IEC 60243-2 pdf download – Electric strength of insulating materials — Test methods — Part 2: Additional requirements for tests using direct voltage

IEC 60243-2 pdf download – Electric strength of insulating materials — Test methods — Part 2: Additional requirements for tests using direct voltage

IEC 60243-2 pdf download – Electric strength of insulating materials — Test methods — Part 2: Additional requirements for tests using direct voltage
This standard gives requirements additional to those in IEC 60243-1 for the determination ofthe electric strength of solid insulating materials under direct voltage stress.
1.2 Normative references
Subclause 1.2 of lEC 60243-1 is applicable with the following addition:
IEC 60243-1:1998,Electric strength of insulating materials – Test methods – Part 1: Tests atpower frequencies
Clause 2 of lEC 60243-1 is applicable.
3significance of the test
ln addition to the requirements of clause 3 of IEC 60243-1, the following points shall beconsidered when using direct-voltage tests.
3.1 For a non-homogeneous test specimen,with alternating voltage,the distribution ofvoltage stress within the test specimen is determined by impedance (largely capacitive). Withan increasing direct voltage, the voltage distribution may still be largely capacitive but dependspartly on the rate of voltage increase.The resistive voltage distribution after constant voltageapplication represents the steady-state condition. The choice between direct or alternatingvoltage depends upon the purpose for which the breakdown test is to be used and, to someextent,on the intended application of the material.
3.2Upon direct voltage application,the following currents result: the capacitive current, theelectric absorption current, the leakage current, and, in some cases, partial discharge currents.
In addition, for materials with dissimilar layers or discontinuities, the voltage distribution acrossthe test specimen is also influenced, as a result of interfacial polarization, by charges ofopposite polarity,which may accumulate on the two sides of the interface and create localfields sufficiently strong to produce partial discharges and/or breakdown of the test specimens.
3.3 For most materials,the d.c. breakdown voltage is higher than the peak value of thepower-frequency breakdown voltage; for many materials,particularly those which are non-homogeneous, the d.c. breakdown voltage will be three times higher than the a.c. breakdownvoltage or even more.
4Electrodes and test specimens
Clause 4 of lEC 60243-1 is applicable.
5 conditioning before tests
Clause 5 of lEC60243-1 is applicable.
6 Surrounding medium
Clause 6 of lEC 60243-1 is applicable.
7 Electrical apparatus
7.1Voltage source
The test voltage applied to the electrodes shall be provided by a power supply having thefollowing characteristics and components.
7.1.1 A choice of voltage of either positive or negative polarity shall be provided, one of theconnections to the electrodes being earthed.
7.1.2 The alternating voltage ripple on the test voltage shall not exceed 2 % of the voltage atall values greater than 50 % of the breakdown voltage.The test voltage shall also be free fromtransients or other fluctuations exceeding 1 % of the applied voltage.
When testing test specimens of low capacitance,it may be necessary to add a suitablecapacitor (for example, 1 000 pF) in parallel with the electrodes in order to reduce the influenceof transients in initiating premature breakdowns.
7.1.3 The controls on voltage shall be capable of varying the test voltage smoothly anduniformly between zero and the maximum test voltage and with the requested rate of voltagerise.The rate of voltage rise shall be controllable within ±20 % of the specified rate.Steps involtage rise shall not exceed 2 % of the expected breakdown voltage value.Controls whichautomatically increase the voltage at a selectable rate are preferable.
7.1.4 A current-sensitive interrupting device shall be used for switching off the direct voltagesource.
7.1.5 A current-limiting resistance should be used in series with the test specimen to preventdamage to the high-voltage supply and to limit as far as possible the damage to the electrodeson the test specimen when breakdown takes place. The maximum current permitted willdepend on the material under test and on the amount of damage to the electrodes which canbe tolerated.
NOTEThe use of a very high-valued resistor may result in breakdown voltages which are higher than thoseobtained with a lower-valued resistor.
7.1.6 When making tests where the value or increase in the value of the current is used as acriterion for breakdown, means of measurement of current through the test specimen shall beprovided.
7.2Voltage measurement
The measurement of applied voltage shall be made across the electrodes. The otherrequirements of clause 7 of IEC 60243-1 shall be met.
Clause 8 of IEC 60243-1 is applicable.
Mode of increase of voltage
Unless otherwise specified, the voltage shall be applied in accordance with IEC 60243-1subclauses 9.1 (short-time test),9.3 or 9.5 (slow and very slow rate-of-rise tests) or 9.6(prooftests).
10Criterion of breakdown
Clause 10 of lEC 60243-1 is applicable to direct voltage tests.Breakdown may be identified bya sudden increase in current or by the current exceeding a certain specified value.
11 Number of tests
Clause 11 of IEC60243-1 is applicable.
12.1 Unless otherwise specified, the report shall include the following:
a) a complete identification of the material tested, a description of the test specimens and themethod of preparation;
b) the polarity of the test voltage;
c) the median (central value) of the electric strengths and/or breakdown voltages;