CPC Continuity through Metal Clad box

[Nigel_Cro]

I hope I can explain this properly - here goes...

13A Fused spur in a metal clad box, surface mounted. 2.5mm PVC singles on the supply side entering the box. 2.5mm three core SWA on the load side leaving via a standard gland.

So - Line, Neutral & CPC in either side of the fused spur - easy.

CPC from the supply side into the lug on the back of the box.

CPC from the banjo on the gland into the load side.

The question is - the fused spur does not isolate the CPC as there is continuity through the box - does this matter, or is there something blindingly obvious I am missing??

Thanks.

 

[yorkspark]

The cpc is there for safety and is not allowed to be switched. ideally it also wants a link from the backbox to the faceplate as well.

 

[Nigel_Cro]

Thanks, so the CPC being continuous through the box is as designed, just something to be aware of when testing? The back box has 2 fixed lugs so I thought that was sufficient for the faceplate?

 

[yorkspark]

The back box has 2 fixed lugs so I thought that was sufficient for the faceplate? if you take the plate off for testing then you will lose your earth on the outgoing side/swa, as your relying on the faceplate screws, the incoming earth should go to the faceplate and a link to the back box

 

[Nigel_Cro]

Ok - the earth from the SWA goes into the CPC terminal on the load side of the faceplate, at the moment there is a link from the banjo into the same terminal. So, if the faceplate is taken off there is still earth connectivity both sides of the switch. SWA > Load side terminal > load side terminal > Banjo > back box > back box terminal > supply side terminal > supply side terminal > PVC Single > Consumer Unit. - CPC Continuity achieved??

 

[333rocky333]

The back box has 2 fixed lugs so I thought that was sufficient for the faceplate? if you take the plate off for testing then you will lose your earth on the outgoing side/swa, as your relying on the faceplate screws, the incoming earth should go to the faceplate and a link to the back box

Sounds like the faceplate is linked to the banjo which is glanded i assume into a metal backbox.


However I would still link between the dedicated earth terminals on the front plate and the Back plate, therefore bonding the lot together as well as the 3rd core of the swa if used as Earth.
Not that it matters the wires can be shared between the Supply and load Earth terminals as they are linked by a metal bar as you found out.

 

[333rocky333]

Thanks, so the CPC being continuous through the box is as designed, just something to be aware of when testing

For what reason are your concerns regarding testing

 

[Nigel_Cro]

I would still link between the dedicated earth terminals on the front plate and the Back plate, therefore bonding the lot together as well as the 3rd core of the swa if used as Earth.

Thanks Rocky, the supply side earth terminal on the faceplate is linked to the post in the back box. Third core of the SWA is in the load side earth terminal. Banjo on the gland is also in the load side earth side terminal.

I think it is all ok.

 

[EFLImpudence]

the wires can be shared between the Supply and load Earth terminals as they are linked by a metal bar as you found out.

I am not sure the OP has found out that yet.

 

[Nigel_Cro]

I am not sure the OP has found out that yet.

You are right...

My initial assumption was that in a fused spur the CPC was not continuous i.e. you turn the switch to 'off' and it disconnects all three conductors, I assumed that the functionality being provided was to totally isolate everything on the load side of the switch. If you have no line and no neutral any fault state on the load side is going to be 'safe' so no point/need to have a path back to earth?? Whereas, if the switch is 'off' and there is a fault state on the supply side there could still be a problem on the load side? I suppose this is why you take so much care to prove dead?

Thanks everyone - every day is a school day!