Siemens SENTRON 3VL series - Page 63

Application planning
5.5 Use in IT systems
3VL IEC molded case circuit breakers
System Manual, 11/2013, 110 0110 - 02 DS 03
63
Fault situation
The most critical fault for molded case circuit breakers in ungrounded IT systems is a double
ground fault on the infeed and load side of the molded case circuit breaker. If this fault
occurs, the entire phase-to-phase voltage is applied via one pole of the molded case circuit
breaker.
Figure 5-2 Double ground fault (ground fault and short-circuit to frame)
Explanation of the illustration
● Faults ① and ② simultaneously:
– Double ground fault on the load and infeed side
– Single-pole short-circuit, the full phase-to-phase voltage of 690 V is applied to main
contact L1
– Selection of the molded case circuit breaker according to their suitability as defined in
IEC 60947-2, Annex H
● Fault
③
– 2 or 3-pole short-circuit
– Multi-pole short-circuit, a voltage of 690 V /V 1.73 = 400 V is applied at the main
contacts
– The design of the molded case circuit breaker is in accordance with I
cu
/I
cs
See also
Standards and specifications (Page 308)