Earth Leakage : Earth leakage takes place when, through some fault in an appliance or its connections. Electric current escapes to earth i.e. the body of the appliance become 'live' and may give severe shocks. A minor earth fault is called as a earth leakage and under these conditions the appliance continues to work and gives mild shocks. However, if there is a major earth fault the loss of current to the earth causes the fuse to blow out if the body of appliance is earthed.

Increasingly, earth leakage circuit breakers (ELCBs), are being used to detect this loss of current as they can be sensitive to a loss of as little as 5 mA.

They can be bought to fit a 16 A socket or as individual components for each equipment or appliance. ELCBs can be too sensitive: for example, if neon indicator lamps are wired from live to earth, the tripping of EL,CB would take place very often. This kind of problem can be solved by rewiring the indicator lamps between live and neutral, though "nuisance tripping" can also be caused by slight moisture e.g. with refrigerators. A typical ELCB circuit is shown in Figure.

Figure: ELCB circuit diagram.

Hazards in Laboratories In the event of a short circuit or leakage occurring, the circuit breaker will cut off the mains power supply and will continue to break the circuit until the fault is rectified.

It can be seen, then, that ELCBs are more sensitive than fuses and are more convenient to use. They are useful for protecting circuits supplying power tools, etc. where the risk of shock is high. However, ELCBs only protect the live conductor; for example, if the live conductor is short-circuited to earth.

ELCB would not prevent a fatal shock if both neutral and live conductors were touched by someone standing on an insulated surface. Now check your understanding of the principles of earthing electrical equipment by answering the following questions. You need only tick the answers you think are correct and compare them with ours.