I'm in the process of changing my plastic light switches for metal ones. the box mounted in the wall is plastic. If i wire the earth cable to the light switch (face plate) so as to make this safe is that sufficient for it to be safe or do i need to change the plastic box for a metal one so i can earth it?
So long as the earth is connected correctly to the switch there should be no problem but do make sure the earth wire is earthed as it is unlikely the earth loop impedance will have been checked when it was installed.
The standard method is to use an instrument called an earth loop impedance meter which puts a load between Line and earth for something like 20 ms and has an inbuilt system to display the results as the impedance. During the test it can raise the earth to a high voltage for few seconds so one has to be careful no one can get a shock while doing the test. The other method is to use a low ohm ohmmeter these have to use at least 200ma during the test and unless you go into the consumer unit and disconnect wires there the long wire method has to be used. i.e. a wire from incoming earth to a screw on the switch then subtract the resistance of test wire from the result.
Of course using a multi-meter and measure against nearest socket earth pin would give some sort of result and is better than nothing. All except the earth loop impedance tester would be done with all power turned off at main switch. It is all this testing that makes it so expensive to get electricians to do what seems very minor work.
For lights with a 6 amp MCB (Type B) one would expect less than 7.5 ohms Impedance or 3.75 ohms long wire method. With AC we call it impedance with DC its called resistance and for what you are doing you can think of it being the same.
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