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Fault situation

The most critical fault for molded case circuit breakers in ungrounded IT systems is a double-phase-to-ground fault on the infeed and load ends of the molded case circuit breaker. If this fault occurs, the entire phase-to-phase voltage is applied across one pole of the molded case circuit breaker.

Two faults on the load side - double-phase-to-ground fault

One fault on the line side and one fault on the load side (double-phase-to-ground fault)

Ungrounded transformer

Molded case circuit breaker

Exposed conductive part

RA

Contact resistance of exposed conductive part ground

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