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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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 18 November 2012 07:55 PM
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PatHammond

Posts: 215
Joined: 04 December 2008

Not been on the forum for a longtime; hello again everyone

Background: EICR for entire housing assoc built 1990. EICRs undertaken by 3rd party company gives C2 for all sockets on ground floor that aren't RCD protected; fair enough. As part of the EICR the company has listed a limitation for all circuits on insulation resistance testing which seemed a bit of a cop out to me. Any thoughts why they don't do IR tests? Anyway I digress.

As the maintenance electrician I am now addressing the C2s by retrofitting RCBOs on the offending circuits whilst undertaking the required testing on the circuits which in a number of cases is throwing up the expected problems like broken rings and low IR readings less than 1 meg. For broken rings I'm installing 20A RCBOs instead of 32A. But what about the low IR readings. I can do an hour or so investigations to find the problem but there is a point at which I need to go back to the housing assoc and recommend a more comprehensive investigation of the low IR readings. My feelings are that I may just put it down as a non compliance with current regs on the minor works cert if the IR is greater than or equal to 0.5 M ohms and less than 1meg. Would you agree with this.


And at what point should I be flagging it as a potentially dangerously low IR readings. Less than 0.5 meg? Just where do I draw the line with this

Is any danger mitigated by the new RCBO ie it will be safer with an RCBO than it was with just the type B.

Does anybody have any thoughts on any of this.

Many Thanks

Pat

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 18 November 2012 08:16 PM
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jcm256

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How will you know without testing is/are the ring breaks a clean cut off i.e. the neutral might go down another path. Of course, you will know because a common cause of RCD trips usually occurs when the RCD fitted, for the first time as they show up Neutral-Earth faults not previously known to exist. Don't test just with a neon lamp type tester and say it's ok and walk away put a load on. And the loop test.
Only trying to be helpful, you seem like a sound chap, trying to protect the previous chap .
Good luck
Regards
 18 November 2012 08:30 PM
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spinlondon

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Shouldn't be a code C2 for all ground floor socket-outlets, should be C3 for most, and only C2 for those used to supply mobile equipment outdoors.
If the IR readings are too low, the RCBOs will operate.
Which conductors are providing the low readings?
Anything below 1MΩ should be flagged up as being dangerous.
To be honest the poor IR readings are probably a greater danger than the lack of RCD protection for the socket-outlets.
 18 November 2012 09:55 PM
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Martynduerden

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Originally posted by: spinlondon

Shouldn't be a code C2 for all ground floor socket-outlets, should be C3 for most, and only C2 for those used to supply mobile equipment outdoors.


Opinions spin, Opinions I would likely code them C2 if they were likely to be used for outdoor equipment.

Bear in mind that in a good amount of properties have all ground floor sockets are on one circuit therefore I would view it as a circuit code as opposed to an item code.

And of corse all ground floor sockets could be reasonably used for outdoor equipment.

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 18 November 2012 09:57 PM
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PatHammond

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All the ground floor and kitchen are on 2 circuits and all could be used for equipment outdoors and I'm not going to argue the toss with a 3rd party inspector. I agree that low IR can be more of a problem than no RCD hence my query about why a national I&T company are putting it as a limitation.

As for the less 1meg IR being dangerous; when I first trained up under 16th edition the reg was 0.5meg so it was akright back in 2008 not that long ago. At a recent NICEIC inspection I had this discussion with my area engineer about whether an IR of 0.8Meg on a circuit should have been a C2 or C3. He reckoned you could put it as C3 but then bring forward the date of the next recommended inspection.

Anyway, what would you do if you find a circuit with an IR 0.5 meg. Do you fit the RCBO and make recommendation for the low reading to be investigated. Do you not make any changes to the circuit until the problem is sorted out or do you fit the RCBO draw up the minor works but state on the minor works that it doesn't comply with the regs.

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 18 November 2012 11:27 PM
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spinlondon

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There was me thinking that an EICR should be conducted to the current edition of the Regulations.
How does one justify applying a code for a non-compliance with an out of date Regulation?
Then again if you apply codes for non-compliance with a Regulation that doesn't exist, I guess anything goes.
What do you do if someone asks you to quote the particular Regulation that's not being complied with?

Pat, if all you've been tasked with doing, is installing RCBOs, then yes make a note of the poor IR reading on the MEIWC.
If you've been tasked with ensuring the installation is safe, then sort out the poor IR.
 19 November 2012 06:39 AM
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Jobbo

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If any limitations are including in a EICR, than a reason why, should be included for justification. (Perhaps they couldn't isolate a particular circuit perhaps).
 19 November 2012 08:16 AM
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PatHammond

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Absolutely Spin, EICRs are undertaken against the current version of the regs so IR>1meg is a non compliance but then we have a sliding scale of severity of the danger this presents starting with a C3 and ending with a C1 and I'm asking where does one draw the line with IR.

I was tasked to fit the RCBOs but so I can just do this and note low IR on the MWEIC but because the company who did the EICR put a limitation on all IR testing on all 30 properties I'm the one now who is picking up these other non compliances and am wondering how to proceed.

Perhaps the question I should be asking is: what code would you give the following.

IR = 0.95 Mohms
IR = 0.5 M ohms
IR = 0.1M ohms

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 19 November 2012 09:24 AM
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spinlondon

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I would suggest that you apply Ohms law, to determine at what level of IR each RCBO will operate.
Depending upon the sensitivity, you may find all of the values listed will cause an RCBO to operate.
 19 November 2012 09:40 AM
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AJJewsbury

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I would suggest that you apply Ohms law, to determine at what level of IR each RCBO will operate.
Depending upon the sensitivity, you may find all of the values listed will cause an RCBO to operate.

15mA @ 230V = about 15.3k Ohms (or 0.0153' Meg)

1 Meg = 0.23mA (presuming no capacitive components)

- Andy.
 19 November 2012 11:17 AM
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unshockable

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Hi all

Not sure about the OT but surely IR limits were the historical intersect with the ESQCR?

The installation as a whole needed to give a sufficiently high IR to stop large cumulative earth currents circulating on the TNS networks. In an era of PNB, PME, non-linear loads etc they have less relevance?

Can anyone remind me of the limits imposed historically by the ESQCR and the wiring regs?

Ducking down and taking cover.

Simon
 19 November 2012 11:28 AM
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rocknroll

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Can anyone remind me of the limits imposed historically by the ESQCR and the wiring regs?


In general >0.5 was never seen as a problem then and is still the case across the world even now.

regards

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 19 November 2012 11:37 AM
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Legh

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That is the problem with prescriptive evaluations when doing a PIR/EICR. You end up with a definitive answer but to get there you would have to disconnect every load, disconnect and link out every switching device from every circuit to establish a compliant insulation value of the cables.
Not a very practical solution. and doesn't really follow the ethos of GN3.

It is IMO quite common to have some circuits for various reasons with IR values lower than the latest up to date prescribed values. That's not to say that a sample for suspect circuits is not investigated, but does involve the inspector in taking a view on whether it is dangerous or not.

I think that might be something to do with experience .....

Legh

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 19 November 2012 11:44 AM
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Legh

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Originally posted by: unshockable

Hi all

Not sure about the OT but surely IR limits were the historical intersect with the ESQCR?

The installation as a whole needed to give a sufficiently high IR to stop large cumulative earth currents circulating on the TNS networks. In an era of PNB, PME, non-linear loads etc they have less relevance?

Can anyone remind me of the limits imposed historically by the ESQCR and the wiring regs?

Ducking down and taking cover.

Simon


I think the acceptable earth leakage for a running installation is 1/10000 of the supply fuse rating. I've got the old paperwork somewhere.

That would be for a 100A service fuse 10mA or approximately 0.23M ohms.

Legh

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 19 November 2012 12:30 PM
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potential

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Originally posted by: Legh


That would be for a 100A service fuse 10mA or approximately 0.23M ohms.



Legh

I think you are a decimal point out.
@230V a 10mA current using Ohm's law would indicate a resistance of 23,000 ohms which is 0.023 Mohms.
 19 November 2012 12:33 PM
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Legh

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Originally posted by: potential

Originally posted by: Legh

That would be for a 100A service fuse 10mA or approximately 0.23M ohms.

Legh


I think you are a decimal point out.

@230V a 10mA current using Ohm's law would indicate a resistance of 23,000 ohms which is 0.023 Mohms.


I think you could be right....I was thinking of putting 23kOhms but thought that might be confusing....
Thank you for the correction.

Legh

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 19 November 2012 05:10 PM
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KFH

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Just a point on the old regs, the 14th Edition said not less than 1M for wiring. It does mention 0.5 M for equipment casing to live. It also suggests breaking large installations into groups of outlets each containing not less than 50 outlets.

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 :-)
 19 November 2012 05:43 PM
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potential

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When I started testing wiring in the 60s it was accepted that weather conditions should be taken into account when recording readings.
Almost certainly many of the low resistance readings encountered are caused by condensation forming on the wiring and associated boxes etc.
 19 November 2012 08:13 PM
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spinlondon

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Is it not the case, that the longer you conduct the test, the water evaporates due to heat in the conductors and the reading increases?
 19 November 2012 10:10 PM
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PatHammond

Posts: 215
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any one fancy answering this question; what EICR codes would you give the following



IR = 0.95 Mohms

IR = 0.5 M ohms

IR = 0.1M ohms

IR= 0.05

IR (N-E) =0 ohms

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