Mains electricity part 2: safety

Name the safety devices in mains electricity

• Fuses
• Earth wires
• Insulation or double insulation
• Circuit breakers

What is the structure of a fuse?

It is usually a cylinder or a cartridge which contains a thin piece of wire made of a metal with a low melting point

How does a fuse work?

• When a current that is too large passes through the circuit

• the fuse wire becomes overheats and melts. The fuse 'blows'.

• Breaks the circuit by stopping the current

What is a purpose of a fuse?

Prevent overheating in circuit/current exceeding a value/switch off the circuit

Prevents you getting a shock and reduces the possibility of an electrical fire

Does a fuse have to be replaced with a new one after the fault causing the increase in current is fixed before the appliance can be used again?

Yes

What is the characteristic the new fuse must have?

The new fuse replacing the melted one must be the same size as the previous one

If the current in a circuit is 2A then what size fuse should you use?

One that allows a current that is a bit larger, e.g in this case 3 or 4 or 5

Modern safety devices, such as those found in the consumer units are often in the form of circuit breakers.
How do these work?

If a current that is too large passes through the circuit, a switch automatically opens making the circuit incomplete

What are the advantages of circuit breakers? (Compared to fuses)

• Can be easily reset

• Turn off circuit more quickly

• Allow does not need to be replaced (unlike the fuse)

A circuit breaker has a rating of 16A.

Suggest a reason why the heater may switch off before it reaches its normal operating current

• Idea that there are likely to be other appliances in same circuit

• Fuse in heater net be rated less than 16A

• Idea that heater may have a thermal safety cut out

• Idea that thermostat turn off heater

What do ALL devices with metal casings have to have?

An earth wire

How do earth wires work?

If the live wire breaks and comes into contact with the casing, the earth wire provides a low-resistance path for the current to the ground/earth. The current is likely to be large enough to blow the fuse and turn the circuit off
Basically prevents buildup of charge

Explain what would happen to devices with metal casings that do not have an earth wire if the live wire is damaged?

Without the earth wire anyone touching the casing of the faulty appliance would receive an electric shock as the current passed through them to earth

What is double insulation?

Many electrical appliances have casing made from an insulator such as plastic, rather than metal. All electrical parts are insulated this wasy so that they cannot be touched by the user.

How is the wire used in an device with double insulation different from another one without (just the metal casing)?

Appliances with double insulation use a two-wire flex (cable with only a live and neutral wire) as there is no need for an earth wire

Switches: in which wire should a switch be placed in? Why?

The live wire so that when the switch is open no electrical energy can reach an appliance

What would happen if the switch is placed in the neutral wire?

Electrical energy would still be able to enter a faulty appliance and could possibly cause an electric shock.