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Topic Title: Changeover switch in plastic enclosure
Topic Summary: Earth terminal
Created On: 31 October 2017 01:31 AM
Status: Read Only
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 31 October 2017 01:31 AM
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Posts: 371
Joined: 01 June 2007

I have a generator changeover switch to install and want to check something beforehand. It is in an insulated enclosure but has an earth terminal in the enclosure. Do I need to run an earth cable into the enclosure with my mains in, then an earth cable out with my mains out and terminate these together in the earth terminal along with the generator earth or just connect my generator earth together with an earth out to the MET? To me the earth terminal is serving no purpose other than a connector.
It got me wondering as I have seen plastic enclosures before with earth terminals incorporated and wondered if it was necessary to run an earth cable to them if the terminal is provided even though it is t actually earthing anything ?

 31 October 2017 08:36 AM
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Posts: 9921
Joined: 22 July 2004

Well depending on the cables into and out of the enclosure there may be CPCs to terminate there, and the box makers do not know. BS7671 requires a CPC to any place like a light switch where at some point in the future a metal bodied part may be fitted - so the plastic box has a place to tie the CPC to for the future.
You don't have to use it - indeed in some situations like an RCD in front of a TT installation it is more dangerous to bring the CPC into the box than not ! (as a CPC to pre RCD meter tail fault may not draw enough current to blow the company fuse, just to liven up all exposed metal in the building )
The more common problems with generator/inverter earthing arise when the genset maker assumes the installer will provide an NE bond externally, and give an earth free floating output, but the user does not, so RCDs fire just fine on the self test button, but don't actually trip on earth faults. The clue is that N and E are several volts apart and wander with the load and dampness. And that RCDs dont trip..
More than once I have had to add an NE bond in or near a changeover switch or connector to fix this sort of silliness.

regards Mike

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