4.4

• electric shock occurs if current flows from an electric circuit through a person's body to earth
• the typical resistance of dry skin is 10000 ohms and for wet skin it is lowered to 1000 ohms
• the size of the current and length of time for which it acts determine the strength of the shock
• to avoid risk of an electric shock:

  1. switch off the electrical supply before starting repairs
  2. use plugs that have an earth pin and cord grip
  3. do not allow appliances or cables to come into contact with water
  4. do not have long cables trailing across a room
• to reduce the risk of fire:

  • use the correct fuse
  • do not attach too many appliances to a circuit
  • do not overload circuits
  • appliances that use large amounts of power should not be connected to a lighting circuit designed for low current use
• neutral wire is earthed at the local sub station so there is no p.d between it and the earth
• the live and neutral wires each run in two complete rings around the house and the power cokets are tapped off from them
• a fuse protects a circuit and is always placed in the live wire. it should ensure the current carrying capacity of the wiring is not exceeded
• trip switches are now used instead of fuses and when the current exceeds the rated value of the circuit breaker, the electromagnet becomes strong enough to separate a pair of contacts and break the circuit. the trip switch setting should be chosen to be a little higher that the calue of the current in the device being protected
• the residual current circuit breaker (rccb) is and adapted circuit breaker which is used when the resistance of the earth path between the consumer and the sub station is not small enough for fault current to blow the fuse
• the third wire - earth is a safety precaution to prevent electric shock should an appliance develop a fault