Electrical Safety: Fuses & Earthing Flashcards
All 7 cards in this deck
What is the function of the earth wire in an appliance with a metal case?
It connects the metal case to earth, keeping the case at the same potential as earth so a person touching it cannot get a shock.
What are the steps by which an earth wire and fuse make an appliance safe when the live wire touches the metal case?
e.g. a metal kettle with a 13 A fuse
- The metal case is a conductor connected to earth, so the resistance between live and earth is very low.
- A very large current flows to earth through the low-resistance earth wire.
- The large current melts the thin fuse wire, which breaks and disconnects the mains supply to the kettle.
How does a fuse work?
It contains a thin wire whose temperature depends on the current in it; a very large current raises it beyond its melting point so the wire breaks and disconnects the supply.
Why must switches and fuses be connected in the live wire?
So that when they open, the appliance is disconnected from the mains voltage and no part of it can be at mains voltage.
How do you choose a suitable fuse rating for an appliance?
e.g. a radio with a normal current of 2.3 A, choosing from 2 A, 5 A, 10 A, 13 A
Pick the fuse just above the expected current, not below it and not much higher.
e.g. the 5 A fuse.
True or false? Touching only the neutral wire of an appliance connected to the mains is as dangerous as touching only the live wire.
False. The neutral wire is at about 0 V, the same as earth, so there is almost no potential difference to drive a current through you.
What does a circuit breaker do when the current in a circuit becomes too large?
It trips, switching itself off automatically to break the circuit and cut off the supply.