Minimum IR values

Thanks for the reply Andy,

looking through GN3 which states that the installation should preferably be tested as a whole and for new a new installation results should yield higher than 20Mohm, which should easily be achieved if the wiring was installed correctly, so usually this problem won’t exist when carrying out I&T.

GN3 also states that simple installations that contain no distribution circuits should preferably be tested as a whole but doesn’t say it’s a requirement, so on a simple domestic installation directly connected to the origin if the global test comes to less that 20Mohm, each individual circuit can be tested individually and if all circuits are satisfactory then no worries.

However with more complex installations where there are distributions circuits supplied from an LV switchboard for example, according to 643.3.2 distribution circuits should be tested separately with all final circuits connected and  with current using equipment disconnected. 

I interpret this to state the whole consumer unit, including the cable supplying the consumer unit should be tested together and the results of the distribution circuit itself with all the parallel resistances for all final circuits recorded on the schedule of test results for the distribution circuit, which as I stated in another post could result in all final circuits being satisfactory but the distribution circuit itself being under the recommended value, which seems a bit odd!

I hope that make sense I’m not the best with words, just trying to make sense of it all

lyledunn said:

you will note that the minimum stipulation is in chapter 64 which is concerned with initial verification

That is a good point, but a whole board IR test seems to be at odds with the requirement in 641.1 to I&T during erection. Would you not do your dead tests before connecting a final circuit at its DB?

but the distribution circuit itself being under the recommended value, which seems a bit odd!

If the overall IR (dist cct + DB + its final circuits) was under 1MΩ (and plastic wiring) then at least once circuit's IR must be way lower than normal (100s of Meg) - 1MΩ is more of 'definitely wrong' (for fresh clean wiring) than "recommended".

   - Andy.

Just for an example

Installing new 8 way single phase consumer unit and new supply to consumer unit is from LV switch board.

measured values of IR at consumer unit are as follows.

L-E : 1. 137Mohm 2. 126Mohm 3. 140Mohm 4. 168Mohm 5. 187Mohm 6. 216Mohm       7. 128Mohm 8. 119Mohm

All of the individual measurements are well over the GN3 recommendation of 20Mohm. However when testing the IR value for the distribution circuit in line with regulation 643.3.2 the reading would be in the region 18.34Mohm as all final circuits would be connected and tested together in parallel along with the supply cable.

would this then not be deemed complaint for a new installation as it is under the recommended value stated in GN3?

I have no idea what is connected to your circuits, what length they are, how many connections there are, what external influences there might be that would permit a judgement to be made. Big difference between 10m of 2.5mm2 T/E connecting to a single socket and 150m of 1.5mm2 connecting to a boat load of light fittings. 

You have established the reason for the value lower than 20Mohms (who made this up) is due to the parallel arrangement. If class one equipment was connected, say luminaires of some kind, they could have a permissible as new IR value of 1Mohm. It wont take too many before the IR value is on the floor if they are included in the test.

Just to be clear, GN3 also states that the reason IR tests are conducted is also to confirm that the insulation is not damaged. I have a 50m length of 1.5mm2 T/E cable in the training centre with every conceivable damage inflicted to it including a molten mess of sheath and inner core insulation deftly caused by a blow lamp that still indicates a +999Mohm value.

Yes the converse is important to remember, not only is the test definition necessarily very wishy washy, its usefulness is not always clear - not all faults show on an IR tester at any detectable level. The classic being the live nail problem, so long as the wall is dry and the nail only hits the live core then it may well not fail an IR test on inspection. This does not make it safe !!

It does find dead shorts OK but then so would a battery and bell set.

Far more important is an earth continuity test of some kind, either R2 and wander lead or R1 +R2

Mike.

the GN3 recommendation of 20Mohm.

I'm afraid I don't have a copy of the latest GN3 - is that 20MΩ recommended for each final circuit, or just as a 'replacement' for 1MΩ below which you really should be investigating even though it nominally complies? (i.e. acknowledging that 1MΩ is a pretty weak test for modern plastic wiring systems). If the latter then presumably it should be applies in the same context as 1MΩ - i.e. distribution circuit + DB + outgoing final circuits.

   - Andy.

Yes 20Mohms global. It frustrates me sometimes that we have a national standard that is almost usurped by guidance from the very people that feed into it. 

lyledunn said:

Yes 20Mohms global

Except for complex installations ... but I agree there is no definition of complex.

At the end of the day, the standard (BS 7671) has a different precedence in a court of law to the guidance.

The issue at hand, is that the industry wants a "rule of thumb" to apply across the board, and "hang their hat on", to remove the need for anyone to think about what they are seeing in a test result out there in the real world, but with the wide variety of installations served by the standard and the guidance, sadly the mutually exclusive nature of the "desire" vs reality shows through.

The wording in GN 3 can be read two ways, and one way is that subdivided (to a suitable level) each division having insulation resistance exceeding 20 MΩ is OK is one of those ways.

My view, is that, to understand the arbitrary limits of insulation resistance testing isn't something limited to a classroom, or book, using succinct words, but best learned by getting out there and doing the work in a variety of real installations.

When you consider that the limit in BS 7671 (at various times 1 MΩ and other close values) is only an arbitrary "industry rule of thumb" and doesn't relate directly to BS IEC 60479-1 Effect of current on human beings and livestock, my explanation above will hopefully seem clear enough