Technical data

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Appendix

Insulation Resistance Tests

When circuit breakers are subjected to severe operating conditions, an insulation resistance test

may be performed as indicated in NEMA standard publication No. AB4-1991. An insulation

resistance test is used to determine the quality of the insulation between phases and phase-to-

ground. The resistance test is made at a dc voltage higher than the rated voltage to determine the

actual resistance of the insulation. The most common method employs a "megger" type instrument.

A 1000-volt instrument will provide a more reliable test because it is capable of detecting tracking on

insulated surfaces. Investigate resistance values below 1 megohm. The insulation test shall be

made:

n Between line and load terminals of individual poles with the circuit breaker contacts open

n Between adjacent poles and from poles to the metallic supporting structure with the circuit

breaker contacts closed

This test may be made with the circuit breaker in place after the line and load conductors have been

removed, or with the circuit breaker bolted to a metallic base which simulates the in-service

mounting.

Electrical Tests

These tests require equipment for pole resistance measurement and for overcurrent and

instantaneous tripping in accordance with NEMA Standard publication No. AB4. They are not within

the scope of normal field operation.