Insulation Resistance...

[junksamiad]

I am just working through my understanding of the insulation resistance test, and I want to find out the best way to do the combined Live-Earth test where the Line and Neutral conductors are connected (joined, or however it is best described) through one probe of the tester, and the other probe is connected to the Earth conductor (or terminal).

I have seen a few videos on you tube, one where a guy is using a Megger (but with an older probe attached) to combine the Line and Neutral, where he puts a croc clip on the Neutral and then clips the other end to his tester probe which is then on the Line, thus creating the link. Of course, the other probe is then on the Earth. This seems logical, but I am using a Kewtech KT63 which doesn't give much on the end of the probe to connect a croc clip to it. Just wondering if there is another accepted way out in the field of doing this?

Thanks in advance for any help and advice.

 

[Deleted member 9648]

You can connect the line and neutral together in a connector block, then poke your probe in.
I would disconnect the CPC if I were testing a single circuit.

What would be your reason for disconnecting the cpc?

 

[goasis]

Had a fault once on new install where a plumber had kindly screwed through a cable when fitting a rad. The screw had only made contact with the live and then into the fabric of the building (brick) IR testing showed up the fault, however, when I removed the cpc from the earth bar the fault dissappeared. The fault path was presumably through building fabric, metal backboxes and back, probably via socket circuit as it was local to the fault. 1 reason to leave cpcs connected when testing IR.

 

[junksamiad]

Thanks for the responses. Everyone's feedback has helped me fill in a few gaps and iron out a few uncertainties, which is very helpful.

Thanks again.

 

[Simonslimline]

Test between live conductors and live conductors and earth. If the cpc is not connected to the means of earthing then the test is not being carried out between live conductors and “Earth”.

 

[Deleted member 26818]

What would be your reason for disconnecting the cpc?

Because I want to know the IR of the circuit conductors, not the IR of every CPC, bonding conductor, earthing conductor, washing machine, dishwasher, toaster, microwave, fridge freezer, kettle, water pipes, gas pipes and whatever else is connected to the earthing system.

 

[johnduffell]

Because I want to know the IR of the circuit conductors, not the IR of every CPC, bonding conductor, earthing conductor, washing machine, dishwasher, toaster, microwave, fridge freezer, kettle, water pipes, gas pipes and whatever else is connected to the earthing system.

But that's the whole point of ir testing! You want to make sure there's no potential path between any live conductors and anywhere else conductive.
Otherwise it would be a case of 'sorry you got an electric shock from the taps, i didn't test that' or just RCD tripping issues.
Obviously if you get a low reading you would then start disconnecting things to narrow it down, but for the record test result you need to include as much as possible on the "non live" side.

 

[Deleted member 9648]

Because I want to know the IR of the circuit conductors, not the IR of every CPC, bonding conductor, earthing conductor, washing machine, dishwasher, toaster, microwave, fridge freezer, kettle, water pipes, gas pipes and whatever else is connected to the earthing system.

But you are testing the IR of the live circuit conductors....you are just including actual earth rather than just a single copper wire cpc.
The logic of your post defeats me.

 

[Deleted member 26818]

You are aware that Insulation Resistance decreases as the length of a cable increases?
Leaving the CPC of the circuit being tested connected to the earthing system will produce an unrealistic low value.

 

[Deleted member 9648]

You are aware that Insulation Resistance decreases as the length of a cable increases?
Leaving the CPC of the circuit being tested connected to the earthing system will produce an unrealistic low value.

And disconnecting it could miss a fault, unless you carry out a separate test from the live conductors to true earth, pointless to disconnect it.

 

[Ian1981]

If a number of circuits in parallel are being tested at once such as a global test, then yes the insulation resistance will decrease or simply take longer to reach a high value but for simply testing one circuit live conductors to the means of earthing, then I’m sorry but that statement is so incorrect and does not comply with the regulations and misses the point of Why we are testing to earth as a whole and not simply the cpc.

 

[goasis]

You are aware that Insulation Resistance decreases as the length of a cable increases?
Leaving the CPC of the circuit being tested connected to the earthing system will produce an unrealistic low value.

Surely there's nothing "unrealistic" about it, if there is a low resistance between the live conductors and any part of the earthing system in the install then that is worth knowing about.
BTW please view this as part of a discussion, I'm along way off knowing it all, here to learn!!

 

[davesparks]

Thanks teletrix and wirepuller. I guess I will do the ir correctly from now on.

Interested to hear if anyone else was taught my way by disconnection of cpc from board. Or is it just me?

I was taught to use the method most appropriate to what I'm doing.
If it's testing for inspection and testing purposes then test with all cpcs connected.
If its fault finding etc then sometimes you want/need to test with the cpc disconnected.

 

[a-z electrics]

The most realistic IR test ie mimicking actual working conditions is surely to test with ALL wires connected? I seem to remember somewhere the guidance WAS to do fully connected testing for IR? It's certainly how I was taught .. ok so the N-E readings on an rcd group will be the same.. so? You only need doing it once then.. if its an issue, THEN do infividuals.