Electrical Safety

• Mains electricity is potentially lethal

  • potential differences as small as 50 volts can pose a serious hazard to individuals
• Risk of electrocution sign

  A) danger

  B) electric shock risk

  
• Common hazards include (1):

  • 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
• Common hazards include (2):

  • 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
• Excess plugs

  A) plugs

  B) connected

  C) extension lead

  D) overheating

  E) fire

  
• 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
• 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
• 3 pin plug and earth connector (1)

  A) earth

  B) neutral

  C) live

  
• 3 pin plug and earth connector (2)

  A) neutral

  B) earth

  C) live

  D) fuse

  E) cable grip

  F) live

  G) neutral

  H) earth

  I) metal cased

  
• 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
  • the wires are covered with an insulating material, such as rubber to prevent electrocution
• Conducting wire

  A) conducting

  B) insulating

  
• 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
• Double insulated appliances - 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
• Many electrical appliances have metal cases
• Metal cases 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 of metal cases
• If the live wire comes into contact with a metal case the earth wire does this:

  • 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)

  
• 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
• 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:

  A) power

  B) voltage

  
• 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
• Cables have flexible plastic sleeves

  • made from plastic/rubber
  • some are double insulated
• Insulation layers stops the core of the live wire from conducting other wires
• Damaged insulation - no longer protected against current/voltage flow into the body aka electrocution
• Too much current or cables fet too hot so the cables overheat

  • creates electrical fire
• Damp conditions

  • water is a very good conductor
  • dry skin has a higher resistance than wet skin
• If an appliance doesn’t have a plastic coating it needs to be earthed
• If a live wire touches the outer casing, the current will flow down the earth wire rather than a person
• Circuit breakers - a system powered with electromagnets

  • if there’s too much current the electromagnet pulls apart the contact and breaks the circuit
  • can be resetted each time
• Fuses protects a circuit from too much current

  • if too much current flows through it, the fuse wire melts and the circuit is broken
• Fuses should be picked so that it allows a slightly higher current to flow than the appliance region but as close to it as possible
• Where to place a fuse?


  In series with the live wire before all the components