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tommys88

Evening chaps,
In the process of taking my 2394/2395 testing course and just trying to fully get my head around insulation resistance testing predominantly in occupied domestic property's where it isn't possible to dissconnect all loads from the circuit.
I am aware of the need to link out the line and neutral to then test to CPC, by linking out what would people recommend the best way to do that is a simple crocodile clip from the busbar to neutral bar?
Then is it a case of having all breakers on and testing all circuits on the rcd or best to individually test at the top of each mcb?

Cheers
 
If you can turn everything off you can link up L and N , L and CPC , N and CPC at board with all circuit breakers on and test , obviously making sure any voltage sensitive stuff is removed fro circuits, flicking light switches and testing to make sure you are testing all sequence of switching. Then if there is an issue you turn each circuit off till you find the fault of there is any and go from there.
 
What does your course tutor suggest?

You'll be should be aware that the more resistances in parallel, the lower the overall insulation resistance. Therefore with installations containing a large number of circuits, measured in parallel, a 'pessimistically' low value of insulation resistance might be obtained.

In a new installation, I would measure each circuit individually to verify the circuit. In some circumstances, prior to replacing a CU for example, a 'global' insulation resistance test might be prudent, might be a quick way to ensure there's no underlying faults to be encountered.

Therefore I would isolate the circuit live and neutral and connect together and test to earth bar (in a CU for example). It would always be best to isolate the circuit, from loads and test appropriately.

Kewtech manufacture those crocodile clips you mention, but in my experience the actual clips introduce a varying resistance into the test.

Also note, that leaving RCD's & RCBO's connected would affect the test.
 
Actually not pessimistically low but meeting the Requirements of BS7671. The figures quoted are for installations or DBs in larger installations rather than individual final circuits.
 
Cover your back for the exam. Disconnect the line neutral and CPC completely then use a connector block between line and CPC and then test at every accessory.
 
Actually not pessimistically low but meeting the Requirements of BS7671. The figures quoted are for installations or DBs in larger installations rather than individual final circuits.

Where installations containing large numbers of circuits, are measured in parallel, a pessimistically low value of insulation resistance might be measured. Consideration should be given to testing in smaller sections, with fewer circuits. Citation; notes from my 2392-10 course. If that's incorrect, I would be grateful for any correction.

PS I'm not being sarcastic, any correction would be useful
 
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PS I'm not being sarcastic, any correction would be useful
Absolutely no problem at all. (Apologies for only getting round to responding to your post now.)

I suggest you read in particular Regulation 612.3.2
 
Absolutely no problem at all. (Apologies for only getting round to responding to your post now.)

I suggest you read in particular Regulation 612.3.2

Thanks for your reply, and apologies myself for not replying sooner.

I've read before reg 612.3, Table 61 minimum values and particularly 612.3.2 '....with all it's final circuits connected'. However, I still refer to my course notes, about consideration of splitting up an installation into smaller sections to test for insulation resistance, because of the 'resistors in parallel effect'.

My understanding from my training, I might be wrong here, that a figure below 2Mohm should be further investigated albeit an acceptable figure if Table 61 minimum result is read. Therefore larger installations would need breaking down into smaller sections?
 
The 2M figure is a rule of thumb. In reality if I got a figure anywhere that low it would ring alarm bells.
Daz
 
I always test individual circuits but ALWAYS do a universal test of the whole installation also, as this can pick up faults that may otherwise be overlooked.
 
I always test individual circuits but ALWAYS do a universal test of the whole installation also, as this can pick up faults that may otherwise be overlooked.
I likewise on a CU for example, just to record individual readings for future reference. However, those readings in the low 10's, could give an overall low reading, which would need consideration (to Table 61 minimum). Like Daz never had such a low reading (2Mohm) yet! Except when I've discovered neutral earth fault.
 
I had a reading <1Mohm on a new coil of t&e a few years back, visually inspected it and couldn't see any obvious issues. Needless to say I didn't use it!!
 
Blimy that's not good. What did supplier day? Presumably they had other reels with the same problem if they were from the same batch.
Daz.
 
Blimy that's not good. What did supplier day? Presumably they had other reels with the same problem if they were from the same batch.
Daz.
I remember they were a bit funny about it actually, they swapped it, never heard anything else about it. Does anyone else IR test new cable BEFORE installation or is it just me?
 
I remember they were a bit funny about it actually, they swapped it, never heard anything else about it. Does anyone else IR test new cable BEFORE installation or is it just me?
Nope, might do now, I always IR my new installations, but then the horse has bolted :)
 
Good idea to test new cable. I haven't but think I might.
Re IR tests I prefer to test each circuit. For the time it takes to test 6 or 8 circuits its my preference.
But in a normal size domestic even a parallel test I'd expect to get a sensible reading never seen 2meg or close to it unless a fault.
 

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