Mains electricity hazards

Mains electricity can cause electric shock if current passes through a person. It can also cause fire if cables or connections become hot enough to ignite surrounding materials.

Damaged insulation and overheating

The insulation around a cable normally prevents contact with its conducting wires. If the insulation is damaged, a live conductor can be exposed. Touching it can provide a path for current through the body, causing an electric shock.

Damaged insulation can also allow conductors to touch, causing a short circuit. A large current can then flow and heat the cable or its surroundings, creating a fire hazard. Cables that overheat can damage their own insulation and start fires, even if the insulation was not damaged initially.

Damp conditions

Water containing dissolved substances can conduct current. In damp conditions, water can provide a conducting path through a person or between a live part and a person, increasing the risk of electric shock.

Overloading plugs and sockets

Connecting several appliances to a plug, extension lead, single socket or multiple socket can cause too much total current to flow through its cables and connections. This is called overloading. The excessive current heats the cables and connections; if they become hot enough, they can cause a fire.

Mains wiring and live-wire switching

A mains circuit connects an appliance to a supply using three wires. During normal operation, current flows through the appliance between the live and neutral wires. The earth wire provides a separate safety connection.

• The live wire, also called the line wire, carries an alternating potential relative to earth.
• The neutral wire provides the return path for current during normal operation.
• The earth wire is a safety connection and normally carries no current.

Switching the live wire

A switch must be connected in the live wire. When the switch is opened, it breaks the connection between the appliance and the live supply.

If a switch is placed only in the neutral wire, opening it breaks the normal current path but leaves the appliance connected to the live wire. Parts of the appliance can therefore remain at a live potential and present an electric shock hazard.

Fuses

A fuse protects a circuit from excessive current. It contains a thin wire that heats up as current passes through it. If the current is excessive for long enough, the wire melts and breaks the circuit, stopping the current.

The fuse is then described as blown. Its wire cannot carry current again, so the fuse must be replaced before the circuit can operate. The replacement must have a suitable rating.

Choosing a fuse rating

A fuse’s current rating should be above the appliance’s normal operating current, so that the fuse does not blow during normal use. It must also be low enough to protect the circuit and its cabling from excessive current.

When the appliance’s power and supply voltage are known, its normal current can be calculated using:

Here, $$I$$ is current in amperes, $$P$$ is power in watts and $$V$$ is voltage in volts.

For example, an appliance rated at $$460\ \text{W}$$ on a $$230\ \text{V}$$ supply has a normal current of $$2\ \text{A}$$. A $$3\ \text{A}$$ fuse is above this operating current and could be suitable, provided it is low enough to protect the circuit and its cabling.

Fuses and double insulation

A fuse does not need an earth connection to operate: excessive current through its wire heats and melts it. A fuse can therefore protect the circuit and cabling of a double-insulated appliance, even though the appliance has no earth wire.

Trip switches

A trip switch protects a circuit by automatically opening it when the current exceeds a set value. Opening the circuit interrupts the current, reducing the risk that the wiring will overheat and cause a fire.

After the cause of the excessive current has been dealt with, the trip switch can be reset to close the circuit again. Unlike a fuse, it does not need to be replaced after it operates.

Choosing a trip-switch setting

The setting must be high enough to allow the normal operating current, but low enough to protect the circuit and its cables from excessive current. If the setting is too low, the switch may trip during normal operation. If it is too high, it may not protect the cables adequately.

When the power and supply voltage are known, the normal current can be calculated using:

Here, $$I$$ is current in amperes (A), $$P$$ is power in watts (W), and $$V$$ is supply voltage in volts (V).

For example, equipment using $$1200\ \text{W}$$ from a $$240\ \text{V}$$ supply has a normal current of $$1200/240 = 5\ \text{A}$$. A $$6\ \text{A}$$ setting would allow this current, provided it is low enough to protect the circuit and its cables.

Appliance casing protection

An appliance fault must not leave a surface that a person can touch dangerously live. Two ways to protect the user are to insulate accessible surfaces from live parts or to connect a conducting casing to earth.

Double insulation

A double-insulated appliance uses non-conducting insulation to separate its accessible surfaces from live electrical parts. This prevents those surfaces from becoming live, so the appliance does not require an earth wire.

Earthing a conducting casing

An appliance with a conducting outer casing can have the casing connected to earth. If a fault causes the live wire to touch the casing, the earth connection provides a low-resistance path for a large fault current.

The large current causes a suitable fuse to melt or a trip switch to operate. Either device disconnects the supply, reducing the risk of electric shock from the casing.