Siemens SENTRON 3VL System Manual page 72

Application planning
6.5 Use in IT networks
Fault situation
The most critical fault for circuit breakers in ungrounded IT networks is a double ground fault
on the incoming or load side of the circuit breaker. If this fault occurs, the entire phase-to-
phase voltage is applied via one pole of the circuit breaker.
Illustration of a double ground fault
Figure 6-2
Explanation of the illustration
Faults ① and ② simultaneously:
● Double ground fault on the load and incoming side
● Single-pole short-circuit, the full phase-to-phase voltage of 690 V is applied to main
contact L1
● Selection of the circuit breaker according to their suitability as defined in IEC 60947-2,
Annex H
● No calculation of the IT value possible or necessary
Fault ③
● 2 or 3-pole short-circuit
● Multi-pole short-circuit, a voltage of 690 V / 1.73 = 400 V is applied at the main contacts
● The circuit breaker is dimensioned as in other networks
72
I
Ground and housing fault
3VL molded-case circuit breakers
System Manual, 03/2009, 110 0110 - 02 DS 01