IEC 60071-1:2011 pdf free download – Insulation co-ordination
5.11.3Standard rated lightning impulse withstand voltage
In Table 3, standard rated lightning impulse withstand voltages associated with each standard ratedswitching impulse withstand voltage have been chosen in consideration of the following:
a) for equipment protected by close surge arresters, the low values of lighting impulse withstand level are applicable.They are chosen by taking into account the ratio of lightning impulseprotective level to switching impulse protective level likely to be achieved with surgearresters, and by adding appropriate margins;
b) for equipment not protected by surge arresters (or not effectively protected)，only the higher values of lightning impulse withstand voltages shall be used.These higher values are basedon the typical ratio of the lightning and switching impulse withstand voltages of the externalinsulation of apparatus (e.g. circuit-breakers，disconnectors，instrument transformers，etc.).They are chosen in such a way that the insulation design will be determined mainly by theability of the external insulation to withstand the switching impulse test voltages;
c) in a few extreme cases,provision should be made for a higher value of lightning impulse withstand voltage.This higher value shall be chosen from the series of standard values givenin 5.6 and 5.7.
6.1General requirements
Standard withstand voltage tests are performed to demonstrate,with suitable confidence, that theactual withstand voltage of the insulation is not lower than the corresponding specified withstandvoltage.The voltages applied in withstand voltage tests are standard rated withstand voltagesunless otherwise specified by the relevant apparatus committees.
In general，withstand voltage tests consist of dry tests performed in a standard situation (testarrangement specified by the relevant Apparatus Committees and the standard reference atmosphericconditions).However，for non-weather protected external insulation，the standard short-durationpower-frequency and switching impulse withstand voltage tests consist of wet tests performed underthe conditions specified in lEC6oo6o-1.
During wet tests, the rain shall be applied simultaneously on all air and surface insulationunder voltage.
lf the atmospheric conditions in the test laboratory differ from the standard referenceatmospheric conditions, the test voltages shall be corrected according to lEC 60060-1.
All impulse withstand voltages shall be verified for both polarities, unless the relevant apparatuscommittees specify one polarity only.
When it has been demonstrated that one condition (dry or wet) or one polarity or a combinationof these produces the lowest withstand voltage, then it is sufficient to verify the withstand voltagefor this particular condition.
The insulation failures that occur during the test are the basis for the acceptance or rejection ofthe test specimen. The relevant apparatus committees or technical committee 42 shall definethe occurrence of a failure and the method to detect it.
When the standard rated withstand voltage of phase-to-phase (or longitudinal) insulation is equalto that of phase-to-earth insulation，it is recommended that phase-to-phase (or longitudinal)insulation tests and phase-to-earth tests be performed together by connecting one of the twophase terminals to earth.
6.2Standard short-duration power-frequency withstand voltage tests
A standard short-duration power-frequency withstand voltage test consists of one application ofthe relevant standard rated withstand voltage to the terminals of the insulation configuration.
Unless otherwise specified by the relevant apparatus committees, the insulation is considered tohave passed the test if no disruptive discharge occurs. However,if one disruptive dischargeoccurs on the self-restoring insulation during a wet test, the test may be repeated once and theequipment is considered to have passed the test if no further disruptive discharge occurs.
When the test cannot be performed (such as for transformers with non-uniform insulation), therelevant apparatus committees may specify frequencies up to few hundred hertz and durationsshorter than 1 min.Unless otherwise justified, the test voltages shall be the same.