Instrumentation • Electronics
Caution – leakage currents!
Herbert Blum investigates leakage currents in fault-current protected environments.
T
o provide for protection of personnel, residual current operated circuit breakers are seeing increased use in electrical installations. These often trip unnecessarily due to leakage currents caused by electrical systems.
The result is machine downtime that, however, can
be prevented - with knowledge about leakage currents and targeted countermeasures to ensure efficient yet safe operation. Because power-line filters are significant causes of ground currents, they deserve special attention.
But these are theoretical values, which can deviate due to unsymmetrical loading or a higher frequency (> 50 Hz). Thus, it is advisable to measure current to ground with filters installed and in operation. Many frequency inverters are delivered with integrated
filters (footprint filters). These are generally simple filters with small chokes and large capacitors between the phase conductors and ground that cause large leakage currents.
Filtering effect
The filtering effect of the large Y-capacitors can generally be replaced only with larger inductances. For example, a one- stage filter with large Y-capacitors must be replaced with a two-stage filter with two chokes. Often there is also an EMC statement of conformity for
the enclosed filters. This, however, is valid only for an ideal installation and short motor cables. Longer motor cables require a new EMC measurement.
They also generate a larger capacitance to ground, which can result in larger leakage currents. These additional asymmetrical currents can lead to the magnetic saturation of the filter chokes. As a result, the filter loses a large part of its effectiveness and the system exceeds the permitted EMC limits. A remedy can be provided with shorter cables or an
output filter. This sine-wave filter should be inserted directly at the inverter’s output. It effectively attenuates leakage currents above 1 kHz by reducing the slew rate of the motor voltage. If multiple inverters are used in a system, it can be
Fig. 1. RCDs disconnect the circuit and thus prevent accidents. (source: SCHURTER AG)
If a person directly touches a live conductor, the fault current flows to ground. An upstream RCD (residual current protective device) detects this fault current and immediately disconnects the circuit. But a RCD cannot distinguish between normal residual
currents and dangerous fault currents. Leakage currents can thus trigger a RCD.
Leakage currents in filters
In EMC filters, capacitors from all conductors are wired to ground. Current is continually flowing through each of these Y-capacitors. In an ideal three-phase power network with sinusoidal voltages, the sum of all these currents is zero. In practice, however, there is a continuous leakage current
to ground due to strong distortion in the grid voltage. Most filter manufacturers specify the maximum expected leakage current so that it is easier to select a suitable filter.
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Fig. 2. SCHURTER sine-wave filters series FMAC SINE. (source: SCHURTER AG)
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