Insulation Resistance

[JohnW2]

I find that when I am testing the whole installation, it would normally be a new installation measuring form the tails. When doing work on older installation this is not always possible or practical. So I generally test each circuit individual. (I mostly work in the domestic sector)

Thanks. The impression I get is that this is not what most people do (i.e.I thought they tended to measure the whole installation, at least for starters) - am I wrong in this belief? I would have thought that it's usually going to be worth first testing the entire installation (at least, the entire cu/db) - since, if that gives a high reading, then one doesn't have to spend time testing individual circuits.

But If measuring from the tails again the guideline would be 2Mohms.
You could have say 6 circuits on a board with the overall resistance reading being 1.85Mohms. But once all circuits are tested individually there would be no fault found and all the readings complied.

I suppose this all comes down to what one means by 'no fault'. IR values even remotely that low (indeed, any which are not >>100M&#937 are clearly 'not right' - but I presume when you say 'no fault', you mean that the low IRs are due to damp/dirt/whatever, rather than to deterioration of any components of the installation.

Kind Regards, John.

 

[PrenticeBoyofDerry]

I having nothing against measuring the system as a whole or even testing earth busbar and neutral busbar across each line conductor of the circuit.
Just you would at sometime have all conductors out for any RFC and that test could be completed and for the sake of an extra 20 minutes, you could also have the conductors of most CU/fuse boxes out tested and in again. This would allow full documentation of results for any reports being made.
I would consider a fault being when a very low reading was given, a few ohms, which would indicate damaged cables, incorrect connections.

 

[JohnW2]

I having nothing against measuring the system as a whole or even testing earth busbar and neutral busbar across each line conductor of the circuit. Just you would at sometime have all conductors out for any RFC and that test could be completed and for the sake of an extra 20 minutes, you could also have the conductors of most CU/fuse boxes out tested and in again.

Fair enough,although you could obviously save yourself a little bit of time/effort if an initial 'whole installation' IR was high enough to indicate that nothing further required. Perhaps more importantly (to my mind) is that it seems prudent to limit the disconnection/reconnection of terminations to what is absolutely necessary.

I would consider a fault being when a very low reading was given, a few ohms, which would indicate damaged cables, incorrect connections.

I'm not sure I would actually call that 'IR testing' - it's more like basic fault finding Depending on exactly what you mean by 'a few ohms', one probably would not expect to see that low a leakage resistance in a circuit which was actually functioning. Indeed, if it was a circuit protected by a 30mA RCD, then any L-E or N-E resistance less than about 8,000Ω would have been tripping the RCD and (depending on exactly what it means) 'a few ohms' L-N or L-E might well have been tripping MCBs (and quite probably starting fires, since kilowatts of power would be getting dissipated, continuously, in whatever was the leakage path)!

Kind Regards, John.

 

[PrenticeBoyofDerry]

Fair enough,although you could obviously save yourself a little bit of time/effort if an initial 'whole installation' IR was high enough to indicate that nothing further required. Perhaps more importantly (to my mind) is that it seems prudent to limit the disconnection/reconnection of terminations to what is absolutely necessary.

I hear this said a lot, but assuming the reason why the IR test was being conducted in first place, was to do a PIR on an installation.
The possibility is that circuits are going to be disconnected at the DB/CU, so readings of continuity can be documented on the schedules, so doing and IR test on each circuit is not going to add much to the overall time taken and you then have readings for each individual circuit rather than a whole.
If you were only measuring the IR as to assure of it's compliance, fair enough test the whole installation. If you were in fact diagnosing a fault , it will be inevitable that at some stage the circuits would need to be split up.

 

[JohnW2]

I hear this said a lot, but assuming the reason why the IR test was being conducted in first place, was to do a PIR on an installation.
The possibility is that circuits are going to be disconnected at the DB/CU, so readings of continuity can be documented on the schedules, so doing and IR test on each circuit is not going to add much to the overall time taken and you then have readings for each individual circuit rather than a whole.
If you were only measuring the IR as to assure of it's compliance, fair enough test the whole installation. If you were in fact diagnosing a fault , it will be inevitable that at some stage the circuits would need to be split up.

I take some of those points. However, the fact remains that if the IR of the whole installation is high, that not only proves compliance but also indicates that there are no faults detectable by IR on any of the circuits - since the IR of each circuit must be at least a bit higher than the IR of the whole installation.

Kind Regards, John.

 

[PrenticeBoyofDerry]

However, the fact remains that if the IR of the whole installation is high, that not only proves compliance but also indicates that there are no faults detectable by IR on any of the circuits - since the IR of each circuit must be at least a bit higher than the IR of the whole installation.

Kind Regards, John.

I totally agree and in those circumstances we can be happy that the IR readings of each circuit will comply to BS7671:2008.

 

[JohnW2]

I totally agree and in those circumstances we can be happy that the IR readings of each circuit will comply to BS7671:2008.

Indeed, and that surely is going to be quite common with a routine PIR? Not only that, but even if you were having to use forms which demanded an IR value for each circuit (although I don't thing that probably happens), you could still write something - e.g. if the IR for the whole installation were, say, 175MΩ, then you could very honestly write ">175MΩ" (which surely would be enough to satisfy anyone) for each one of the individual circuits.

Kind Regards, John.

 

[PrenticeBoyofDerry]

You could document those figures, but I personally would take all the readings.
As stated before, it is always handy to see previous test results and if all circuits are given under the same reading. It would not be over helpful in determining if one circuit or another had deteriorated since the last test.
You would see it as a whole but not as an individual circuit.

 

[JohnW2]

You could document those figures, but I personally would take all the readings.
As stated before, it is always handy to see previous test results and if all circuits are given under the same reading. It would not be over helpful in determining if one circuit or another had deteriorated since the last test.
You would see it as a whole but not as an individual circuit.

Again, fair enough, but my personal view is that it depends on what the figures actually are - if they are very high, then they are really not going to be of much interest/value when it comes to the next PIR.

As I've intimated before, in my installation, provided I'm very careful to exclude everything which could affect the IR tests, the 'whole installation' IR on each of my sub-circuits/CUs (of which there are a good few!) is nearly always 'unmeasurable' (>500M&#937. Under those circumstances, absolutely nothing would be gained by measuring individual circuits (since each one would inevitably also be ">500MΩ" as far as my meter was concerned). Even if the figure was only 200MΩ or so, I'd think I'd still be very doubtful as to whether there was any point in measuring individual circuits.

However, it's obviously a matter of individual choice/decision.

Have a good weekend!

Kind Regards, John.

 

[PrenticeBoyofDerry]

However, it's obviously a matter of individual choice/decision.

Have a good weekend!

Kind Regards, John.

True and the same to you John