Fuses and Earthing Flashcards
What happens if a fault develops in which the live wire touches the metal case?
Because the case is earthed, when too great a current flows through the live wire through the case and out the earth wire the surge in current melts the fuse
Why would a fuse melt?
When the amount of current is greater than the fuse rating
What does the melting of a fuse (or tripping the circuit breaker in the live wire) do?
Cuts off the live supply and breaks the circuit
This isolates the whole circuit making it impossible to get an electric shock from the case
What else do earthing and fuses help prevent?
Fires caused by the heating effect of large current
Asleep protect the circuits wiring in appliance from getting fried if there’s a current surge
What should fuses be rated?
As near as possible but just higher than the normal operating current
Why does the fuse ratings needed for cables usually increase with the cable thickness?
The large the current the Thicker the cable
What does earthing mean?
The casing is attached to an earth wire
What can an earthed conductor never do?
Become live
What does it mean if an appliance is double insulated?
It has an insulating material (e.g. Plastic) casing and no metal parts showing
What does anything that’s double insulated not need?
An earth wire (only needs live and neutral)
What are cables that only carry live and neutral wires called?
Two core cables
What are circuit breakers?
Electrical safety devices used in some circuits that protect the circuit from damage if too much current flows
What do circuit breakers do when they detect a surge in current in the circuit?
They break the circuit by opening a switch
Why are circuit breakers more convenient than fuses?
They can be reset by flicking a switch on the device whereas fuses have to be replaced once melted
What is one type of circuit breaker used instead of a fuse and earth wire?
Residual Current Circuit Breakers (RCCBs)
