Earthing Q - new pattress won't fit original metal conduit

[blackworx]

Hi folks,

Read the wiki and searched the net, but can't find an answer to the following.

As part of room renovation I'm swapping a single socket outlet for a double.

Our flat is 1970's vintage and there's no separate CPC in the ring (just black and red wires, and each socket face plate's earth connector is simply wired to the back box) so I think the earth is provided via the steel conduit itself? From what I've read this is no longer considered good practice, but unfortunately I don't have the budget to rewire right now.

Problem I have is as below - the gap between the conduit ends doesn't fit the knockouts in the new double pattress.

I don't much fancy filling the hole in and going back to a single socket. If one of the conduit sections is not screwed to the back box (via the original brass glands) am I correct in thinking the sockets further round will still have an earth connection, but there won't be a continuous CPC right the way round the ring, which in turn could create a dangerous situation if another break in the CPC were to happen in future?

 

[ban-all-sheds]

unfortunately I don't have the budget to rewire right now.

But you have the budget for a room renovation.

Do you have the budget for an electrician to check that circuit? For all you know another bodger has been there before you and broken the continuity of the conduit somewhere else. Or corrosion has made it ineffective.

Failing that, do you have the budget for a funeral?

Problem I have is as below - the gap between the conduit ends doesn't fit the knockouts in the new double pattress.

Then you must replace it with one which does fit.

I don't much fancy filling the hole in and going back to a single socket.

Irrelevant - you must not have a break in continuity there. If only a single box will fit then you have 3 options.

1) Stick with a single socket.

2) Use a single to double converter.

3) Install a new double socket above or alongside the current location, and extend the wiring to it.

If one of the conduit sections is not screwed to the back box (via the original brass glands) am I correct in thinking the sockets further round will still have an earth connection, but there won't be a continuous CPC right the way round the ring, which in turn could create a dangerous situation if another break in the CPC were to happen in future?

Not allowed.

Dangerous and illegal.

Don't even think about doing it.


But whatever you do you need to get that circuit checked. In fact, you need to get them all checked - you could be living in the middle of a lethal hazard.

 

[blackworx]

Thanks for detailed response. Pretty much as I thought. I did have a double converter on there originally, but wanted it recessed. Back to the drawing board, or, as you say, find a box that fits.

But you have the budget for a room renovation.

For all you know another bodger has been there before you

A little less paternalistic/judgemental would be nice though. I'm not an idiot and I'm certainly not a bodger. If I was a bodger I would've just screwed the damn thing in and not given it a second thought. As for the snide budget remark, the budget for the renovation is spent; this potential wiring issue having been discovered part way through the work, ergo no budget for rewiring right now.

Thanks all the same for responding though.

 

[bernardgreen]

Unfortunately BAS is correct,

You do need to get the installation checked.

The condition of the thread inside the end of the conduit suggests that corrosion is occurring between conduit and bush. I assume that there were bushes screwed into the ends of the conduit to hold the old box tightly against the conduit. That is essential for the continuity of the earth connection.

If there were no bushes or they were loose then you MUST have an electrician check the whole installation and you should consider a re-wire before decorating.

 

[Taylortwocities]

A little less paternalistic/judgemental would be nice though. I'm not an idiot and I'm certainly not a bodger.

You may not be a bodger, but many DIYers on here are.

When dishing out guidance on here the first concern is your safety.
You might get a little dig on the way, but that's life….

 

[blackworx]

Oh I understand BAS is 100% correct and have no problem with his conclusion, but it'll be a month before I can afford a spark to come look at it. I am literally broke from the renovation spending.

Yes there were bushes, all firmly in place, and the conduit thread is in good condition - clean and smooth - it's just dirty with plaster dust right now.

 

[blackworx]

You may not be a bodger, but many DIYers on here are.

Again, no argument with that. My argument is only with BAS's direct implication that I am, in fact, a bodger.

When dishing out guidance on here the first concern is your safety.
You might get a little dig on the way, but that's life….

Hmmm, I get the why, but still in my view there was little need (on the available evidence of my original post) for the how. I manage to dispense advice in my areas of expertise without jumping straight in with the sarcasm; at least not until I'm sure the person I'm speaking to is an idiot.

 

[bernardgreen]

Then best option is to put the old socket box back. Ensure the bushes, conduit threads and the surfaces of the box are clean and bright before screwing the bushes in tightly.

It would sensible to stretch / amend the budget to put re-wiring before decorating as re-decorating after a re-wire is just more expense.

 

[OwainDIYer]

Not a problem - and easier if you kept the original box and bushes.

Using the original box as a template, get a new metalclad box without knockouts and drill it to suit the original conduit spacing and bush size.

Box without knockouts: MK K830 ALM

Then do a continuity test from the socket back to the main earth terminal.

 

[ericmark]

I have carefully drilled boxes with hole saw rather than use knock-outs where they will not line up otherwise. Care is of course required and I have not been relying on conduit for the earth.

It is possible you could thread a wire back to socket before/after or knock out more plaster and fit banjo earth tags and a brass 6mm bolt.

so your not relying on the metal of the box to continue the earth.

The most important thing is to test. Electricians use an earth loop impedance tester you can either hire one of those or buy the cheap alternative in theroy these only test as being better than 1.5Ω and really you want it better than 1.44Ω for a system without RCD protection. But although not quite good enough likely if better than 1.5Ω then there is a good connection and will also be better than 1.44Ω or 1.37Ω which from June is the new limit for a B32 MCB.

What one has to consider is it worth paying out £50 for plug in tester or get an electrician to do the job properly?

The problem is steel does corrode (Rust) and develop bad connections so using a socket tester once a year is likely a good idea anyway. As is fitting RCD protection as well.

Do be wary these

do NOT do the same job. There is no loop test and it is the loop test which is important. Although it says "Missing Earth" it uses that little current to test it even wet string as an earth wire would pass the test. It needs to say loop test on it even the ones with loop test at 1.6W don't really do that good of a job but do at least go some way towards proper testing.

The one an electrician uses draws 15mA to 4A according to range selected the plug in one uses 7mA and price jumps from £45 to £450 between the two meters. But I hear what you say about short of cash and I am trying to give alternative methods.

But I repeat it's not the connecting of the earth it's the testing of the earth once connected that matters.

The other option is if no earth switch off the sockets there is no half way. If it has a earth great if it don't switch it off.

 

[Risteard]

the earth is provided via the steel conduit itself? From what I've read this is no longer considered good practice

I have to disagree here. Although it is true that it is no longer usual practice to rely on the conduit as a cpc (typically due to designers being concerned that the containment system will not be correctly installed) it is absolutely permitted and should not be regarded as poor practice.

 

[sparkwright]

I tend to agree with most of what Ristead has said.

I have successfully drilled new holes in a new back box (which already had knockouts in) without it going horribly wrong.

I would recommend getting more boxes than you need though, as if you mess it up the boxes are then useless.

I used a step/cone drill to get the holes the right size.

If you're a good electrics DIYer you may be able to add your own seperate CPC through all the conduits if you want to, or leave it for another time.

You do NOT want debris going down those conduits though.

You must make sure both conduits make good, firm contact with the box.

You must ensure there is an earth lead between the back box and the socket front.

You must, must make sure those lock rings go back in place and make full contact with the metal box.

Personally I think a professional should be doing this. However, you have shown a good understanding in how important the conduit is in providing the earth.

And there are some really lousy 'professionals' out there who could easily make a right mess of this.

So be warned, make sure whoever does it does it properly.

 

[RF Lighting]

Get a different make of box, or try it upside down. It's the 25mm knockout that's cocked you up.

 

[blackworx]

Thanks all.

Get a different make of box, or try it upside down. It's the 25mm knockout that's cocked you up.

Sadly not the case re the 25mm knockout - if only it were that easy. However that's a good point re trying a different make. I'd assumed they'd all be standard spacing. I'll take the old box along the next time I'm going to a big shed and see if there's one that's a match.

Failing that I'll go with drilling a 'blank' back box to match the conduit as suggested, and get a spark in at the end of the month to inspect and test.

The room I'm currently working on is only the first of three that are covered by this ring and I'm expecting to renovate both the other rooms over the next few months. So while it will be a PITA if work needs done after I've finished decorating, at least it's only one room I've wasted effort on and not all three.

 

[sparkwright]

It's the old half inch imperial conduit, so it's never going to connect to a modern box with 20 mm knockouts SUCCESSFULLY.

As I said, you can drill new holes in a new box, but the main problem is the pre-formed knockouts might open out as you're drilling. Can be done with care though.