A 1472, A1579 Leakage current clamp
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4.1.4 Problems caused by leakage currents
Too high leakage currents can cause very critical problems in electrical
installations. The most often problems are nuisance tripping of protective
devices and safety problems due to increased fault, earth and touch
voltages.
Nuisance tripping of protective devices
Nuisance tripping occurs if the sum of leakage currents exceeds the
tripping level of the residual current protective device.
In circuits with a lot of installed equipment the sum of leakage currents in
normal operation can be close to the trip out limit. In this case even a small
added leakage current (caused by switching on/off loads or connection of a
new equipment) can trigger the nuisance tripping.
It must be considered that leakage currents can vary in time and the
tripping could be only occasional.
A solution is to add RCD(s) and divide the circuits in a way that the leakage
currents in normal operation stay below 20% of the nominal value of each
RCD. For further information refer to
METREL handbook Guide for testing
and verification of low voltage installations
.
Problems because of non- typical shape of leakage current
Leakage currents can have different non-sinusoidal shapes. They are
caused by nonlinearity of loads, nonlinearity of insulation materials and
presence of harmonic voltages and currents in the power systems:
high frequency leakage current
d.c. pulsed and smooth d.c. leakage current
arcing leakage current
harmonic leakage currents
transient and other leakage currents of short duration
The amount of the mentioned leakage currents in power systems is
increasing because more and more electronic controlled devices are
connected to the power system.
It must be considered that standard protection devices and measuring
equipment is not suited for monitoring and measuring of the mentioned
currents.
Impact on protection devices
Atypical shapes of leakage current can blind standard RCDs. For instance
general type RCD’s are not sensitive to leakage currents with pulsed DC
components or smooth DC current. High frequency leakage currents
caused are not dangerous for human safety but can result in wrong
operation of the RCD. It could happen that a dangerous fault leakage occur
at the same time and is not detected by the RCD.
Impact on measuring equipment
It is preferred to use at least TRMS measuring equipment for measuring
leakage currents. For more demanding applications instruments that
include harmonic analysis, time diagrams and DC sensitive measuring
equipment must be used.
