Applications
3.5 Use of 3VA UL molded case circuit breakers in IT systems according to IEC 60947-2
3VA molded case circuit breakers with UL and IEC certification
Manual, 01/2019, L1V30435333-01
149
3.5.3
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.
①
Ungrounded transformer
②
Molded case circuit breaker
③
Exposed conductive part
R
A
Contact resistance of exposed conductive part ground
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 I
cu
/I
cs
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.
