Beckhoff SCT Series, Documentation

Page: 37 / 41

Application example
SCT5xxx 37 Version: 1.0
system-related leakage currents are mainly capacitive. However, the RCD cannot distinguish between the
different leakage currents. Therefore, it can already trip when the sum of all system-related leakage currents
is above the tripping threshold. This is also possible during normal operation.
Fig. 25: Electrical equivalent circuit diagram of a speed-controlled motor
As shown in the figure, different frequency components can occur in the differential current from DC up to
several kHz. When analyzing the measured differential current, the system-related differential current must
always be considered as well, because this is present despite perfect insulation and cannot be technically
separated. Also, due to inductances (e.g. motor), high current peaks can be generated during the switch-on
processes, which can lead to relay tripping at the RCDs and RCMs.
In general, the different frequency components can be interpreted as follows:
Frequency Typical causes
50 Hz Power supply, mains filter (low-pass), interference suppression capacitors
50 Hz + 150 Hz Single-phase frequency converter with internal EMC filter
150 - 1050 Hz Three-phase frequency converter with internal EMC filter
2k - 50k Hz Long shielded motor cable
50k - 150k Hz Insufficient EMC filter
2k - 150k Hz Long shielded motor cable + insufficient EMC filter
When commissioning a differential current monitor, it is important to know the actual system-related leakage
current. Only in this way can appropriate warning thresholds and relay tripping thresholds be sensibly set.