Gfci
Actually it is a GFCI (ground fault circuit interrupter). There are three connections in the circuit: power, return, ground.
During normal operation all of the current flows between power and return. A GFCI measures the current on the power feed and the return. If there is an imbalance, the breaker flips. Essentially, it means that the current is going somewhere else to ultimately get to ground (potentially through you). A difference of about 6 ma will trip a GFCI...note that your normal breaker is 15 amps or 30 amps or more of current delivered across the power/return circuit.
A current of 60 ma or higher (at 120v, 60 hz) can cause ventricular fibrillation. This is measured based upon the touch point being a dry hand. If you are swimming, your body impedence is actually much lower and ventricular fibrillation can occur at a much lower current.
So, the bottom line is that everything should be GFCI protected and the GFCIs should be tested to verify that they are working correctly.
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