Steel capping thickness

[blackfacespikeyhair]

Hi all,

I am thinking about installing a light fitting in my loft which will involve chasing a cable into the wall in the room below. I understand that this circuit will require RCD protection or mechanical protection on the cable (or both). I have an old fuse box which takes rewirable fuses (Wylex B-type? - not sure). As far as I know it is not possible to fit an RCD to this type of box. Is this right?

If so, my only option is mechanical protection. This link, http://www.esc.org.uk/forum/forum_nd_thread.html , suggests a minimum protection of 3mm of steel (scroll down to question NDQ12). Can anyone confirm that this thickness is necessary? If so, does anyone know where I can buy capping this thick? The capping that I have seen in the shops is all very thin (ie probably 0.5mm or less). Perhaps I could use multiple layers of thin capping - is this considered acceptable?

Any other suggestions welcome! Thanks in advance for your help.

 

[hairyben]

I've had steel plate made up by a fabricator, don't think there's anything on the market except steel conduit.

Easy way is to take the circuit out the board and through an RCD, eg a RCD switch-fuse-spur unit.

Another option is to use flexishield type cable.

 

[Taylortwocities]

Capping is NOT suitable.
You cant run a cable down the wall below and RCD protect it unless there is a socket , light switch etc in its run to indicate that its there.

Your option is:
bury the cable more than 50mm from the wall surface or
Use steel conduit and it must be properly earthed throughout its length.

 

[blackfacespikeyhair]

Thanks for your replies.

Is there such a thing as oval steel conduit? I haven't seen it anywhere. If I were to use round conduit it would mean either cutting into the masonry or re-plastering the whole wall to an inch thick! Don't really fancy either of these options.

If I were to add an RCD FCU to the circuit, would it be acceptable to use earthed SWA and plaster directly over it? Alternatively, could I have an RCD FCU and then use ordinary T&E in PVC conduit, if the cable were in a safe zone?

One concern I have about adding the RCD is that, potentially, the loft light circuit would be the same as that on the floor immediately below. If the RCD were to trip, both floors would be in darkness which could present its own hazards. Are there any regulations about this? I could in theory connect to a different lighting circuit but it would be an extra hassle.

Thanks again.

 

[ban-all-sheds]

I am thinking about installing a light fitting in my loft which will involve chasing a cable into the wall in the room below.

I take it your lighting circuit loops at the switches?

If I were to add an RCD FCU to the circuit, would it be acceptable to use earthed SWA and plaster directly over it?

Where would you install the glands?

Alternatively, could I have an RCD FCU and then use ordinary T&E in PVC conduit, if the cable were in a safe zone?

Yes.

One concern I have about adding the RCD is that, potentially, the loft light circuit would be the same as that on the floor immediately below. If the RCD were to trip, both floors would be in darkness which could present its own hazards.

You could use the RCD FCU just on the new branch you install, then if it tripped it would only take out that branch.

Or you could spur off a socket circuit, so the loft lights wouldn't be on a lighting circuit at all, and you could position the FCU to create a zone for the cable to run in.

Are there any regulations about this?

Yes, and the fact that you think that a trip would create a hazard means that you think it would not comply.

 

[blackfacespikeyhair]

I take it your lighting circuit loops at the switches?

There is a junction box under the floorboards in the room below the loft. I was intending to connect to that.

Where would you install the glands?

I hadn't quite worked it out yet, but I was considering another junction box connected to the one mentioned above. This could be an external type with round entry points or possibly (and I feel like I am about to get told off here!) a metal mounting box with a blanking plate over the top. The latter might make the earth connection easier to do.

You could use the RCD FCU just on the new branch you install, then if it tripped it would only take out that branch.

Trouble with this is that, at some point in the future, an RCD could be added to the whole circuit. This safety feature would then be lost.

you could spur off a socket circuit, so the loft lights wouldn't be on a lighting circuit at all

This sounds like a good idea but I thought it was frowned upon to have fixed lighting off a socket circuit...?

the fact that you think that a trip would create a hazard means that you think it would not comply

I wasn't aware of any particular regulations but I thought it an obvious risk. It seems to me that there are a number of factors to take into account when evaluating the danger of this scenario. I would have to consider the likelihood of the RCD tripping, the consequences of it tripping, and the expense/hassle of avoiding this problem in the first place. The reason I asked the question was because I wanted to know if it was up to me to make a judgement on this or if I were required by law to configure things in a particular way.


Thank you for all your comments and suggestions.
Tim

 

[ban-all-sheds]

I take it your lighting circuit loops at the switches?

There is a junction box under the floorboards in the room below the loft. I was intending to connect to that.

But isn't that for the lighting of the floor below? If your lighting circuit(s) loop through the fittings then in the loft will be the circuit cable for the upstairs lights and by far the easiest way to install a loft light is to come off that rather than one a floor below and have to chase the wall...

Where would you install the glands?

I hadn't quite worked it out yet, but I was considering another junction box connected to the one mentioned above. This could be an external type with round entry points or possibly (and I feel like I am about to get told off here!) a metal mounting box with a blanking plate over the top. The latter might make the earth connection easier to do.

If you're going to have a flush box with a blanking plate you could have a flush box with an RCD FCU and not need to use armoured cable...

You could use the RCD FCU just on the new branch you install, then if it tripped it would only take out that branch.

Trouble with this is that, at some point in the future, an RCD could be added to the whole circuit. This safety feature would then be lost.

It would be no different to the situation you'd be in if the circuit was already RCD protected.

you could spur off a socket circuit, so the loft lights wouldn't be on a lighting circuit at all

This sounds like a good idea but I thought it was frowned upon to have fixed lighting off a socket circuit...?

Nothing wrong with it.

the fact that you think that a trip would create a hazard means that you think it would not comply

I wasn't aware of any particular regulations but I thought it an obvious risk.

Yes - that's the whole point/problem/dilemma.

The regulations require [abridge]that every installation shall be divided into circuits as necessary to avoid danger and minimise inconvenience in the event of a fault and to take account of danger that might arise from the failure of a single circuit such as a lighting circuit.[/abridge]

And that's it - nothing specific is prescribed, or proscribed, it's down to the designer to decide. There are reasonably widely accepted best practices, like not having too much on one RCD, hence the emergence of the concept of a "17th Edition CU" with multiple sections, or with RCBOs, and people are starting to split lighting circuits L/R instead of up/down etc.

It seems to me that there are a number of factors to take into account when evaluating the danger of this scenario. I would have to consider the likelihood of the RCD tripping, the consequences of it tripping, and the expense/hassle of avoiding this problem in the first place. The reason I asked the question was because I wanted to know if it was up to me to make a judgement on this or if I were required by law to configure things in a particular way.

Yes it's up to you as designer to use your skill and judgement to design what to the best of your knowledge and belief is in compliance with BS7671:2008. So if you start out thinking that what you propose won't avoid danger and minimise inconvenience etc, then by definition you think it won't comply...

If you can do it, the best idea would be a maintained or sustained emergency light connected to the upstairs lighting circuit cables in the loft positioned so that it will illuminate the way out if the power fails. No chasing of walls, no spurs, no concealed cables needing RCD, no safety issues of being plunged into darkness...

 

[blackfacespikeyhair]

But isn't that for the lighting of the floor below?

Yes it is but it also supplies the light in the room I have been talking about (immediately below the loft). This light is on a wooden beam and the supply cable is clipped to that beam. The cable continues into the wall (a different wall from the one I have been talking about), then to a switch, then finally on to the junction box. So the live supplying that light is already switched. The neutral is joined inside the switch housing. No cables currently run into the loft so there is no easy connection to be had there (sadly).
It may help if I tell you that the space I have been describing as the "loft" is a relatively small space above what is already an attic room. The wooden beam I mentioned supports the roof. The loft has only become accessible since I cut a hole in the attic ceiling a few weeks ago. The purpose of this is add thermal insulation above the attic. During this work I don't want any trailing cables which is why I would like to add fixed lighting.

If you're going to have a flush box with a blanking plate...

This is not what I meant. The idea I had for the metal mounting box was to screw it to a plinth between the joists under the floorboards. The swa gland would attach to a circular cut out of the mounting box. I would then have, dare I say it, a choc block inside the mounting box to connect the wires from the swa to the normal t&e wires. The t&e would then continue to the original junction box. The earth from the t&e would be connected to the mounting box thereby making an earth connection for the armour of the swa. So the metal mounting box would act as a glorified junction box.

It would be no different to the situation you'd be in if the circuit was already RCD protected.

I agree - that's the problem. Your suggestion of having RCD protection on the branch rather than the whole circuit is fine but it seems like a temporary fix. If at some point in the future I (or someone else) were to add RCD protection to the whole circuit, the problem of having lights on both floors potentially going out at the same time would return. My feeling is that it would be better to avoid this from the outset.

Another question occurs to me here. If the RCD were on the branch, there would be nothing protecting the cable which supplies it. So I would have a new cable which is unprotected. Doesn't the 17th say that all new cabling should be protected?

Nothing wrong with it.

Excellent. I like this idea!

If you can do it, the best idea would be a maintained or sustained emergency light ...

I like this idea too!


Off to think...

Thanks again ban-all-sheds. You have been really helpful.
Regards,
Tim

 

[securespark]

Just to clarify - nothing wrong with lighting from a circuit feeding socket outlets as long as the lighting cable is fused down (as it would be if you used an FCU or RCD FCU).

 

[GaryMo]

Just to clarify - nothing wrong with lighting from a circuit feeding socket outlets as long as the lighting cable is fused down (as it would be if you used an FCU or RCD FCU).

Apart from a very thin argument that it wouldn't comply with section 314 (division of installation).
Using a power tool and the MCB or RCD which supplies the ring final trips when the loft lighting is being replied upon springs to mind.