To Earth or Not To Earth that is the question

[AmateurFiFi]

Just in the process of redecorating our Bedroom in our mid 20's house.

I was looking to change the surface mounted plastic light switch to a brass flush mounted one.

I have, however come up against a possible problem. The current switch is wired in DI singles (two only line and switched line) no earth. They arrive at the back of the surface box in metal conduit (similar in size to 20mm but undoubtably imperial) that does not appear to be earthed - Resistance >200M Ohm). It is easy to to replace these with T+E as there is a functional earth (Resistance circa 0.1 Ohm) in the ceiling rose that is easily accessible in the loft.

The current original back box is the wrong size to use for the proposed switch - or even a plastic switch and will have to be changed. Due to the current position on the wall and the conduit entry point it will not be possible electrically attach the new back box to it.

The walls are "plastered" with a black mortar undercoat and we do not want surface mounted trunking.

The question I would like to answer is must this conduit be earthed and if so how. There is sufficient conduit above the ceiling to attach an earthclamp to and then run a suitable size cable to one of the lighting junction boxes that is easily available.

When i reuse the conduit does it have to be earthed if used to

 

[riveralt]

It is easy to to replace these with T+E as there is a functional earth (Resistance circa 0.1 Ohm) in the ceiling rose that is easily accessible in the loft.

Seems to be the simplest answer to your question.

 

[AmateurFiFi]

Main question - that I think got lost in my ramblings is:

Should I earth the conduit - and if so whats the best way

Thanks in advance

 

[EFLImpudence]

As it is accessible in the loft - yes.

 

[riveralt]

Main question - that I think got lost in my ramblings is:

Should I earth the conduit

I would say have said no.

But EFL says yes because of loft access.

EFL assuming the OP fits T&E as I suggested while I can find 522.6.101 and 522.6.103 - also 543.2 can you point me to the regulation regarding this one.

 

[EFLImpudence]

Surely by definition it is an exposed conductive part.

 

[JohnW2]

Should I earth the conduit

I would say have said no. But EFL says yes because of loft access. EFL assuming the OP fits T&E as I suggested while I can find 522.6.101 and 522.6.103 - also 543.2 can you point me to the regulation regarding this one.

As far as the regs are concerned, I'm with you on this one. IF the cable is in a safe zone [522.6.102(v)] and is RCD protected [522.6.102], there is obviously no requirement for earthed conduit, and I'm not aware of any requirement that if T&E goes through a 'metal tube' (or otherwise comes in contact with metal) that the metal has to be earthed. I therefore don't see a regulatory requirement for the conduit to be earthed if it is in a safe zone and only RCD-protected T&E is going through it.

Having said that, and even though any risks are exceedingly small, if the cable is 'going through a metal tube' and that tube can easily be earthed, it obviously would do no harm, and would theoretically add (very slightly) to safety, to earth it - so "why not?".

Kind Regards, John

 

[JohnW2]

Surely by definition it is an exposed conductive part.

Interesting! Even when there are two layers of insulation between it and any live conductors? [It's sort-of 'Class II'] Would you saythe same of any metal with which DI cable did, or could, come into contact with? [if so, people might have to go against current wisdom and earth their metal draining boards, since I'm sure that the cables of kettles, mixers etc. may occasionally come into contact with them!!]

Kind Regards, John

 

[riveralt]

Having said that, and even though any risks are exceedingly small, if the cable is 'going through a metal tube' and that tube can easily be earthed, it obviously would do no harm, and would theoretically add (very slightly) to safety, to earth it - so "why not?".

I suppose I was thinking that EFL's response could open the OP to a lot more work than he anticipated since if we did apply the 'why not' principle then one must assume that all the loft accessible conduits must also be bonded.
Personally, if I was the OP I would rip out the old conduit -alternatively I would not bond the conduit. I would remove the old cable, cut back the conduit so it was no longer exposed, fit new T&E and if felt necessary provide some form of barrier over the old conduit.

 

[JohnW2]

I suppose I was thinking that EFL's response could open the OP to a lot more work than he anticipated since if we did apply the 'why not' principle then one must assume that all the loft accessible conduits must also be bonded.

I suppose so, but the 'why not?' does not necessarily have to be extended to 'why not all of it', I suppose!

However, EFLI has thrown a cat in amongst the pigeons by suggesting that the conduit constitutes an exposed-c-p. I personally don't think that is correct but, if it were, it obviously would have to be earthed. Let's see how he responds to what I wrote!

Kind Regards, John

 

[EFLImpudence]

Interesting! Even when there are two layers of insulation between it and any live conductors? [It's sort-of 'Class II']

True.
I wasn't coming at this from any degree of insistence but I would not have replied with a definite "No".
It just seemed sensible. Someone in the loft could lean on it.
If anyone disagrees - fair enough.

Would you say the same of any metal with which DI cable did, or could, come into contact with?

No.

[if so, people might have to go against current wisdom and earth their metal draining boards, since I'm sure that the cables of kettles, mixers etc. may occasionally come into contact with them!!]

You would be able to see that and the result of providing an earth path for someone receiving a shock would be more hazardous.

 

[EFLImpudence]

It would be earthing and not bonding.

And - cutting it off would be the most satisfactory thing to do.

 

[JohnW2]

Interesting! Even when there are two layers of insulation between it and any live conductors? [It's sort-of 'Class II']

True. I wasn't coming at this from any degree of insistence but I would not have replied with a definite "No".

Same here. As you will have seen, I didn't give a 'definite No'. On the contrary, I said "why not, if it's easy to do". However, as I implied, suggesting (as you did) that the conduit was "by definition" an exposed-c-p seemed to be rather OTT and likely to have all sorts of potential repercussions!

It just seemed sensible. Someone in the loft could lean on it.

Sure, but given the cable's two layers of insulation, the conduit is probably no more likley to become live than is the draining board I mentioned.

I think we're more-or-less agreed.

Kind Regards, John

 

[OwainDIYer]

The current original back box is the wrong size to use for the proposed switch - or even a plastic switch and will have to be changed. Due to the current position on the wall and the conduit entry point it will not be possible electrically attach the new back box to it.

A photo would be helpful. It may be possible to use a metal back box and attach it to the existing conduit, or there used to be MK adapters that could mount over an old flush conduit box and accommodate a new switch.

You can get wooden surrounds for surface metal boxes which might be cosmetically acceptable
http://www.tlc-direct.co.uk/Products/VLXKBOXSO.html

 

[RF Lighting]

Don't forget, if it's a bit of old slip jointed conduit, then it'll have a sharp end on it just as the T&E bends at a right angle to make the transition from wall to ceiling.

It wouldn't be impossible at all for even an insulated and sheathed cable to fault to the metal.

To me, that's an exposed conductive part.

If you can prove continuity to the back box, then just earthing the box which you must do anyway will be sufficient. If not, drop a bit of conduit singles earth down the tube with the twin and earth, and fit a clamp to the tube in the loft.