Resistance between two circuits

[QuietLeni]

Dear All,

I am trying to fix up our house after an electrician replaced our aging CU (with no RCDs) with a new-fangled double-RCD CU. This was done because we had a new extension and so we needed to separate out the old circuits (on RCD A) from the new (on RCD B).

The circuits on RCD B are all okay, but I had a problem with the circuits on RCD A.

RCD A has the following circuits:

Ring Main

Hot Water system

Downstairs Lighting

Upstairs Lighting

When the CU was first installed, RCD A kept tripping, no matter what we did when we switched on the ring main. As a temporary measure, the electrician put in a big switch instead of the RCD, as the system has worked with no changes for the last 10 years and we knew of no problems during that time.

The issue was fixed when I found a leak from neutral to earth and so we need to put the RCD back instead of the big switch, to make sure that we get our protection installed properly.

Just before I switched over, I checked the Ring Main in the CU with my trusty Megger, which helped me find the problems, expecting to see an open circuit, but instead, I saw about 223 Ohms across Live to Neutral! Hmmm. I then switched off all of the other circuits and found that the Ring Main registered an open circuit (as do all of the others in isolation)!

By process of elimination, though, I found that I ONLY get 200-223 Ohms across the Ring Main when it is off and the Upstairs Lighting circuit is on.

I think that somehow, there is a shared Live or Neutral between the two circuits, which is strange, considering the construction of the house - I would have expected to have found a shared Live/Neutral between the upstairs lighting and the Ring Main out of the two, as the two sets of wiring are in the same place.

Four things:

1) should I be worried?

2) can I put in the RCD safely?

3) how can I track down the issue?

4) which is it going to be? Borrow Live or Neutral?

Kindest regards in advance,

QuietLeni

 

[ProDave]

All I can say is 10 years ago you didn't have an "electrician" you had a cowboy. He should have found and fixed the fault when he fitted the new CU.

Turn ONLY the ring main on. Do any lights come on?

Turn ONLY the lights on. Do any lights not work?

 

[ProDave]

duplicate post by mistake

 

[steptoe]

Its a new CU @ProDave

Read it again

@QuietLeni

Get your installer back, thats all he is,

he is certainly NOT an electrician,

 

[QuietLeni]

All I can say is 10 years ago you didn't have an "electrician" you had a cowboy. He should have found and fixed the fault when he fitted the new CU.

Turn ONLY the ring main on. Do any lights come on?

Turn ONLY the lights on. Do any lights not work?

Thanks for your help, Dave. However, I am where I am and that guy is long-gone now. Oh well.

If I turn on the ring main only, only items on the ring main come on. No lights come on.

If I turn on the lights only, all of the lights work.

I am guessing, from that experiment, that it is a borrowed neutral?

 

[QuietLeni]

Its a new CU @ProDave

Read it again

@QuietLeni

Get your installer back, thats all he is,

he is certainly NOT an electrician,

Steptoe,

Thanks for the input, however, the electrician that installed the new CU is not touching the old wiring, as he did not install it.

Can I have some advice on how to find the issue? Where do I look to try and fix this?  Everything is working as expected, but there is the weird connection somewhere between the mains and the upstairs lighting.

 

[Murdoch]

With all the old circuits on a single RCD, a borrowed neutral will not worry the RCD.

You need a decent spark who knows what he is doing to test the old circuits..... 

What test results are listed on the cert you were given?

 

[ProDave]

Steptoe,

Thanks for the input, however, the electrician that installed the new CU is not touching the old wiring, as he did not install it.

Can I have some advice on how to find the issue? Where do I look to try and fix this?  Everything is working as expected, but there is the weird connection somewhere between the mains and the upstairs lighting.

That "electrician" does not deserve to use that description. He is at best a clueless installer able to wire a new circuit as long as it works he is okay.  NOBODY should be undertaking a consumer unit change if they are not able to find and fix any faults with the original wiring.  This is bread and butter stuff and 99.9% of the proper electricians on this forum would have properly tested the existing wiring and fixed the faults.

The best solution is to get a properly COMPETENT electrician to test it, find the faults and fix it.  Sadly NO bit of paper or qualification will tell you if anyone is properly competent to find and fix a fault on old wiring.

And yes I miss read it that the extension and new CU was done 10 years ago.  If it has only just been done, I would be sending the second electrciian's bill to the first "electrcian"

If you haven't paid the first guy yet, I would not be in a hurry to do so.

 

[QuietLeni]

With all the old circuits on a single RCD, a borrowed neutral will not worry the RCD.

You need a decent spark who knows what he is doing to test the old circuits..... 

What test results are listed on the cert you were given?

Thanks for the help. I guess that I am inclined to agree with you.

I am sad, however, that no one wants to even suggest any way of finding out what the issue is.

 

[steptoe]

Thanks for the help. I guess that I am inclined to agree with you.

I am sad, however, that no one wants to even suggest any way of finding out what the issue is.

Its not as simple as telling you how to find the fault,

You need quite extensive testing equipment and a good knowledge of how to use it.

 

[Andy™]

only way to find the fault is to get someone who knows what they are doing to do some fault finding, which your first 'electrician' should have been more than capable of

 

[Murdoch]

Thanks for the help. 

I am sad, however, that no one wants to even suggest any way of finding out what the issue is.

Employ a local competent spark, through recommendation, and get them to fault find properly.... Something a diyer can't just do without knowledge, proper test kit, and the experience to use it.

 

[phil d]

QuietLeni, it's virtually impossible to tell someone how to fault find over the internet, yes we could tell you how to find the switch wire on a 3 plate lighting circuit, we could tell you how to test for ring continuity, it's basic stuff, however fault finding requires experience and knowledge that can't be imparted in a few minutes, it takes years. You could have an amateur with the latest most expensive test meter, or an experienced electrician with an older more basic meter, the electrician will find the fault first, it's who is "driving" the meter that matters,

Put it another way, I'm a police trained advanced driver, in an older slower car I can easily outdrive a youngster in a fancy new Audi, yes he may lose me on a straight road,but I'll be all over him like a rash when we hit the bends,why? Because at the end of the day, I'm the trained driver, he isn't!

You've been badly let down by one so called "electrician" don't let it put you off, there are loads of decent sparks about, if you are worried about getting another in, ask if he works on results, if he doesn't fix the problem, then you don't pay him, if he's any good he'll accept those terms.

You could spend hours poking around in the "dark" and eventually you may blindly "stumble" upon the problem, the trouble is if you don't know what could be causing in then you could wander on by and miss it completely.

Please for your safety and sanity, get a decent spark in.

 

[QuietLeni]

Well... I think that I have an answer to where the problem is, as I have found that it is a shared neutral from the mains to a shaver socket in the bathroom.

When I plugged a light into the mains and had everything upstairs switched off/disconnected, I had an open circuit. I then got someone to switch on each of the lights upstairs in turn. Still an open circuit. Then, when my partner plugged a shaver into the shaving socket, I then got 400 Ohms. So the issue is there.

Now as the shaving socket works only when the upstairs light circuit is on, the situation must be a borrowed neutral from mains to that socket.

This now makes sense, as the shaver socket was put in by a guy that We thought was going to be great, as he had good references, but we found lots of little things wrong with his work around the bathroom once he finished. It seems like this was another piece of his Wild West sharp shooting.

Now that I know where to look, I need to fix it.

 

[Murdoch]

A shaver socket is almost certainly selv. On this basis plugging something in won't make any difference

 

[phil d]

A shaver socket is almost certainly selv. On this basis plugging something in won't make any difference

Some of these units have a switch in so it only energises the primary winding when something is plugged in, however for a small isolating transformer 4oo ohms seems a high reading, definately something not right here.

 

[Andy™]

look at the wiring in the shaver? if its borrowed neutral id expect the L&N to arrive in different cables

 

[phil d]

look at the wiring in the shaver? if its borrowed neutral id expect the L&N to arrive in different cables

Yes, you'd think so wouldn't you,people do have some funny ideas, I once had a heated "discussion" with a customer over wanting to chase a wall out to fit a high level socket. "there's a perfectly good live and neutral behind this redundant light switch" he argued, pointing to a red and a black cable, he was convinced it was L & N, not live and switch wire. I had another where he'd wired a florry fitting across a live and switch wire in a junction box, the tube just rolled instead of striking up, the bathroom light glowed dimly when off, but when you switched it on, the florry went off, but he was adamant it wasn't down to his wiring.

 

[Risteard]

A shaver socket is almost certainly selv. On this basis plugging something in won't make any difference

A shaver socket definitely IS NOT SELV. It utilises electrical separation which is completely different - it is low voltage rather than extra-low voltage.

 

[QuietLeni]

Some of these units have a switch in so it only energises the primary winding when something is plugged in, however for a small isolating transformer 4oo ohms seems a high reading, definately something not right here.

When I say, "shaver", I think that it was a toothbrush charger.

Well, here is a picture of the offending article.

Why would anyone use a borrowed neutral in a regular lighting circuit?

i guess that I can disconnect it and see what happens to the lights. I am guessing that they will go out?