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Lightning Impulse withstand test is a destructive test carried out on high voltage switchgear. It is classed as a destructive test as each shot damages the insulation and lays carbon tracks.

The Energy Networks Association Technical Specification (ENA-TS) 41-36 states the BIL (basic Impulse level) Required for the Voltage rating of the piece of switchgear rated up to 36kV.

For example an 11kV switchgear will have a BIL of 95kV.

Switchgear which include a Vacuum circuit breaker must have a second means of isolation, ie a disconnector. This disconnector can be a two position (mains on, off) or three position (mains on, off, or earth on) disconnector operated off load which is in series with the Circuit breaker.

To pass the test 15 shots at +95kV and 15 shots at -95kV are fired at all positions of the disconnector and only 2 out of the 15 (and not the last 5) shots can flash over to earth.

For example if the disconnector was in the off position one side will be connected to the impulse generator and the other side will be earthed. Positive and Negative shots will be fired and the impulse can not flash through the air gap to the earthed side of the disconnector.

This proves the isolation gap inside the disconnector has a BIL rating of 95kV.

Therefore if the incoming side of a switchboard is live and the outgoing side is isolated for maintenance. There can be a transient impulse on the live side of the circuit and it will not flash across the isolating gap and endanger anyone working on the dead side of the circuit

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