Fault Situation - Siemens 3VA1 100 A Manual

3.5.2

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
②
③
R
A
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
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.
3VA molded case circuit breakers
Manual, 04/2015, A5E03603177010-02
Ungrounded transformer
Molded case circuit breaker
Exposed conductive part
Contact resistance of exposed conductive part ground
3.5 IT system applications of 3VA molded case circuit breakers
/I
cu cs
at 690 V.
Applications
139