08/07 AWB1230-1407GB 5 – 5
5.1.6 Use in IT systems
5.1.7 Regulations
In IEC 60947-2 or EN 60947-2 “Low voltage switchgear Part 2:
circuit-breakers” for the use of circuit-breakers in an unearthed or
impedance earthed network (IT systems) an extra test to IEC
60947-2 Appendix H is required.
Subsequently the tests with 1.2 times the highest setting of the
short time delayed overcurrent trip (S trip) or the undelayed
overcurrent trip ( I trip) when no S trip is available, as single pole
short-circuit switch-off capacity I
IT
are to be verified. This is for a
maximum of 50 kA. The tests are to carried out with the phase
voltages of the highest rated operating voltage Ue for use in the
network.
With this the worst case fault that could occur in the IT system is
covered, with a double earth fault on the load and incoming sides
See following illustration:
햲 Fault 1
햳 Fault 2
햴 Frame
햵 Impedance
햶 Transformer
Explanation:
– After fault 1 fault 2 then occurs.
– With that there is then a double earth fault on the load and
incoming sides.
– On the main contacts in phase L1 is then the full phase voltage of
e.g. 690 V.
– At the same time the contact must carry a high short-circuit
current.
L2
L1
L3
d
b
c
a
e
690 V AC
690 V
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