Electrical Safety (CIE IGCSE Physics)

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

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

   

• 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
  • Excess current from overloading of plugs, extension leads, single and multiple sockets when using a mains supply - If plugs or sockets become overloaded due to plugging in too many components the heat created can cause fires

Mains Electricity

   

• Mains electricity is the electricity generated by power stations and transported around the country through the National Grid
  • Everyone connects to the mains when plugging in an appliance such as a phone charger or kettle

• Mains electricity is an alternating current (a.c.) supply
• In the UK, the domestic electricity supply has a frequency of 50 Hz and a potential difference of about 230 V
  • A frequency of 50 Hz means the direction of the current changes back and forth 50 times every second

• Mains electricity, being an alternating current, does not have positive and negative sides to the power source
  • The equivalent to positive and negative are called live and neutral and these form either end of the electrical circuit

Three-pin Plug & Earth Connection

The plug socket and inside of a three-pin plug showing the three wires and their connections. The live and neutral wires deliver the electricity to the device. The Earth wire is for safety

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

   

   

Insulation & 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 this happens:
  • 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

• 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

• A trip switch, found in the Consumer Box (where the electricity enters the building) does the same job as a fuse
  • When the current is too high the switch 'trips' (automatically flicks to the off position)
  • This stops current flowing in that circuit

Choosing Which Fuse to Use

• 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