Detail of GB/T 1408.2-2016

Introduction of GB/T 1408.2-2016

CB/T 1408 Insulating materials - Insulating materials - Test methods for electric strength includes the following three parts: ——Part 1: Tests at power frequencies; ——Part 2: Additional requirements for tests using direct voltage; ——Part 3: Additional requirements for 1.2/50μs impulse tests. This part is Part 2 of GB/T 1408. This part is drafted in accordance with the rules given in GB/T 1.1-2009. This part replaces GB/T 1408.2-2006 Electrical strength of insulating materials - Test methods - Part 2: Additional requirements for tests using direct voltage; the following main technical changes have been made with respect to GB/T 1408.2-2006: ——The full text of GB/T 1408.1-2006 is modified to GB/T 1408.1-2016; ——10 cited documents including GB/T 1981.2-2003 are deleted; ——The contents of report is modified; —— The “Bibliography” is added. This part is identical with IEC 60243-2: 2013 Electric strength of insulating materials - Test methods - Part 2: Additional requirements for tests.(Edition 3) This part was proposed by the China Electrical Equipment Industrial Association. This part is under the jurisdiction of National Technical Committee on Insulating Materials of Standardization Administration of China (SAC/TC 301). The previous editions of this part are as follows: ——GB/T 1408.2-2006. Insulating materials - Test methods for electric strength - Part 2: Additional requirements for tests using direct voltage 1 Scope This part of GB/T 1408 gives requirements additional to those in GB/T 1408.1 for the determination of the electric strength of solid insulating materials under direct voltage stress. 2 Normative references The following referenced documents are indispensable for the application of this document. For dated references, only the edition cited applies. For undated reference, the latest edition of the referenced documents (including any amendments) applies. GB/T 1408.1-2016 Insulating materials - Test methods for electric strength - Part 1: Test at power frequencies (IEC 60243-1:2013, IDT) 3 Terms and definitions For the purposes of this document, the terms and definitions given in GB/T 1408.1-2016 apply. 4 Significance of the test In addition to the requirements of Clause 4 of GB/T 1408.1-2016, the following points shall be considered when using direct-voltage tests. For a non-homogeneous test specimen, with alternating voltage, the distribution of voltage stress within the test specimen is determined by impedance (largely capacitive). With an increasing direct voltage, the voltage distribution may still be largely capacitive but depends partly on the rate of voltage increase. The resistive voltage distribution, after constant voltage application, represents the steady-state condition. The choice between direct or alternating voltage depends upon the purpose for which the breakdown test is to be used and, to some extent, on the intended application of the material. Upon direct voltage application, the following currents result: the capacitive current, the electric absorption current, the leakage current and, in some cases, partial discharge currents. In addition, for materials with dissimilar layers or discontinuities, the voltage distribution across the test specimen is also influenced, as a result of interfacial polarization, by charges of opposite polarity, which may accumulate on the two sides of the interface and create local fields sufficiently strong to produce partial discharges and/or breakdown of the test specimens. For most materials, the d.c. breakdown voltage is higher than the peak value of the power-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. breakdown voltage or even more. 5 Electrodes and specimens Clause 5 of GB/T 1408.1-2016 is applicable. 6 Conditioning before tests Clause 6 of GB/T 1408.1-2016 is applicable. 7 Surrounding medium Clause 7 of GB/T 1408.1-2016 is applicable. 8 Electrical apparatus 8.1 Voltage source The test voltage applied to the electrodes shall be provided by a power supply having the following characteristics and components. A choice of voltage of either positive or negative polarity shall be provided, one of the connections to the electrodes being earthed. The voltage ripple on the test voltage shall not exceed 2 % of the voltage at all values greater than 50 % of the breakdown voltage. The test voltage shall also be free from transients or other fluctuations exceeding 1 % of the applied voltage. When testing test specimens of low capacitance, it may be necessary to add a suitable capacitor (for example, 1000 pF) in parallel with the electrodes in order to reduce the influence of transients in initiating premature breakdowns. The controls on voltage shall be capable of varying the test voltage smoothly and uniformly between zero and the maximum test voltage and with the requested rate of voltage rise. The rate of voltage rise shall be controllable within ±20 % of the specified rate. Steps in voltage rise shall not exceed 2 % of the expected breakdown voltage value. Controls which automatically increase the voltage at a selectable rate are preferable. A current-sensitive interrupting device shall be used for switching off the direct voltage source. For many materials, a dangerous voltage may persist across the test specimen for a considerable time after the d.c. test voltage is removed. Disconnection of the power supply to the direct voltage source does not necessarily result in the output voltage or the voltage at the electrodes being reduced to zero. For this reason, it is essential that the electrodes be short-circuited and connected to earth for a period equal to minimum twice the total charging time to ensure that the charge is dissipated. For some large test specimens it will be necessary to maintain the shorted condition for 1 h or even longer. A current-limiting resistance shall be used in series with the test specimen to prevent damage to the high-voltage supply and to limit as far as possible the damage to the electrodes on the test specimen when breakdown takes place. The maximum current permitted will depend on the material under test and on the amount of damage to the electrodes which can be tolerated. Note: The use of a very high-valued resistor may result in breakdown voltages which are higher than those obtained with a lower-valued resistor. When making tests where the value or increase in the value of the current is used as a criterion for breakdown, means of measurement of current through the test specimen shall be provided. 8.2 Voltage measurement The measurement of applied voltage shall be made across the electrodes. The other requirements of Clause 8 of GB/T 1408.1-2016 shall be met.

Contents of GB/T 1408.2-2016

Foreword I 1 Scope 2 Normative references 3 Terms and definitions 4 Significance of the test 5 Electrodes and specimens 6 Conditioning before tests 7 Surrounding medium 8 Electrical apparatus 9 Procedure 10 Mode of increase of voltage 11 Criterion of breakdown 12 Number of tests 13 Report Bibliography