Electrical Safety (Cambridge International IGCSE Physics)

Electrical hazards

Spec point — 4.3.6 · State the hazards of: (a) damaged insulation (b) overheating cables (c) damp conditions (d) excess current from overloading of plugs, extension leads, single and multiple sockets when using a mains supply

Electrical hazards

• Mains electricity can be dangerous if safety procedures are not followed

  • Voltages as low as 50 V can pose a serious risk of electrocution

• Common electrical hazards include:

  • damaged insulation
  • overheating cables
  • damp conditions
  • excess current from overloading of plugs, extension leads, single and multiple sockets when using a mains supply

The risk of electrocution is indicated by hazard signs but other risks which would not be signposted are listed below

Damaged insulation

• If the insulation around an electrical cable is damaged, the metal part of the wire may become exposed
• If a person touches the exposed wire, they could be subjected to a lethal electric shock
• There is also a chance that current will flow between the exposed wire and any piece of metal it comes into contact with

Overheating of cables

• If an excess of current flows in a wire, this can lead to overheating
• This could cause the insulation to melt, or even cause a fire

Damp conditions

• Damp conditions can be dangerous in the presence of electricity since water is an electrical conductor

• If moisture comes into contact with a live wire, this could set up:

  • a short circuit within the device, which could cause a fire
  • a conductive path for current to flow through a person to the earth, which could cause electrocution

Excess current from overloading

• An excessive current may flow if too many plugs, extension leads or sockets are connected to the mains supply
• The heat created could cause the insulation to melt, or even cause a fire

Electrical fire due to excessive current

Too many appliances plugged into an extension lead can cause overheating and fire

Mains circuits

Spec point — 4.3.6 · - Know that a mains circuit consists of a live wire (line wire), a neutral wire and an earth wire and explain why a switch must be connected to the live wire for the circuit to be switched off safely

• Explain why the outer casing of an electrical appliance must be either non-conducting (double-insulated) or earthed

Mains circuits

• All electrical appliances are connected to the mains supply

• A mains circuit consists of:

  • a live wire
  • a neutral wire
  • an earth wire

• The insulation covering each wire is colour-coded for easy identification:

  • Live wire – brown
  • Neutral wire – blue
  • Earth wire – green and yellow stripes

Live, neutral & earth wires

A UK plug socket

A mains-powered appliance in the UK contains live, neutral and earth wires

• Every country has a slightly different configuration, with many using two-pin plugs and plug sockets such as in the USA and mainland Europe

  • The earth wire is still present in the two-pin plugs, just more hidden

• The three wires have distinct purposes:

The live wire

• The purpose of the live wire is to carry the alternating current from the mains supply to a circuit
• It is the most dangerous of the three wires
• If it touches the appliance without the earth wire, it can cause electrocution

The neutral wire

• The purpose of the neutral wire is to form the opposite end of the circuit to the live wire to complete the circuit
• Because of its lower voltage, it is much less dangerous than the live wire

The earth wire

• The purpose of the earth wire is to act as a safety wire to stop the appliance from becoming live

• This prevents electric shocks from occurring if the appliance malfunctions or the live wire breaks off and touches the case of the plug

• In order to protect the user or the device, there are several safety features built into domestic appliances, including:

  • double insulation
  • earthing
  • fuses
  • circuit breakers

Double insulation

• The conducting part of a wire is usually made of copper or some other metal

  • If this comes into contact with a person, this poses a risk of electrocution

• For this reason, wires are covered with an insulating material, such as rubber

The conducting part of a wire is covered in an insulating material for safety

• Some appliances do not have metal cases and so there is no risk of them becoming electrified

• Such appliances are said to be double insulated, as they have two layers of insulation:

  • Insulation around the wires themselves
  • A non-metallic case that acts as a second layer of insulation

• Double insulated appliances do not require an earth wire or have been designed so that the earth wire cannot touch the metal casing

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 it would risk being electrocuted

• The earth wire is an additional safety wire that can reduce this risk

• If the case does become live:

  • The earth wire provides a low resistance path to the earth
  • It 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

Fuses & trip switches

Spec point — 4.3.6 · - Explain the use and operation of trip switches and fuses and choose appropriate fuse ratings and trip switch settings

• State that a fuse without an earth wire protects the circuit and the cabling for a double-insulated appliance

Fuses & trip switches

• Fuses and trip switches (circuit breakers) are safety devices designed to cut off the flow of electricity to an appliance if the current becomes too large (due to a fault or a surge)

Fuses

• Fuses are used to protect individual appliances

• Fuses are located in the plug

• Fuses usually consist of a glass cylinder containing a thin metal wire

• A fuse without an earth wire protects the circuit and the cabling for a double-insulated appliance

• 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

Circuit symbol of a fuse

The circuit symbol for a fuse has a wire running through it (not to be confused with a resistor)

• Fuses have ratings, which signify the maximum current that can flow through the fuse wire without it melting
• If the current exceeds that value, the fuse wire melts and the individual appliance is disconnected from the mains supply
• Fuses come in values of 3 A, 5 A and 13 A
• The correct fuse to use is the value just above the current required for the appliance

Choosing the correct fuse

• Suppose an appliance uses 3.1 A:

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

Trip switches

• The current enters the house at the consumer unit(sometimes referred to as a 'fuse box')

• The consumer unit consists of a series of trip switches (or circuit breakers) which control the amount of current supplied to each circuit within the house

  • When the current is too high the switch 'trips' (automatically flicks to the off position)
  • This stops the current flowing in that circuit

Example of a domestic circuit

The consumer unit distributes current to all the circuits in the house

• The main advantages of trip switches are:

  • they provide protection from current surges or faults
  • they can be reset when the problem is fixed