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Minimum IR values

OCB99

Hi everyone, 

First time poster so try and take it easy on me Grin

been looking at threads on this forum for quite a while now and it’s been a great help during my training so thanks everyone.

Just wanted to know your opinions on IR testing and how to apply the minimum value stated in BS7671. The regs state that the values in table 64 applies to a distribution circuit with all final circuits connected. Would that mean if on an EICR for example there were 10 circuits each measuring 9Mohms between two conductors (Very unlikely I know) it would be a fail as the total resistance for 10 circuit would be theoretically 0.9Mohm?

Also would this apply to simple installations where the consumer unit is directly connected to the origin, essentially classing the DNO cable as a ‘distribution circuit’.

Thanks in advance.

Parents

  • 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.

  • in reply to AJJewsbury

    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?

  • in reply to OCB99

    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.

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  • in reply to OCB99

    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.

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