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
A fault develops between two phases and an exposed conductive part or ground.
A voltage of 690 V is applied at the main contacts. This voltage is disconnected by two breaker poles.
The molded case circuit breaker is rated for Icu/Ics at 690 V.
One fault on the line side and one fault on the load side (double-phase-to-ground fault)
Single-pole short circuit, the full phase-to-phase voltage of 690 V is applied to main contact L3.
Selection of the molded case circuit breaker according to their suitability as defined in IEC 60947-2, Annex H.