Domestic Electrical Safety

Mains plugs, fuses and the earth wire

Irish houses use a 230 V, 50 Hz supply and the three-pin Type G plug. Each wire has a job.

The fuse is fitted on the live side of the plug. It is a thin wire that melts if the current is too large, so the circuit breaks. Common values are 3 A, 5 A and 13 A, and 13 A is the largest fuse fitted in a plug.

Choose the fuse just above the normal current, found from $I = P/V$. A fuse far too large would not blow until the wiring is already overheating.

The earth wire connects the metal casing of an appliance to earth. Normally it carries no current. If the live wire touches the casing, a large current flows through the earth wire, which blows the fuse and disconnects the appliance. Without it, the casing would sit at 230 V and you would become the path to earth.

Choosing a fuse and the fault current

When the live wire touches the earthed casing, the current returns through the earth wire, which has very low resistance. By $I = V/R$, the current is very large, far above 13 A. The fuse melts in a fraction of a second.

The same idea applies to an MCB (miniature circuit breaker) in the fuse board. It is a switch that trips automatically when the current is too high, and can be reset. A quick trip needs a large fault current, which needs a low earth fault loop resistance.

The RCD: protecting people

A fuse needs a current of many amps. But a current of only about 30 mA through the body can be dangerous. A fuse or MCB will not react to it.

An RCD (residual current device) compares the current in the live wire with the current in the neutral wire. Normally they are equal. If some current leaks to earth, perhaps through a person, the neutral current is smaller. The difference is the leakage current.

Inside the RCD, live and neutral pass through a toroidal (ring-shaped) iron core. When the currents are equal, their magnetic effects cancel and the flux in the core is zero. A leakage leaves a net changing flux, which induces an emf in a sense coil wound on the core. That signal operates a switch which trips in about 30 ms. Household RCDs trip at a leakage of 30 mA.

Every RCD has a test button. It deliberately creates a small leakage so you can check that the device still trips. Mechanical parts and electronics can fail, so households should test it regularly.

The missing current flows to earth by another path, through the earth wire, or through a person.

Current through the body

Treat the body as a resistor. Dry skin has a high resistance, but wet skin can have a few thousand ohms or less.

The energy grows in proportion to time, so a hundredfold longer exposure gives a hundredfold more energy (about 105 J). This is why fast tripping is crucial.

Why a house needs both

Think of a washing machine with a metal case.

• Earth wire + fuse or MCB: a live wire touching the casing sends a large current through the earth wire. The fuse blows and the casing is made safe.

• Direct contact with live: if a person touches a bare live part, the current goes through the person. As in the example above (a person of resistance $1{,}500\ \Omega$ at 230 V), it may be only about 150 mA, too small for a 13 A fuse. The earth wire is not involved.

• RCD: it detects the leakage, 30 mA or more, and cuts the supply in about 30 ms.

So the earth wire and fuse protect against fault currents in the appliance, and the RCD protects people. They cover different dangers, so we use both.

Back to the kettle: with the casing earthed, the loose live wire sends a large current down the earth wire, the fuse blows and the casing is no longer live. The RCD adds protection if a person is ever part of the path.

Summary

• Irish mains: 230 V, 50 Hz; Type G plug with live (brown), neutral (blue) and earth (green and yellow) wires.

• The fuse (in the live wire) is chosen just above the normal current $I = P/V$; it melts if the current is too high.

• The earth wire gives a low-resistance path to earth, so a live-to-casing fault gives a large current that blows the fuse or trips the MCB. Low earth loop resistance means faster tripping.

• An RCD compares live and neutral currents using a toroidal core and sense coil; it trips at a leakage of 30 mA in about 30 ms. The test button checks it works.

• The body behaves as a resistor: $I = V/R$, $W = I^2Rt$. Short exposure time is crucial.

• Earth wire and fuse protect the appliance; the RCD protects people. A house uses both.