# 4.5.1 Hazards, Fuses & Earthing

### Hazards of Electricity

• Mains electricity is potentially lethal – potential differences as small as 50 volts can pose a serious hazard to individuals

Signs, like the above, warn of the risk of electrocution

• Common hazards include:
• Damaged Insulation – if someone touches an exposed piece of wire, they could be subjected to a lethal shock
• Overheating of cables – Passing too much current through too small a wire (or leaving a long length of wire tightly coiled) can lead to the wire overheating. This could cause a fire or melt the insulations, exposing live wires
• Damp conditions – If moisture comes into contact with live wires, the moisture could conduct electricity either causing a short circuit within a device (which could cause a fire) or posing an electrocution risk

### Fuses

• A fuse is a safety device designed to cut off the flow of electricity to an appliance if the current becomes too large (due to a fault or a surge)

The circuit symbol for a fuse – take care not to confuse this with a resistor

• Fuses usually consist of a glass cylinder which contains a thin metal wire.
• If the current in the wire becomes too large:
• The wire heats up and melts
• This causes the wire to break, breaking the circuit and stopping the current
• Fuses come in a variety of sizes (typically 3A, 5A and 13A) – in order to select the right fuse for the job, you need to know how much current an appliance needs
• If you know the power of the appliance (along with mains voltage), the current can be calculated using the equation:

• The fuse should always have a current rating that is higher than the current needed by the appliance, without being too high – always choose the next size up

Example:

• Suppose an appliance uses 3.1 amps
• A 3 amp use would be too small – the fuse would blow as soon as the appliance was switched on
• A 13 amp fuse would be too large – it would allow an extra 10 amps to pass through the appliance before it finally blew
• A 5 amp fuse would be an appropriate choice, as it is the next size up

### Earthing

• Many electrical appliances have metal cases
• This poses a potential safety hazard:
• If a live wire (inside the appliance) came into contact with the case, the case would become electrified and anyone who touched in would risk electrocution
• The earth wire is an additional safety wire that can reduce this risk

A diagram showing the three wires going to a mains powered appliance: live, neutral and earth

• If this happens:
• The earth wire provides a low resistance path to the earth
• His causes a surge of current in the earth wire and hence also in the live wire
• The high current through the fuse causes it to melt and break
• This cuts off the supply of electricity to the appliance, making it safe

### Author: Jenna

Jenna studied at Cardiff University before training to become a science teacher at the University of Bath specialising in Biology (although she loves teaching all three sciences at GCSE level!). Teaching is her passion, and with 10 years experience teaching across a wide range of specifications – from GCSE and A Level Biology in the UK to IGCSE and IB Biology internationally – she knows what is required to pass those Biology exams.
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