Low IR readings on lighting circuit - 0.654 Mohms

[HappyHippyDad]

I have spent quite a bit of time today trying to understand why I am only getting 0.654Mohms on the upstairs lighting between Live and neutral (at 250V)!!!

L-N 0.654 Mohms
N-E 25.11 Mohms
L-E 24.7 Mohms

I get almost the same when measured at 500V (cant remember exactly but about 0.6Mohms)
I get 105.5 Mohms when I measure between L+N combined and E.
Upstairs contains lights/extractor fans/shaver units

I have looked in every junction box in the loft and every light and switch and dont know what else to do.

Perhaps break the circuit down is the next step?

I just need 0.346 Mohms and then I will be a happy man! :sad:

 

[telectrix]

possibility of something still connected. aerial amp in attic? neons? as long as yout L/N - E reading is acceptable, worry not.

 

[Jay Sparks]

As above, something still connected and it will be the last place you look. If the test is ok when you connect L & N together, then all good.

 

[Marvo]

Yeah, it's a wee bit low, check the bathroom fittings and fan for damp to start with. If not then just split the circuit into sections and track it down. Even getting it to a Meg is questionable, you don't know how stable the readings going to be so I'd be looking for considerably better readings than that.

*edit* got beaten to the post but I like Tel's idea as well.

 

[Octopus]

I have spent quite a bit of time today trying to understand why I am only getting 0.654Mohms on the upstairs lighting between Live and neutral (at 250V)!!!

L-N 0.654 Mohms
N-E 25.11 Mohms
L-E 24.7 Mohms

I get almost the same when measured at 500V (cant remember exactly but about 0.6Mohms)
I get 105.5 Mohms when I measure between L+N combined and E.
Upstairs contains lights/extractor fans/shaver units

I have looked in every junction box in the loft and every light and switch and dont know what else to do.

Perhaps break the circuit down is the next step?

I just need 0.346 Mohms and then I will be a happy man! :sad:

Surely you need 1.346??

Did you disconnect the shaver point?

As Tel says link L&N to E then move on!

 

[HappyHippyDad]

possibility of something still connected. aerial amp in attic? neons? as long as yout L/N - E reading is acceptable, worry not.

As above, something still connected and it will be the last place you look. If the test is ok when you connect L & N together, then all good.

Cheers Guys,

Yes, when I read similar threads they said the same. Why does it make it ok if you have a good Live/neutral combined IR reading? The IR between L and N is still poor and could mean a problem? (Although I'm guessing it doesn't as otherwise you guys wouldn't have said it!). Why does the good reading between L+N combined and E show that it is an appliance still not accounted for and not a wiring/damp/poor insulation problem?

 

[Marvo]

It could still be a damp problem but the higher IR L+N combined to earth makes it more likely to be a small load still connected....although it would be a very small load.

 

[telectrix]

it's all a matter of experience. if you get a lowish reading L-N , and all cables and accessories appear in good order, then it's a judgement call as to whether you accept it or investigate further.

 

[HappyHippyDad]

It could still be a damp problem but the higher IR L+N combined to earth makes it more likely to be a small load still connected....although it would be a very small load.

Thanks Marvo,

So would you investigate until you found the cause or like Tel and others have said move on?

Plus, on the MWC how would I document this?

 

[HappyHippyDad]

it's all a matter of experience. if you get a lowish reading L-N , and all cables and accessories appear in good order, then it's a judgement call as to whether you accept it or investigate further.

In that case I need to investigate further as I dont have the instinct to know whether it is acceptable!

I was going to have a day off tomorrow and perhaps have a pint at the local but now I will hunt for ohms!

 

[Marvo]

Thanks Marvo,

So would you investigate until you found the cause or like Tel and others have said move on?

Plus, on the MWC how would I document this?

I don't know if it's within the regs or not and I'm not sure how you document it on a minor works, in South Africa we work to different standards and paperwork. Just over half a meg at 250v is a bit low for my tastes, I'd investigate to be honest. It's low enough to be a definite problem and it might come back and bite you later if you don't improve the reading.

 

[kingeri]

Did you disconnect the fan/shaver point before testing? Are there LED downlights on the circuit (could be a snubber in there somewhere)? Trying to make sense though I am a tad tipsy!

 

[Jay Sparks]

Did you disconnect the fan/shaver point before testing? Are there LED downlights on the circuit (could be a snubber in there somewhere)? Trying to make sense though I am a tad tipsy!

On a Tuesday.....bl0ody hell.....you really are proper Yorkshire lad!!lol

 

[Ohmagain]

Have had a faulty fittings can cause issues. I installed a brand new ring with all new fittings but got a reading of 168Mohms @ 250 volts. was all down to a one faulty socket.
changed and strait away got >2000Mohms @ 500v

 

[kingeri]

On a Tuesday.....bl0ody hell.....you really are proper Yorkshire lad!!lol

Actually these days tend to stick to weekends but now and again I just can't say no!

 

[Jay Sparks]

Actually these days tend to stick to weekends but now and again I just can't say no!

I was tempted to have a little tipple tonight but by the time I got back from running my big fat --- around a five aside footy pitch....I was just too knackered..lol
May be tomorrow though!!

 

[guest119]

Has a shaver point got a centre tapped (to earth) transformer, primary side?

 

[brman]

As said, it is probably a load you haven't disconnected but it does no harm chasing it down just in case.

Just a hint though, if it is low at 250V it really isn't a good idea to test at 500V or when you do find that load you missed you might just find yourself replacing it free of charge after it has objected to 500V up its chuff....

 

[Rampantchilli]

Cheers Guys,

Yes, when I read similar threads they said the same. Why does it make it ok if you have a good Live/neutral combined IR reading? The IR between L and N is still poor and could mean a problem? (Although I'm guessing it doesn't as otherwise you guys wouldn't have said it!). Why does the good reading between L+N combined and E show that it is an appliance still not accounted for and not a wiring/damp/poor insulation problem?

If the low reading was caused by damp, I would expect to find L-E/N-E readings to be of very similar values as the L-N readings as I'd expect damp to affect all conductors.
I'd say, by your readings that you have load connected somewhere, reconnect the circuit, power up and put clamp meter on it, to see if there is any load being drawn, this may help you identify what's on there.

 

[oldtimer]

As Tel also look for a cupboard or attic light then a smoke detector or a burglar alarm