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Topic Title: Insulation Resistance; how low can you go
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Created On: 18 November 2012 07:55 PM
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 19 November 2012 11:08 PM
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Avatar for Legh.
Legh

Posts: 3435
Joined: 17 December 2004

I have just failed C2 an installation where one circuit was 0.3 Meg. On a previous PIR the IR was 8 Meg so obviously something has changed/deteriorate especially as it is T&E probably no older than 20 years. All other circuits were over 50M. Have now done investigation on the fault and one length of cable probably no longer than 7 Meters has the low IR N-E. Now just have to get them to let me replace it , tilled walls, flat roof, plastered ceiling, what fun :-)


Not always a fault of the wiring. PVC doesn't fail that easily under normal usage.

It could be
1/ New new light fitting
2/ An appliance still plugged in
3/ Class II spd somewhere.
4/ DIY tinkering

I chased a fault around a commercial garden set up for a week and discovered that the IR was down because of a combination of a couple of convector heaters used to warm up the green houses which had automatic sprinklers installed and two faulty fountain pumps.

She was very reluctant to accept her B&Q heater solution was the problem and was convinced that all the wiring had failed.

Legh

-------------------------
Why do we need Vernier Calipers when we have container ships?

http://www.leghrichardson.co.uk

"Science has overcome time and space. Well, Harvey has overcome not only time and space - but any objections."
 20 November 2012 10:37 AM
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BigRed

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well, the last dead mouse frying in the terminal box I tested last week threw out about a 0.4M reading, so I would say yes, investigate.
 21 November 2012 02:44 PM
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aligarjon

Posts: 2733
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Originally posted by: BigRed

well, the last dead mouse frying in the terminal box I tested last week threw out about a 0.4M reading, so I would say yes, investigate.


i took the lid off of a cu a couple of weeks ago and there was a mouse straddling the tails. i nearly fell off my steps.

Gary

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Specialised Subject. The Bleedin Obvious. John Cleese
 21 November 2012 03:12 PM
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AJJewsbury

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i took the lid off of a cu a couple of weeks ago and there was a mouse straddling the tails. i nearly fell off my steps.

Ah, another good point to consider for the working at height risk assessment

Across the incoming terminals of electric cookers used to be a favourite of theirs (I guess they went for the warmth).

- Andy.
 21 November 2012 04:09 PM
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Avatar for Legh.
Legh

Posts: 3435
Joined: 17 December 2004

Originally posted by: AJJewsbury

i took the lid off of a cu a couple of weeks ago and there was a mouse straddling the tails. i nearly fell off my steps.


Ah, another good point to consider for the working at height risk assessment

Across the incoming terminals of electric cookers used to be a favourite of theirs (I guess they went for the warmth).

- Andy.


Surely, once the mouse has 'boiled' down the IR should go back up again, so a temporary IR glitch wouldn't you say?



Legh

-------------------------
Why do we need Vernier Calipers when we have container ships?

http://www.leghrichardson.co.uk

"Science has overcome time and space. Well, Harvey has overcome not only time and space - but any objections."
 21 November 2012 04:16 PM
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vesuvius

Posts: 44
Joined: 19 November 2012

Hi Pat. i am going to assume your installation is a 230/400v installation and you got your results from a periodic inspection.

if all this is true none of your readings would meet the requirements for insulation resistance from GN3 (1Mohm). one of the readings is a dead short.

If it is a periodic you were carrying out i would give a C2 as though the readings are all very low the installation im assuming has been in some condition of working order, perhaps with the exception of the dead short reading.

if this was an initial inspection personally i would C1 all your readings as they are so low i couldnt take account for any degrading over time.

in any case all your readings would warrant further investigation. these are of course my own personnal opinions. I wouldnt be happy leaving any installation with those IR readings I would have to find out whats causing them.
 21 November 2012 04:39 PM
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spinlondon

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If it was an initial verification, there would not be any codes.
 21 November 2012 04:48 PM
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Avatar for vesuvius.
vesuvius

Posts: 44
Joined: 19 November 2012

Originally posted by: spinlondon

If it was an initial verification, there would not be any codes.


Quite right apologies meant 'treat as' great advert for proof reading, long week and its only wednesday.

Edited: 21 November 2012 at 05:39 PM by vesuvius
IET » Wiring and the regulations » Insulation Resistance; how low can you go

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