Continuous residual current monitoring can already detect and signal faults before
the protective device responds. Sudden system disconnection can frequently be
avoided this way. For that reason, residual current monitoring devices are prima-
rily used in systems in which a signal, but not a disconnection, should be carried
out in the event of a fault. In addition, the residual current monitoring counts as a
preventative maintenance measure in electrical systems.
The residual current monitoring devices function the same way as residual current
protective devices do. The summation current transformer covers all conductors
needed to carry current, i.e. the neutral conductor as well, if necessary. In a
fault-free system, the magnetizing effects of the current-carrying conductors are
cancelled out for the transformer, and the sum of all currents is equivalent to zero
(Kirchhoff‘s 1st law of circuits). A residual magnetic eld will only remain in the
transformer core if a residual current is owing as a result of an insulation fault
and is generating voltage. The voltage will be evaluated by the RCM electronics
and the switched contact can be used to control such elements as an acoustic/op-
tical signaling device, a higher-level control system or a circuit breaker. RCMs are
not equipped with a direct shut-off function.
RCMs are widely available with both integral current transformers and external
summation current transformers. External summation current transformers are
available with different inside diameters. This even makes it possible to moni-
tor installations with rated currents of several hundred amperes. Furthermore,
residual current monitoring devices are also associated with adjustable values
for response residual current, response time and, if applicable, the display of the
current residual current value.
The ability to adjust the response residual current and time behavior constitutes
a signicant advantage of the RCMs. This enables system-specic adjustment and
makes it possible to take leakage currents which are permanently present into
account. These leakage currents are caused by capacities in the cables and lines
or in electrical equipment, for instance.
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