Redundant Back Boxes

[OnOff]

First post so treat me gently! I have posted this on another forum but got some unhelpful, IMO, off topic answers.

Anyway,I am looking at a kitchen ring - metal back boxes with 20mm circular PVC conduit running between them horizontally. The conduit extends into each back box by a few mm. Existing wiring is 2.5mm T&E which I'm removing and replacing with singles - concerned that it all might be a bit tight with two 2.5's in some lengths). Some of the back boxes will become redundant and have white plastic blanking plates over them (ones now behind cupboards etc). Is it still necessary to earth these unused back boxes? They will simply have 3 single cores running through them. Seems a pain to break the cpc and terminate in each box. Would they still be considered as an exposed conductive part? Thinking that if the live was compromised I could have live plate screws to touch. Cheers.

 

[Doc Hudson]

Are the back boxes going to have cable joints inside them or just cables passing through? Why don't you just get rid of the back boxes altogether if they are not in use? Will your cables still be running in safe zones?

Doc H.

 

[Apache]

If the cable just passing through then no need to earth. Might be 'neater' to remove as per the Doc!

 

[Doc Hudson]

If the cable just passing through then no need to earth. Might be 'neater' to remove as per the Doc!

But the op is talking about singles with no sheath passing through a metal enclosure though, not PVC T&E Sheathed and insulated. If the boxes not needed better to extend the conduit for the mechanical protection keep in things plastic all round?

Doc H.

 

[Barx]

No real need but if it was me and they were staying, I'd probably leave bit of slack on earth at each box, strip and inch of sheath off, double over and terminate into backbox earth lug anyway. Box earthed and no broken conductors.

Sent from my Desire Z

 

[Apache]

But the op is talking about singles with no sheath passing through a metal enclosure though, not PVC T&E Sheathed and insulated. If the boxes not needed better to extend the conduit for the mechanical protection keep in things plastic all round?Doc H.

But a backbox with a plastic faceplate would give a damn sight more protection to some cables that the outer sheath on some T&E? That is surely all the protection we require here?

 

[Doc Hudson]

Unsheathed cables require some mechanical protection, if the mechanical protection was metal conduit it would require earthing. If the mechanical protection was plastic conduit it would not require earthing. If the unsheathed cables entered a metal knock out box, back box, accessory enclosure or similar I would consider that the metal enclosure would now require earthing. As previously said if there are no terminations and cables are passing through the box easiest solution is remove the metal box out of the equation. It is about ensuring that you are satisfied you have complied with all appropriate regulations. Not how safe we personally consider something is.

Doc H

 

[Apache]

Thanks Doc, I bow to your superior knowledge!

 

[Doc Hudson]

Thanks Doc, I bow to your superior knowledge!

It is the unsheathed singles, (I assumed the OP meant this when they said replacing T&E with singles.), that is the issue here I think. Your solution would be perfectly OK with PVC T&E, which you could just bury in a plaster wall with or without metal capping without bothering about additional earthing. But I would not put unsheathed singles either unprotected in plaster or passing through unearthed metal enclosures.

Doc H.

 

[OnOff]

Thanks for the replies:

1) No, there will be no cable joints in the boxes as such, literally just passing through! The "problem" only applies to one double and one single back box that are redundant.

2) Removing the two boxes would be grief as the 20mm PVC comes in either side and "traps" them.

3) Yes, cables still all in safe zones

4) Extend the conduit across the box - could do on the double maybe but the single has spur cables in it going vertically down to a socket below the worktop, fed though from a socket further along the line i.e. no actual connection in that redundant single box-. So will still have live single browns in that box. Hope that makes sense?

5) I did consider just stripping the cpc sheath and doubling over. Can do this on the double quite easily - it has the corner type lug with plenty of room to do it. Not so sure on the single box though as that just has one brass boss for earthing and getting a doubled over 2.5 single might be an issue.....................a small connector block maybe, with the doubled over cpc in one side then a fly lead to the boss. Think I'll go this route.

Thanks again!

 

[Doc Hudson]

Removal of the back boxes is a relatively easy task for a competent person with a decent set of tools. One of the Fien type multi tools would make trimming the conduit and boxes a straight forward problem. Then a bit of judicious cutting of a section of PVC conduit, would provide sufficient protection for you to then plaster back over the hole.

Doc H.

 

[Barx]

Thanks for the replies:1) No, there will be no cable joints in the boxes as such, literally just passing through! The "problem" only applies to one double and one single back box that are redundant.

2) Removing the two boxes would be grief as the 20mm PVC comes in either side and "traps" them.

3) Yes, cables still all in safe zones

4) Extend the conduit across the box - could do on the double maybe but the single has spur cables in it going vertically down to a socket below the worktop, fed though from a socket further along the line i.e. no actual connection in that redundant single box-. So will still have live single browns in that box. Hope that makes sense?

5) I did consider just stripping the cpc sheath and doubling over. Can do this on the double quite easily - it has the corner type lug with plenty of room to do it. Not so sure on the single box though as that just has one brass boss for earthing and getting a doubled over 2.5 single might be an issue.....................a small connector block maybe, with the doubled over cpc in one side then a fly lead to the boss. Think I'll go this route.

Thanks again!

Yellow insulated crimp eye lug under screw head would also work

 

[PC Electrics]

Yellow insulated crimp eye lug under screw head would also work

was going to be my suggestion too.

You could take all three conductors through the redundant back boxes to their points of use. From here bring another cpc back to the back boxes that need earthing. You might actually struggle to get a doubled over 2.5 into a yellow crimp - it might be worth practicing this method before employing it 'live' (so to speak).

Another question for you: if this is a standard layout 32A RFC, why are you putting in a 2.5 cpc? 1.5 is conventional (because that's what's fitted in 2.5 t&e) and 1.0 would comply (provided it's not a 3036 ocpd).

 

[OnOff]

was going to be my suggestion too.You could take all three conductors through the redundant back boxes to their points of use. From here bring another cpc back to the back boxes that need earthing. You might actually struggle to get a doubled over 2.5 into a yellow crimp - it might be worth practicing this method before employing it 'live' (so to speak).

Another question for you: if this is a standard layout 32A RFC, why are you putting in a 2.5 cpc? 1.5 is conventional (because that's what's fitted in 2.5 t&e) and 1.0 would comply (provided it's not a 3036 ocpd).

I'd not thought of running a "remote" cpc and bringing it back, thanks for that. Keeps the through cables intact which I like. I was thinking too "local" to the box in question!

As to "why" I'm using the 2.5 cpc; it's as simple as that's what I have along with the live and neutral reels of the same size. Excuse my ignorance if it is. The protection at the CU is a 30A Crabtree C50 Type 2.

 

[wozz]

It could be argued i think that boxes are not accessible and therefore do not need earthing, we are talking about exposed conductive parts that are not exposed here. I can see docs point but we are not talking about exposed metal conduit here are we. Also if you deem it necessary to have a connection would you not also have to ensure it was accessible for inspection and testing, the OP mentions behind cupboards, doesnt sound very accessible to me.

Personally I would probably just run a 1.5 back from the nearest socket.

 

[SLIPSHOD & SLAPDASH]

If your cpd is an 30a, c50 what are you doing about rcd protection

 

[OnOff]

If your cpd is an 30a, c50 what are you doing about rcd protection

Fair question. Earthing system is TT. The CU is an old Crabtree metal clad unit and the install dates from circa 1987. The whole board is protected by a single General Electric, 30mA RCD in its own (insulated) enclosure between the meter and CU. Yes, it's a total pain when it trips! Plan is to change the whole board in the new year, all RCBO's and use this as one of my jobs for joining ELECSA (no disrespect to your lot!).

 

[PC Electrics]

Wozz rightly makes the point that the connections will not be readily accessible. Rather than using the earth 'lug' in the box I would put a ring crimp on the cpc and put this under a fixing screw.

Nothing 'wrong' with using a 2.5 cpc if that's what you have got rather than buying in 1.5 specially. Just that it's important you understand that it's bigger than you 'need' and when he looks at your certificate your assessor may pick up on this and ask you the same question.

---------- Post Auto-Merged at 15:23 ---------- Previous post was made at 15:22 ----------

PS: I'm with ELECSA and can heartily recommend them. Members of other schemes would probably say the same.

 

[OnOff]

Wozz rightly makes the point that the connections will not be readily accessible. Rather than using the earth 'lug' in the box I would put a ring crimp on the cpc and put this under a fixing screw.Nothing 'wrong' with using a 2.5 cpc if that's what you have got rather than buying in 1.5 specially. Just that it's important you understand that it's bigger than you 'need' and when he looks at your certificate your assessor may pick up on this and ask you the same question.

---------- Post Auto-Merged at 15:23 ---------- Previous post was made at 15:22 ----------

PS: I'm with ELECSA and can heartily recommend them. Members of other schemes would probably say the same.

Going to go with a separate cpc from an adjacent box. I will just put an asterisk for the cpc size on the Schedule of Test Results and then in the Remarks put 1.5 & 2.5.

I've emailed ELECSA a few times on the subject of joining them as I'm in a bit of an odd situation in that I'm not in the trade as such. Basically an over keen diy'er with a large main property (lots of outbuildings) and also a landlord. As I said elsewhere in introducing myself I first did a year long EAL PART P (35 wks/ 2 eves per week), then the full 2330 which I completed this year and the 17th in 2010. 2391 next! ELECSA could not have been more helpful, chap called Mark Cooper on their Technical Desk. I have to sort out some insurance to go with them even if working on just my own properties. Will mainly be doing my own but maybe the odd friend and relatives. Seems daft to be wanting to power up sheds and garages and other notifiable stuff and paying BDC

 

[batty]

Don't mention BDC.

:slap