In true Blue Peter style one I did earlier see if this helps?
Today testing of electrical installations is taken seriously. So what are we testing for?
We must insure should there be a fault the system will switch of within a set time. There are two main faults.
1) Too much current flowing.
2) Current flowing to wrong place.
To start with too much current flowing not only must it break the circuit it must do it in a safe manor and there is a limit to how much current the devices can handle also there may be a difference between overload and short circuit. So a device may have a overload value i.e. 16 amp and a short circuit value B and a maximum handling current 6kA and there will be tests to ensure all this is able to happen.
a) to carry 16 amp the wire will need selecting as to how hot it will get which will change according to how much heat it can get rid of so clipped to a metal tray 1mmÂ² will do but in a stud wall with thermal insulation 4mmÂ² will be required.
b) To trip in the case of short circuit a â€œBâ€
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