Functions
5.1 Current protection
3VL molded-case circuit breakers
System Manual, 03/2009, 110 0110 - 02 DS 01
51
5.1.6 Differential current protection with RCD module
The circuit breakers with differential current protection are used in a variety of ways to implement a
double function:
● Protection of systems against overload and short-circuit currents.
● Protection of cables and electrical equipment against damage from ground faults.
The SENTRON VL RCD modules are supplied as accessories for the VL160X, VL160,
VL250 and VL400 circuit breakers with thermomagnetic overcurrent trip units. This
combination is referred to a circuit breaker with differential current protection of type A. Type
A means tripping is guaranteed both in the case of faults in sinusoidal alternating currents
and in the case of faults in pulsating direct currents. These units have an adjustable trip time
delay ∆t. The values for the rated fault current I
∆n
can also be adjusted.
In a fault-free system, the sum of the currents in the summation current converter of the RCD
module is zero. A ground fault current occurring in the protected circuit as the result of an
insulation fault produces a differential current that induces a voltage in the secondary
winding of the current transformer. The evaluation electronics monitor the induced voltage
and transmit a trip command to the RCD trip unit if the trip criterion is met. The combination
of circuit breaker with differential current protection is designed in such a way as to cause
the circuit breaker contacts to open if the differential current reaches a given value.
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