Siemens SENTRON - Back-Up Protection

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5.4 Back-up protection
Short circuit and residual currents can be up to several hundred amperes, depen-
ding on the network and system conguration. Thus, for instance, in the event of
an insulation fault to the grounded exposed conductive parts of electrical equip-
ment with a correspondingly low resistance, a short circuit-like current ows via
the residual current protective device. The contacts that open in this event must
be able to withstand this stress.
This requires a correspondingly high rated switching capacity. The rated switching
capacity Im of residual current operated circuit breakers shall, according to the
applicable device standards (EN 61008-1/VDE 0664-10) be at least 500 A or
10 * I
n
, whichever is the higher. The rated switching capacity of our residual current
operated circuit breakers is predominately 800 A and hence clearly exceeds the
minimum requirements of 500 A or 630 A for devices of up to 63 A rated current.
In accordance with the installation regulations (VDE 0100-410; DIN VDE 0100-530),
residual current protective devices can be installed in all system types (TT, NT
and IT). Residual currents that are in excess of the rated switching capacity of the
residual current operated circuit breaker can occur alongside short-circuit currents
in a TN system in particular. An overcurrent device shall be connected as a back-up
protective device to restrict the energy and strength of the short-circuit current to
compatible levels.
The conditional rated short-circuit current is stated on the residual current
operated circuit breaker in the form of a symbol in conjunction with a maximum
permissible rated current for the fuse (gG operational class melting fuses).
This will be explained based on the following example:
The conditional rated short-circuit current of the residual current operated circuit
breaker where a fuse rated at a maximum of 100 A is used is 10 kA.
In cases in which no rated fuse current is stated, a minimum value of 63 A applies
automatically in Germany. A minimum value of 6 kA is required for the conditional
rated short-circuit current.
100
10.000