Insualtuion Resistance - individual circuits V bank of MCBs & RCD readings

[Engineer54]

No idea what you mean by "L.O.C.", but insulation resistance should be tested with all circuits in parallel (and with the means of Earthing connected) rather than each circuit. If you test circuits individually then you would need to calculate the parallel resistance to see if it complied with the minimum acceptable for BS 7671.

Unless i'm misreading this, Pass that by me Again please!!! lol!!

 

[Risteard]

No probs. I was just saying that the minimum insulation resistance quoted in BS 7671 is for the installation/dis board, and as such if individual readings were taken for different circuits then the effect of the parallel resistances on the overall insulation resistance would need to be considered.

 

[Meggerman]

No probs. I was just saying that the minimum insulation resistance quoted in BS 7671 is for the installation/dis board, and as such if individual readings were taken for different circuits then the effect of the parallel resistances on the overall insulation resistance would need to be considered.

That is incorrect. The insulation resistance values quoted are for individual circuits and that is why you record it as such. If you carry out a global IR then all the circuit resistances are in parallel so will be lower than any individual circuit. If global IR gives you an acceptable level then you need do no more and record all as that value.

 

[davesparks]

No probs. I was just saying that the minimum insulation resistance quoted in BS 7671 is for the installation/dis board, and as such if individual readings were taken for different circuits then the effect of the parallel resistances on the overall insulation resistance would need to be considered.

Think you've hot the wrong end of the stick there!

Insulation resistance is tested for each circuit, if you tried doing one global test on a large installation it will come out with a seriously low value.

 

[Pete999]

What is L.O.C.? Please

 

[Engineer54]

That is incorrect. The insulation resistance values quoted are for individual circuits and that is why you record it as such. If you carry out a global IR then all the circuit resistances are in parallel so will be lower than any individual circuit. If global IR gives you an acceptable level then you need do no more and record all as that value.

Global IR's are fine for initial checking, but are totally meaningless as far as official recording of circuit IR values in say EICR etc.... If anyone accepts a global IR value of an installation on any official recording medium, something somewhere is very Wrong!!

 

[Hawkmoon]

What is L.O.C.? Please

Yes that's that new approved test......Licking Of Conductors

 

[Octopus]

Yes that's that new approved test......Licking Of Conductors

Best done when you've mastered safe isolation!

 

[Badged01]

Global IR's are fine for initial checking, but are totally meaningless as far as official recording of circuit IR values in say EICR etc....

Agreed, but it seems to be the cheap and cheerful way of doing it for your £50 drive-by landlords special. Worst still is they only do a L/N to E test so they don't need to disconnect anything!. Guess if you put down what you're doing in the limitations box ..........

 

[Motty]

Yes that's that new approved test......Licking Of Conductors

Brilliant :clap::clap:

Low Ohm Continuity

All circuits, bar circuit 9, individually, came out as >999 M ohm across L-N, L-CPC, and N-CPC. it was only when I connected them all up to the RCDs, and doing a collective test, that the L-N came out as 0.06 M ohm on RCD 2 / Circuits 5-9.

With circuit 9 disconnected, circuits 5-8 all read >999 m ohm, and Circuit 9 caused the drop. testing circuit 9 individually across its respective conductors gave the following:

L-N = 600 M ohm
L - CPC = >999 M ohm
N - CPC = >999 M ohm

I got in a spin when connecting it up and got that 0.06 reading across L-N as a "collective".

Additionally I can't connect the CCU to the main earth as the supply cable hasn't been installed by the DNO yet.

I did leave an extractor fan connected on circuit 9, although off at the 3-pole isolator. As its a load, I should have really pulled the wires out. It had been a long day and I was panicking a bit when I started this topic ...

Cheers for all the replys, you have all given me fresh hope!

 

[Pete999]

Yes that's that new approved test......Licking Of Conductors

Nice one

 

[Risteard]

That is incorrect. The insulation resistance values quoted are for individual circuits and that is why you record it as such.

I disagree. My understanding is that the reason for the box on the forms next to every circuit was simply to make the form look tidier.

However the Regulations require the minimum insulation resistance of 1 Megohm for the installation/part thereof and not for an individual circuit, therefore the effect of parallel resistances need be considered. In fact, Guidance Note 3 even suggests that insulation resistance should be tested with all circuits in parallel and not individually (except to find the cause of low readings of course).

 

[Hawkmoon]

I disagree. My understanding is that the reason for the box on the forms next to every circuit was simply to make the form look tidier.

However the Regulations require the minimum insulation resistance of 1 Megohm for the installation/part thereof and not for an individual circuit, therefore the effect of parallel resistances need be considered. In fact, Guidance Note 3 even suggests that insulation resistance should be tested with all circuits in parallel and not individually (except to find the cause of low readings of course).

If you get a lowish reading on one circuit, say 40 Meg (low but still acceptable) wouldn't it be appropriate to record it individually on the cert?

 

[Meggerman]

I do more than consider it, I test it!! I don't understand your post, you are saying you disagree but then also saying that GN3 suggests IR tests are done with all circuits in parallel.

 

[Meggerman]

I would not call 40M/Ohms low IR

 

[Hawkmoon]

I would not call 40M/Ohms low IR

It is if new install and all other circs are >200 - I was just making a point that if a circuit is unexpectedly low with no fault then it is right to record it - even if just for future monitoring of insulation deterioration.

 

[Octopus]

I use a Metrel and if I do all 3 cores automatically, I get >30

Low IR is less than 2!

 

[Hawkmoon]

I use a Metrel and if I do all 3 cores automatically, I get >30

Low IR is less than 2!

Yes but my point was mainly about individual circuit IR readings and the merits of separately recording them on a cert...

Your 'low' IR being < 2 Meg is ok but if you had a reading of say 4 Meg would you not consider that at least a code 3 to be investigated? At least record on the cert?

Or am I misunderstanding the point of your post?

 

[Octopus]

Yes but my point was mainly about individual circuit IR readings and the merits of separately recording them on a cert...

Your 'low' IR being < 2 Meg is ok but if you had a reading of say 4 Meg would you not consider that at least a code 3 to be investigated? At least record on the cert?

Or am I misunderstanding the point of your post?

Not a code 3, if I was testing as part of a board change I would comment on it and on an EICR I would recommend a retest in 3-5 years. The BIG issue with IR tests is that so many now seem to be done as L&N to E so the "real picture" isn't disclosed.

 

[Engineer54]

In fact, Guidance Note 3 even suggests that insulation resistance should be tested with all circuits in parallel and not individually (except to find the cause of low readings of course).

Are you sure you are reading correctly?? If that is actually what is being suggested in GN/3 then get a black marker and black it out because it's WRONG!!

Nothing wrong with conducting a global IR, but you DON'T record the global IR value, on official test record sheets, as the value for every circuit on a DB/CU, that is just plain bonkers!!

Do you know the reason for recording circuit IR values on official test reporting sheets??