest and two lowest values) have to be discarded (in
this example, shots 1 and 8 are the highest and shots 7 and 9 are the lowest).
Figure A.1 – Example of a sequence of breakdown shots for determination
of the breakdown voltage
A.2 Report
See Clause 11.
2 kV/s
2 kV/s
kV
Resting time
– 16 – IEC 60156:2018 © IEC 2018
Annex B
(informative)
Special test methods for low volume samples
B.1 Low volume sample test
The special test method reported in this annex is suggested for use with low sample volumes.
A limited body of data has shown that the results obtained are comparable to the results
obtained from the method described in the main body of this document. Examples of the
reduced volume test cell are shown in Figures B.1 and B.2.
A fast test on-site may require small portable testers, able to measure the breakdown voltage
of insulating liquids (in either direct current or alternating current). An example of such
instruments is a Cockcroft-Walton generator, which utilizes a small electrode gap cell and
measuring instrumentation. The cell in such an instrument also requires very small quantities
of test liquid.
NOTE The results obtained with such portable instruments cannot be used for diagnostic purposes. Results can
differ significantly unless comparability has been established.
IEC
Key
1 partially spherical electrodes, rounded disk electrode, 50 mm diameter, 2 mm gap
2 oil filled cup, test cell HV insulation
3 cover
4 electrode distance control
5 sample inlet
6 sample outlet
Figure B.1 – Example of low volume test cell, fixed electrode distance of 2 mm with
2 ml active volume under dielectric stress
ø50
Dimensions in millimetres
R15
2,5 Section A-A
IEC
Figure B.2 – Example of low volume test cell, fixed electrode distance of 2,5 mm
(150 ml to 200 ml)
ø36
– 18 – IEC 60156:2018 © IEC 2018
Annex C
(informative)
Representative material for a performance test
The reference analysis may be used as a performance check to prove that the test system is
fit for use according to IEC 60060-3.
The representative material shall be unused, filtered and degassed mineral, silicone or ester
liquids. The minimum quality requirement of the liquid shall be according to IEC relevant
standards.
If the test result does not reach the required > 70 kV value, check the functionality of the
equipment, or prepare a fresh representative material sample and carry out a new
performance check.
Bibliography
[1] IEC 60060-3, High-voltage test techniques – Part 3: Definitions and requirements for
on-site testing
[2] IEC 60052:2002, Voltage measurement by means of standard air gaps
[3] IEC 60060-2:2010, High-voltage test techniques – Part 2: Measuring systems
[4] ISO 4287, Geometrical Product Specifications (GPS) – Surface texture: Profile method
–Terms, definitions and surface texture parameters
[5] Elektrische Festigkeit von Isolieröl, Dissertation von G. J. Pukel TU Graz, 2004,
ISBN 978-3-85133-060-1
[6] M. Baur, M. Pompili, R. Bartnikas, “A comment on the test methods for the breakdown
voltage of dielectric liquids”, IEEE Trans. Dielectric Electric Insulation, Vol. 19,
p 1 482-1 484, 2012
[7] M. Baur, L. Calcara, M. Pompili, “Scatter Reduction of the 50-60 Hz Breakdown
Voltage Test for Insulating Liquids”, IEEE Trans. Dielectric Electric Insulation, Vol. 22,
Issue 5, page 2401-2407, October 2015
[8] M. Baur, J. Knauel, L. Calcara, M. Pompili, “Insulating Liquids Breakdown Voltage
Determination: Test Method Efficiency”, ICDL 2017, paper 1239, Manchester
[9] T. J. Lewis, “Mechanism of electrical breakdown in saturated hydrocarbon liquids”,
Journal of Applied Physics, Vol. 27, pp. 645-650, 1956
[10] E. O. Forster, “Critical Assessment of the Electrical Breakdown Process in Dielectric
Fluids”, IEEE Transactions on Electrical Insulation, Vol. 20, pp. 891-896, 1985
[11] E.O. Forster, C. Mazzetti and M. Pompili “Electrical breakdown in dielectric fluids: a
review of old a new concept”, L’Energia Elettrica, Vol. LXVII, pp. 1-19, 1990
[12] IEC 60296, Fluids for electrotechnical applications – Unused mineral insulating oils for
transformers and switchgear
[13] IEC 60422, Mineral insulating oils in electrical equipment – Supervision and
maintenance guidance
___________
– 20 – IEC 60156:2018 © IEC 2018
SOMMAIRE
AVANT-PROPOS . 22
INTRODUCTION . 24
1 Domaine d’application. 25
2 Références normatives . 25
3 Termes et définitions . 25
4 Appareillage électrique . 25
4.1 Généralités . 25
4.2 Régulateur de tension . 25
4.3 Transformateur élévateur . 26
4.4 Disjoncteur. 26
4.5 Résistances de limitation de courant . 26
4.6 Système de mesure. 26
5 Dispositif d’essai . 27
5.1 Généralités . 27
5.2 Cellule d’essai . 27
5.3 Électrodes. 28
5.4 Agitateur . 28
6 Préparation des électrodes . 29
7 Préparation du dispositif d’essai . 29
8 Échantillonnage . 29
9 Procédure d’essai . 29
9.1 Préparation de l’échantillon . 29
9.2 Remplissage de la cellule . 30
10 Application de la tension . 30
11 Rapport . 30
12 Dispersion des résultats et reproductibilité . 31
12.1 Dispersion des résultats .
...
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