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Topic Title: N-cpc 'faults' advice sought please
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Created On: 16 April 2013 06:12 PM
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 16 April 2013 06:12 PM
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BrucieBonus

Posts: 710
Joined: 20 February 2007

Hi all

Some collective brain power would be much appreciated!

Did an EICR today in advance of board upgrade and other work. Cables are all singles in steel conduit, with conduit as earth. IR tests L-N and L-cpc all perfect. IR tests N-cpc (with N's all out of N bar) as follows:

Lights (2 circuits) 500 MOhms
Cooker 500 MOhms
2 socket circuits and a couple of unused power circuits (probably for old heaters) - dead shorts.
I then tested continuity between N and cpc on these circuits and got:
0.17 Ohms, 0.21 Ohms and 0.34 Ohms

Given that the other IR tests were all 500 MOhms and these continuity readings are very low, I'm not convinced that there is an insulation resistance problem...... am I right to question this? Or am I just being too hopeful!?!

I was unable to investigate any further (splitting rings etc) due to time constraints today

The board has 3036's and the Ze was 0.14. The flat is in a big block. Unable to get access to main intake today.

Any thoughts?

Thanks

BB
 16 April 2013 06:52 PM
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dbullard

Posts: 1166
Joined: 02 March 2006

Originally posted by: BrucieBonus






2 socket circuits and a couple of unused power circuits (probably for old heaters) - dead shorts.

I then tested continuity between N and cpc on these circuits and got:

0.17 Ohms, 0.21 Ohms and 0.34 Ohms



Any chance of items plugged in or still connected ??? would point to that or old boxes covered in plaster and redecorated with the neutral touching the bb's and then coming back on the conduit.

Is the supply PME / TNC-S


Just my humble thought.

Regards

Daren

-------------------------
..... Dont pee in my pocket and tell me its raining ......


www.quest-electrical-sw.co.uk
 16 April 2013 07:17 PM
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michaelbrett

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Joined: 28 December 2005

BB

A stupid question, you did isolate the supply wiring via the Main Switch at the Consumer Unit prior to testing?

Regards

Mike
 16 April 2013 08:14 PM
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BrucieBonus

Posts: 710
Joined: 20 February 2007

Thanks for quick replies:
Daren
No items plugged in (N-L tests all good)
Accounted for all the sockets - so don't think any had been plastered in - and it would have to be 1 on each circuit anyway to give those results - it's only a 2 bed flat
Supply I think will be TNCS - but unable to get access today

Mike

I did switch off, but it's a very old board, so it did occur to me that the main switch might not be linked. So I then took out each circuit from the N bar. 3 tested fine and the rest... rubbish

I'm beginning to think that there must be N-cpc faults.... ARRRG!

BB
 16 April 2013 10:46 PM
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antric2

Posts: 1068
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Could there be any swfcu or with neons or old timers hidden away anywhere in a cupboard somewhere that were used to switch the heaters on maybe.
 16 April 2013 10:58 PM
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peteTLM

Posts: 3187
Joined: 31 March 2005

over the years ive had faceplate screws chafe/ pin neutrals against back box, trapped against the edge of the box, skirting screwed through the cable neutral and earth but not live, breakdown of socket causing short, cable eroded by being buried directly in concrete, rats chewed cable, floorboard put down against cable squashing it against central heating pipe..............all neutral/ earth faults.

Drop all the sockets off, draw a route map by doing a simple long lead continuity, and then individually IR each leg. Find it and look at whats happened in the vicinity.

Regards,

-------------------------
----------------------------------------
Lack of planning on your part doesn't make it an emergency on mine....

Every man has to know his limitations- Dirty Harry
 16 April 2013 11:19 PM
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Parsley

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Hidden junction boxes, was the fault to the cpc only or the met?

Regards
 17 April 2013 09:57 AM
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AJJewsbury

Posts: 11553
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No items plugged in (N-L tests all good)

If the appliance is plugged in, but (SP) switched off, the L-N test will show good, but the element or whatever will still be connected to N and PE - so a fault there could still cause you problems on a N-PE test.

I with pete though, my money is on faceplate screws nicking the N - if they hit L it goes bang, so gets noticed and fixed. If they hit N there are no obvious symptoms, so the installation gradually increases the number of N-PE faults over time.

I guess another possibility is that someone has got the N and PE connections the wrong way around (not their usual brand of socket), so N gets connected to the conduit via the faceplate screws (presuming they have actually used a fly-lead for the earth). Again symptomless most of the time.

- Andy.
 17 April 2013 11:19 AM
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WiredScience

Posts: 102
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Originally posted by: BrucieBonus

I then tested continuity between N and cpc on these circuits and got:

0.17 Ohms, 0.21 Ohms and 0.34 Ohms

BB


Assuming you have a proper N-E short, and 2.5/1.5 cable, my calculations estimate distance to faults of approx 7m, 9m, and 14m respectively. Can you guess / trace the cable route?
 17 April 2013 11:52 AM
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AJJewsbury

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Good call, but OP said "Cables are all singles in steel conduit"... so probably a fair bit longer than that.
- Andy.
 17 April 2013 01:22 PM
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BrucieBonus

Posts: 710
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Thanks again for the replies everyone - lots of food for thought.

I'm back there on Monday (to do the Pete TLM method ) so will report back. I'll also check again for any hidden connected items

It just struck me as too much of a coincidence that all 3 socket circuits had the 'same' fault - but of course there is no reason why not! Install not been touched for a very long time.....

Parsley - no jb's (concrete floor/ceiling) and as conduit is the cpc for all circuits, I could only test to the MET

Cheers all

BB
 17 April 2013 04:13 PM
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Parsley

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Originally posted by: BrucieBonus

Thanks again for the replies everyone - lots of food for thought.



I'm back there on Monday (to do the Pete TLM method ) so will report back. I'll also check again for any hidden connected items



It just struck me as too much of a coincidence that all 3 socket circuits had the 'same' fault - but of course there is no reason why not! Install not been touched for a very long time.....



Parsley - no jb's (concrete floor/ceiling) and as conduit is the cpc for all circuits, I could only test to the MET



Cheers all



BB


Sorry BB missed the steel conduit, I shouldn't post after 9PM!

Regards
 17 April 2013 05:07 PM
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Simannjo

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Assuming an accessory screw nicking the Neutral, rather than drop all the sockets n stuff off, check N-CPC resistance on low ohms at all sockets and the lowest reading is where the fault is (or very close). It often goes to zero...

If more than one N-CPC fault exists on the circuit when the first fault has been rectified, the reading will change and then its a hunt to find the point of the lowest reading again...
 17 April 2013 05:28 PM
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BrucieBonus

Posts: 710
Joined: 20 February 2007

Hey - that's a great tip!! Thanks

(why do I never think of these things???)
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