Replacing a Plastic Light Switch with a Metal One

[MikeyBUK]

I wonder if anybody can help. I need to replace an old plastic box light switch that is part of a 2 way switch. I've bought a metal box switch and the guide clearly indicates the need to earth the metal switch box. The cable going into the old box is 1 mm 3 core and earth cable with the earth cable unsheathed and unused.

Am I OK to use this earth for the box? Read a few DIY texts and they indicate I should only earth using a sheathed earth. Any advice gratefully received

 

[bernardgreen]

I assume you mean the earth wire is bare but inside the grey overall sheath.

Yes you can use this but you should put green yellow sleeve on it,

BUT before you change the switch you MUST ensure that the earth wire is connected to earth at the far end. And that that connection is an effective earth. One way is to measure the resistance ( impedance ) between the earth wire at the switch and the earth of a socket with a multi-meter or ohm meter. Do it with power turned OFF to avoid false readings. The resistance needs to be less than one ohm.

 

[kai]

Once the main switch is off, you need to temporarily move the live core of the lighting circuit involved from the top of the breaker, and park it in the earthing bar in the consumer unit - then measure between live and earth at the light switch position, and obtain a very low reading there.
Once that is OK, install new lightswitch, then replace the live core at the fusebox into the top of the breaker, and replace front of CU, and turn power back on.

 

[bernardgreen]

My suggestion avoids the DIYer going into the CU with the dangers that involves.

By using the live as a "wander lead" down to the consumer unt earth bar can give a false "good" result and lead to a dangerous situation.

If the CPC at the switch is not connected to the earth bar at the CU but has been used as a Live conductor elsewhere on the lighting circuit then your test will falsely show the CPC as good . Connecting that "false good" CPC to the switch plate will make the plate live..

Using the live as a wander lead is a valid test but ONLY if the installation is known to NOT have any mis-use of a CPC as a live or neutral lead.