The domestic mains supply is one of three phases generated at the power station. The generator windings are arranged in what is called a "star" configuration. This means that one end of each phase winding is connected together. This common connection is connected to Earth.
So, instead of having six wires from the generator (two for each phase winding) there are four. The common connection, called neutral, and the other three wires, one for each phase. Usually identified as red, yellow and blue.
A domestic mains supply will have two wires coming in from the generating station (or from a switch-yard or sub-station or whatever else it might be called), one wire from a phase and one wire from the neutral. The phase is often called the line, "live" or "hot".
Although a connection to Earth is made at the consumer's premises, it is not necessary for any appliance to function. It will be ignored for the moment.
In normal operation, current from the generator passes through whatever load is switched on and back to the generator. The fact that the current changes direction several tens of times a second (it's a.c.) is irrelevant.
Normal operation is illustrated in these two pictures.
If, because of a fault, the mains supply gets connected to a conducting part of the load, the metal body of a washing machine for example, the appliance becomes "live". In other words there will be a voltage difference (the mains voltage) between the appliance and Earth.
Anyone touching the appliance and standing anywhere, or on anything, with a conductive or partially conductive path to Earth will be subject to an electric shock.
The next two pictures illustrate this.
To minimise the risk from such faults any exposed conductive parts of an appliance are connected to Earth via a third pin on the appliance's plug.
Under a fault condition there can then be no voltage between the appliance's exposed conductive parts and Earth. The fault, however, still exists.
Extra protection is afforded by including in the supply a device which is known by several names. Those I know about are:
GFI ground fault interrupter.
GFCI ground fault circuit interrupter
RCD residual current device
RCCB residual current circuit breaker
ALCI appliance leakage current interrupter
ELCB earth leakage circuit breaker
There is also one called an RCBO which is a combination of an RCD and an MCB (miniature circuit breaker).
It shouldn't be necessary, but experience has taught me to err on the side of prudence so I shall explain what a "miniature circuit breaker" is.
A miniature circuit breaker, or indeed any other 'ordinary' circuit breaker, miniature or not, is simply an electro-mechanical replacement for a fuse. Effectively a re-settable fuse.
I have come across people, people who should have known better, who thought that they gave protection against earth leakage faults. They don't.
A GFI, or whatever other fancy name you know it by, is a differential current detector. It detects any difference between two currents. The current in the supply line, the "hot" lead, and the current in the neutral lead. If a difference of greater than its design figure is detected it then behaves as a circuit breaker and switches off the supply.
The next two pictures hopefully make this clearer.
