RESIDUAL CURRENT DEVICES BASICS AND TUTORIALS

In North America, getting an occasional shock by the 120-volt household mains supply is almost a rite of passage. In Europe, where the mains supply is 230V or 240V, getting “bit” by the mains supply might lead to your last rites.

The higher voltage is much more dangerous because it produces more current given the same impedance.

In some parts of Europe, the situation is exacerbated by the fact that the utility companies use a T-T (terra-terra) earthing system whereby the electrical service is grounded at the service entrance or utility pole and at the point of consumption as well.

The ground fault return path is taken to be the earth, and if it happened to be a less than ideal conductor, then so be it. The problem is that if the impedance of the return path for fault currents is high enough, then the current is proportionately lower.

Since the circuit breakers that are supposed to protect the circuit from large short circuit currents have an inverse-time relationship with the current — the larger the current, the faster they act — they will not act as quickly as they would if the grounding conductor or circuit protective conductor were used to create a low-impedance path to the source.

Thus, more damage can occur and personnel are at greater risk. Add to that the smaller, higher impedance wires used there because of the higher voltage and lower currents, and you have a recipe for mishap.

But the Germans, being the clever people they are, invented a solution to help curb the risk. Their earliest solution was to build high-precision “Swiss watch” 4X breakers. Whereas the typical circuit breaker required 7.5 to 20 times the rated current in order to trip instantaneously, the 4X breaker would trip instantaneously at four times its rated current.

These breakers improved the situation but didn’t completely resolve the problem. Their second pass produced a new type of device that would detect ground faults of as little as 500 milliamps. Later, the sensitivity would improve to trip at 100 milliamps, and then improve again to trip at 30 milliamps.

These devices use a donut-shaped current transformer through which all of the current-carrying conductors are run. If the vectorial sum of the outgoing current and the return current is equal to zero, then no control voltage is generated because the magnetic fields of the currents would cancel.

However, in the event of a ground fault, not all of the outgoing current would be returned through the current transformer, thus signaling a problem. The voltage created by the current transformer would be used to trigger the circuit breaker to open.

These so-called residual current devices, or RCDs, helped resolve the problems with ground faults in T-T systems.

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