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.
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