High Voltage Circuit BreakerHV AC Switchgear

Dielectric combined voltage tests (BIAS test) on circuit breakers

In this test the circuitbreaker is submitted to two separate voltages at the same time: at one terminal, a power frequency(PF) voltage and at the other terminal, either a switching(SW) or lightning impulse(LI). The test simulates the actual voltage conditions that an open CB may experience.
The peak of the impulse wave coincides with the opposite peak of the PF voltage, and the total voltage between the terminals will be the sum of the two voltages.
For SW bias tests, the PF voltage on the opposite terminal corresponds to the rated (phase-to-ground) voltage of the system. This reflects real conditions, since switching overvoltages often occur around the peak of the PF voltage wave. For LI bias tests, on the other hand, the power PF corresponds to 70% of the rated (phase-to ground) voltage. The reason is that LI overvoltages occur quite randomly in time, and the standards have settled for a compromise between the least and the most severe stresses.
In pic show a ABB highvoltage CB in dielectric bias test:
A:PF applied on terminal A.
B:SW or LI applied on terminal B. Synchronized with the max value of the negative PF voltage.

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ABB application guide

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