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# Fuses and the earth wire Watch

1. Hey guys
So I'm doing ocr gateway additional science and in p4 I really don't understand plugs and fuses, specifically how the earth wire causes the fuse to blow.

Thanks for the help.
2. (Original post by lucaboyd24)
Hey guys
So I'm doing ocr gateway additional science and in p4 I really don't understand plugs and fuses, specifically how the earth wire causes the fuse to blow.

Thanks for the help.
A fuse is designed to limit the current in a section of electrical circuit, it does this because the thin wire it's made of heats up and eventually melts (or 'burns-out') when a high current passes through it.

in an appliance with a metal case (e.g.a washing machine) a fault might develop in which the live conductor makes contact with the metal case. if the metal case is connected to electrical earth potential then a high current will flow through the fuse as soon as that fault occurs and cause the fuse to melt.
If the metal case is not connected to earth and same fault occurred the case would remain connected to electrical live - which would be very dangerous for people.

The primary purpose of fuses is to limit current and prevent cables overheating and catching fire... but it also protects users of metal cased appliances from electrocution... provided the appliance case is earthed.

hope that helped.
3. (Original post by Joinedup)

if the metal case is connected to electrical earth potential then a high current will flow through the fuse as soon as that fault occurs
Makes sense thanks just why does it make a high current go through the live wire?
4. (Original post by lucaboyd24)
Makes sense thanks just why does it make a high current go through the live wire?
there's a very low resistance path to earth. a live wire has touched the case so the current isn't having to go through the resistance of the heater element (or whatever the appliance has in it)

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