Application planning
6.5 Use in IT networks
3VL molded-case circuit breakers
72
System Manual, 03/2009, 110 0110 - 02 DS 01
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
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Figure 6-2
Ground and housing fault
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
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Страница 350: ...Correction sheet 3VL molded case circuit breakers 350 System Manual 03 2009 110 0110 02 DS 01 ...
Страница 358: ...Glossary 3VL molded case circuit breakers 358 System Manual 03 2009 110 0110 02 DS 01 ...
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