Resistance between two circuits

[phil d]

It's a bleeding mess, that's what it is, that JB want's doing properly for a start!

 

[QuietLeni]

I will get a better joint box and set the layout correctly.

I removed the borrow neutral from the circuit and as I thought, the lights stopped working in the bathroom only - the other lights in the Upstairs circuit stayed on. I am now wondering why that guy didn't use the neutral from the rest of the lighting circuit, rather than trailing a whole cable up from the mains (which is mainly kept in the void between the ground and first floors).

All I could think of was that he was using the mains circuit because the bathroom light was attached to an extractor fan, but still, why use a borrowed earth from the mains for that?

I think that I need to do is find the rest of the upstairs light circuit and use the neutral from that (unless I am missing sonething)?

 

[Lurch]

How far into your apprenticeship are you?

 

[Geoff1946]

Yes that junction box is a mess. I sincerely hope that the rest of your "old" installation is better, and even more that you know how to do better. However, I do think you deserve credit for finding the problem. That shows good reasoning and investigative skills. As has already been said, those aren't simple to communicate.

 

[pewter]

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.

What did you find?

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)!

Is it not a load you are reading?

What reading do you get between the neutral of each of thse circuits?   , it would be low ohms if it was borrowed.

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.

When you tested between the Live of one circuit and the neutral of another, had you disconnected the neutrals from the board?

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.

If it is you should have nice low reading when you mesaure from each neutral condctor to the other?

I wouldnt measure L-N if i wanted to check if N-N are connected !!!

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

 

[QuietLeni]

Yes that junction box is a mess. I sincerely hope that the rest of your "old" installation is better, and even more that you know how to do better. However, I do think you deserve credit for finding the problem. That shows good reasoning and investigative skills. As has already been said, those aren't simple to communicate.

Yes, The guy obviously was just trying to get the system running and couldn't bother to do a good job. In fairness, though, the system has worked for 8 years without needing any maintenance and this has only come to light because of the new CU and me being proactive in trying to fix the issue.

I just hope that I can find the cable from the rest of the lighting circuit and use that for the neutral instead of the mains one. Why use a mains ring main? Was he worried about the current draw from the shaver or extractor fan?

 

[Doc Hudson]

Yes, The guy obviously was just trying to get the system running and couldn't bother to do a good job. In fairness, though, the system has worked for 8 years without needing any maintenance and this has only come to light because of the new CU and me being proactive in trying to fix the issue.

I just hope that I can find the cable from the rest of the lighting circuit and use that for the neutral instead of the mains one. Why use a mains ring main? Was he worried about the current draw from the shaver or extractor fan?

Phrases such as 'something has worked for 8 years' and the guy 'just trying to get the system running' suggest an acceptance of cowboy attitudes in my opinion. I would say there are two possible causes of your situation. 1) someone who hasn't got a clue what they were doing has wired it up or 2) someone who knew full well they were doing a half baked bodge job wired it up.  Either way I would be wanting a comprehensive inspection of every circuit on the whole installation done, to check there are no more nasties lurking. This should have been done when the CU was changed with an installation certificate issued for the work. I could wire a socket outlet with no earth connected and live and neutral reversed and leave it with a customer thinking it apparently working OK for years without maintenance. But it would still be a bodge job leaving someone at risk of shock. Borrowed neutral, (or more strictly correct "two fuses/MCB's supplying power to the same section of cable"), is a very hazardous arrangement leaving risk of electric shock if only one fuse is isolated during any circuit alteration work.

Doc H.

 

[Andy™]

In fairness, though, the system has worked for 8 years without needing any maintenance and this has only come to light because of the new CU and me being proactive in trying to fix the issue.

the system may have worked for the last 8 years, however it has always been wired wrong and potentially dangerous, just you didnt previously have anything to detect that fault. luckily the problem has been discovered before someone was electrocuted

tbh, id be wanting to test the entire house. id expect more bodges somewhere else...

 

[Murdoch]

Yes, The guy obviously was just trying to get the system running and couldn't bother to do a good job. In fairness, though, the system has worked for 8 years without needing any maintenance and this has only come to light because of the new CU and me being proactive in trying to fix the issue.

"This has only come to light because of the new CU"

i am puzzled .... When was this cu fitted?

 

[QuietLeni]

"This has only come to light because of the new CU"

i am puzzled .... When was this cu fitted?

In the last few weeks. The previous CU was fitted in 1997.

the system may have worked for the last 8 years, however it has always been wired wrong and potentially dangerous, just you didnt previously have anything to detect that fault. luckily the problem has been discovered before someone was electrocuted

tbh, id be wanting to test the entire house. id expect more bodges somewhere else...

Agreed. We are getting the whole house checked by an inspector to make sure that the CU and circuits are given a certificate.

 

[Murdoch]

Who fitted the latest CU?

Why didn't you disclose this earlier as this would have avoided about half of the replies.

 

[Lurch]

Why didn't you disclose this earlier as this would have avoided about half of the replies.

TBF I sort of got that from the OP but it wasn't fantastically worded so I think it got missed by a lot of people.

 

[Murdoch]

TBF I sort of got that from the OP but it wasn't fantastically worded so I think it got missed by a lot of people.

So if the opening post had included this fact, then we wouldn't have gone round and round in circles ...... One suspects its a DIY job, without the necessary test kit, or knowledge  to test, nor document what they've done ............ 

But I could be wrong!

 

[Lurch]

So if the opening post had included this fact, then we wouldn't have gone round and round in circles ...... One suspects its a DIY job, without the necessary test kit, or knowledge  to test, nor document what they've done ............ 

But I could be wrong!

First line of the OP;

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.

 

[Murdoch]

First line of the OP;

Hum..... I'm not sure I believe him.... would you fit a split load board, put all the old circuits on 1 RCD, all the new circuits on the 2nd RCD and not touch or test any of the old circuits?

I wouldn't do either of these things.......

And what about the paperwork, etc 

He's saying somebody else is going to test and certify...

Something is really smelly!

 

[Lurch]

Hum..... I'm not sure I believe him.... would you fit a split load board, put all the old circuits on 1 RCD, all the new circuits on the 2nd RCD and not touch or test any of the old circuits?

I've met plenty of odd electricians! Who knows.

 

[Murdoch]

I've met plenty of odd electricians! Who knows.

People that do these things are either builders or DIYer's .................................

 

[kerching]

I've met plenty of odd electricians!

I have never met one who isn't !

WHY CHOOSE A JOB WHERE YOU CANNOT SEE, HEAR, SMELL, TASTE  OR FEEL,that which can easily kill you on a daily basis

i wanted to be a Pathologist......at least they are guaranteed their work is dead 

 

[Lurch]

i wanted to be a Pathologist......at least they are guaranteed their work is dead 

Probably still have to check. Maybe not by not by licking your finger and touching it though?

 

[kerching]

Cold water in the ear or scratching an eyeball used to be an approved method

or get uncle Jimmy to,walk,around the slab for a few minutes