Leakage currents in fault-current protected environments
In addition to fuses and circuit breakers, today there is increasing use of residual current operated circuit breakers (also called RCDs, residual current devices) in electrical systems. Fuses protect electrical systems primarily against short circuits and fires, whereas RCDs provide for reliable protection of operating personnel. They register fault currents flowing to ground, for example caused by defective insulation, and cut them off before anyone can be harmed. The problem is that a RCD cannot distinguish between residual currents that arise in normal operation and those that occur due to dangerous fault currents. Frequency inverters in particular, which are needed for energy efficient operation of motors, cause large residual currents.
In addition, the capacitance of the cables and power-line filters that are necessary to maintain electromagnetic compatibility (EMC) create additional ground currents. The sum of all leakage currents can thus trigger a RCD and shut down all loads on the same cable harness. This results in machine downtime, causes loss of production and thus considerable expenses. However, there are measures for dealing with high leakage currents to ensure efficient yet safe operation.
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