Testing Nightmare

[Wiksey]

This definitely comes under the heading of jobs I wish I'd never started...

Can someone please for the sake of my sanity tell me where I am being an idiot?

Short version - on a lighting circuit, how can R1+R2 = 0.37 to 0.72 ohms with switches on but in places give 16 to 409 ohms with switched off?

Long version below:

There's a long back story that's not entirely irrelevant, but I'm doing initial testing on a house that has no completion certificate with a view to seeing how much remedial work is required to bring the whole install up to the right standard to get it signed off.

Yesterday I found some things that are concerning or need fixing but today I ended up totally stumped.

So, testing a lighting circuit..

Safe isolation - tick

Connect Line (R1) and CPC (R2) at CCU

I would at this point expect to go along the circuit testing at each point finding small resistances and of the switch elements get a small value with the switch in the on position but "out of range" with the switch in the off position. However, at some points with the switch off I was getting readings ranging from a few ohms to about 400 ohms.

There doesn't seem to be any logic to their position in the circuit (they are spaced out with ones in-between not giving me any trouble.

I tried various combinations of disconnecting and partial disconnections - individual wires didn't show faults, only when part of the complete circuit.

There were no interconnections at switches or junction boxes.

Junction boxes were all intact and correctly wired.

This is the upstairs lighting circuit, and I did a quick test of the downstairs and found the same thing.

If I am being an idiot, please feel free to tell me, but someone please tell me what I've done wrong (apart from start the job) as it's genuinely doing my head in!

Thanks

Wiksey

 

[Andy™]

bet you didnt have the neutral isolated at the board....

draw it out then it will be obvious

 

[Wiksey]

bet you didnt have the neutral isolated at the board....

draw it out then it will be obvious

If only it were that simple - honestly I wish it was.

All 3 conductors were disconnected at the CCU. I swapped to measuring R1 + RN and didn't find any problems. So presumably whatever the problem is is something to do with the CPC.

If I get a chance I'll get a rough sketch scanned in later.

 

[Rob_the_rich]

borrowed neutral between upstairs and downstairs lighting circuits?

 

[ProDave]

What about I/R testing on these circuits? any anomalies there?
 

 

[Wiksey]

borrowed neutral between upstairs and downstairs lighting circuits?

Right, you're getting me thinking here (esp. when Andy also questioned the Neutral situation). I'll have to check that there's no interconnection between the two lighting circuits.

What is still making me think it's something else though is that it wasn't found at every point on the circuit and that between L & N there was no problem at all.

Never say never though, I will check. Thanks

What about I/R testing on these circuits? any anomalies there?
 

After finding what showed as a connection between the conductors I didn't move onto I/R testing as I assumed (yes, I know I shouldn't assume, and am now wishing I hadn't!) it would fail. Will check, thanks.

 

[Lurch]

If there  is no borrowed neutral and the IR is OK then I would be inclined to strip it down completely and test each cable individually. Probably quicker and easier in the long run.

 

[Sharpend]

when energised did you try switches individually to ensure no anomalies? 

 

[Murdoch]

Have you tried the lights with the power on?

What is the cu configuration? Rcbo's ? Dual RCD? Single RCD?

Need a bit more info really

 

[Rob_the_rich]

Surely live tests after dead tests?

Or is the house occupied?

Agree with more info.

 

[steptoe]

its not much help, the others have already suggested loads,

but please stop using CCU, I keep thinking cooker connection unit

:|

 

[Evans Electric]

Weird readings are usually through lamps ,  fans , fluorescent ballasts ,  12V transformer down lights , doorbell transformers, neutrals ,  TV signal boosters in loft connected to the lighting .

 

[Rob_the_rich]

Could be water in there somewhere too.

 

[Evans Electric]

its not much help, the others have already suggested loads,

but please stop using CCU, I keep thinking cooker connection unit

:|

I keep thinking ...cuckoo.... :C

 

[Evans Electric]

Might pay to do as Lurchio says .... disconnect half the circuit and try again until you isolate the bit thats throwing your readings .   A good learning exercise too

 

[Wiksey]

If there  is no borrowed neutral and the IR is OK then I would be inclined to strip it down completely and test each cable individually. Probably quicker and easier in the long run.

Thanks. When I get there next I'll double and triple check for interconnections, then try I/R, but I'm thinking the same, that it needs stripping bit by bit

when energised did you try switches individually to ensure no anomalies? 

The argument put forward by the current owner is "well it works, so it must be ok"...

Surely live tests after dead tests?

Or is the house occupied?

Agree with more info.

Agreed, usually live tests after dead tests, but, yes the house is occupied and has been for for about 8 years... just never signed off... no completion certificate... we are or at least were looking at buying...

Have you tried the lights with the power on?

What is the cu configuration? Rcbo's ? Dual RCD? Single RCD?

Need a bit more info really

Currently a 16th Edition CU with no RCD on the lights (next job once all faults rectified is a new CU to latest standard)

its not much help, the others have already suggested loads,

but please stop using CCU, I keep thinking cooker connection unit

:|

Sorry

Was taught CCU, will try calling it a CU if that helps?

Weird readings are usually through lamps ,  fans , fluorescent ballasts ,  12V transformer down lights , doorbell transformers, neutrals ,  TV signal boosters in loft connected to the lighting .

Thanks, I will keep this in mind as I start dismantling!

Neutrals seems to be a common suggestion here, I'll keep a close eye on these too.

 

[Wiksey]

Could be water in there somewhere too.

I've not seen any signs of corrosion or damp in the electrics, but thanks for the suggestion, I will keep it in mind.

I keep thinking ...cuckoo.... :C

You callin me cuckoo ;-)

Again, I'll try a different abbreviation, with no cooker or bird confusions...

Might pay to do as Lurchio says .... disconnect half the circuit and try again until you isolate the bit thats throwing your readings .   A good learning exercise too

Yep, defo a good learning exercise, but also driving me nuts and I'm going on holiday in the morning.

Thanks, I think I'll be onto this disconnecting suggestion pretty quickly when I get back on site.

 

[Sharpend]

Thanks for replying, but didn't ask what the home owner thought, I asked if you had tried the lights yourself? I guess from your answer that you haven't. 

 

[Rob_the_rich]

Central Consumer Unit? Customer's Consumer Unit? Google says Coronary Care Unit.

Must have been bunking off on my short course when they mentioned that?

How did you manage to persuade them to test before buying?

 

[Wiksey]

Thanks for replying, but didn't ask what the home owner thought, I asked if you had tried the lights yourself? I guess from your answer that you haven't. 

Ohh, sorry. I start getting a bit random when it gets late!

All the lights work as you would expect with no anomalies. I wouldn't swear to having tried every light myself, but they have lived in the house for more than 8 years and "have never had a problem"... at least, not until I turned up!