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

[Engineer54]

As did I (I said the whole "installation or part thereof"). Simple installations with no distribution circuits should be tested as a whole.

And I don't agree that it is nonsense.

Recording the Global IR for each circuit ''IS'' 110% Nonsense!! As i have stated earlier above, Global IR tests are fine for a general check(s), but meaningless as far as recording purposes go.

A major reason for circuit IR values being recorded is to determine if the IR value or values over the working life time of the circuit is noticeably degrading, not just to test if it's compliant at each time of testing.
It was drummed into us as apprentices, that you are looking for any ongoing downward tendencies, in order that they maybe investigated and rectified before a categoric failure occurs...

But then if you don't come from a maintenance/industrial based background you wouldn't know that. It seems they don't teach you much about the reasoning behind periodical testing these days, even within the dedicated testing and inspection courses such as 2391 or the latest 2394/95...

 

[Risteard]

Of course I understand about degradation, but if all of the circuits in parallel read say 50 Megohms (50 million Ohms), then we know that every circuit is in excess of this and there is nothing to prohibit us from recording this. Lower insulation resistances are often indicative of long runs etc. and not of insulation failure.

 

[davesparks]

But that doesn't indicate anything useful in the records.
If the global test shows 500meg on year one, 450 on year two, 300 on year three then the conclusion is that the whole installation is going to be knackered in a few years.
However if it is done properly and each circuit recorded, the trend on the one circuit can be identified and dealt with.

 

[Engineer54]

Of course I understand about degradation, but if all of the circuits in parallel read say 50 Megohms (50 million Ohms), then we know that every circuit is in excess of this and there is nothing to prohibit us from recording this. Lower insulation resistances are often indicative of long runs etc. and not of insulation failure.

Why did i just know you were going to come back with this kind of, load of old codswallop...

But unless you conducted individual circuit IR tests, you wouldn't know that!! One circuit could, or should i say, will pull down the global IR value, but you would have no idea of this fact. You could/would be ''recording'' an inaccurate value without even knowing about it.

Please tell me this is not your normal method of conducting an installations IR testing??

As a matter of interest, Is there anyone else here, that conducts a global IR test and uses that value, for recording individual circuit IR values on official test record sheets???

 

[Badged01]

....Is there anyone else here, that conducts a global IR test and uses that value, for recording individual circuit IR values on official test record sheets???

Don't know about on here, but I stumble across lots of re-EICR's where that is exactly what has been done before. And usually its just a test between L&N to E!. It's the quick fly-by-night solution ......

 

[Engineer54]

Don't know about on here, but I stumble across lots of re-EICR's where that is exactly what has been done before. And usually its just a test between L&N to E!. It's the quick fly-by-night solution ......

Hey, i can understand the above when were talking about EICR's and lighting circuits, (L&N -E) especially with commercial/industrial situations where lighting is at high level. But using global IR testing as a cover all for all recording purposes even initial/first testing of an installation, just isn't on. I don't care what it say's or doesn't say in BS7671 or it's guides. ...It's as you say, a ''fly by night solution'' and a nonsense!!

 

[Risteard]

It's actually a fly-by-night solution to record the individual circuit insulation resistances without actually working out what the parallel resistance is. It is impossible to comply with BS 7671 by this method.

There would be no means of knowing whether the installation complies.

 

[davesparks]

Risteard - what is the purpose of an IR test and what is the purpose of recording the result once carried out?

 

[Pete999]

Risteard, cn you explain your reasoning regarding your last post?

 

[Hawk]

Simple installations that contain no distribution circuits should be tested as a whole.

 

[davesparks]

Simple installations that contain no distribution circuits should be tested as a whole.

Explain please?

 

[Engineer54]

It's actually a fly-by-night solution to record the individual circuit insulation resistances without actually working out what the parallel resistance is. It is impossible to comply with BS 7671 by this method.

There would be no means of knowing whether the installation complies.

Are you actually being serious in what your saying here, or is this a wind-up??

 

[Badged01]

Don't know if its a wind-up, but I'm starting to wonder whether people actually understand the concept of what IR testing is about!

Im too polite, so we need some straight talking on the subject from that Glenn fella ..........

 

[Richard Burns]

Now GN3 specifically states that for both initial verification and for periodic testing that insulation resistance testing should be undertaken as a block measurement of a group of circuits (such as distribution boards), with simple installations measured as an entire unit.
The preference is to measure insulation resistance between live conductors and live conductors and live conductors and earth (with cpcs connected), but there is the option to test between earth and the live conductors connected together if the first method is not practical.
I have only just realised that initial verification permits group testing, though I would normally use this for EICRs.

From the point of view of the regulations this is permitted and correct.

For fault finding and diagnosis over time the testing of individual circuits provides a much better indication of the condition of circuits and allows detailed investigation and assessment to be undertaken. It will also provide a much better indication of the state of the installation on a circuit level.

If you have measured IR on separate circuits and levels are low then it is possible that the overall insulation resistance measurement may be below that specified in the regulations (overall resistance is lower than the lowest value of parallel resistances), but I believe that if all individual circuits meet the regulatory limits then the installation is satisfactory (this is perhaps suggested in GN3, but not stated).

I would normally test individual circuits on initial verification and group testing on periodic inspections.
However either way round can be done in either circumstance and provide an allowable result that can be recorded.
Perhaps a quick calculation may be useful if individual IR results are low just to check total resistance as a reference but it is very easy to drop below the limits on lots of circuits (e.g 10 circuits at 15MΩ would drop very close to 2MΩ overall).

 

[Engineer54]

Now GN3 specifically states that for both initial verification and for periodic testing that insulation resistance testing should be undertaken as a block measurement of a group of circuits (such as distribution boards), with simple installations measured as an entire unit.

Does it, well yet another area of GN3 to be totally ignored and kicked into touch!! Never heard such load old crap, who writes this S***e for the ITE??