S
Sharkster
Title says it all really. I've just qualified and unfortunately my college bombed through IR testing but before I even touch anything I want to make sure I understand it thoroughly. Can anyone please compare what I've copied and pasted below to the diagrams and made sure I've understood it correctly before I blow anything up. Sorry the wording on the scans hasn't come through very clear. Thanks for any help it's really appreciated.
HOW INSULATION REISTANCE TESTING WORKS
Please note the explanation here is if youre testing the individual circuits in an installation WITH NO VOLTAGE SENSITIVE EQUIPMENT LEFT IN OR NOT BYPASSED.If you’re doing the initial verification on the whole installation, then you have to use the testing at the meter tails method.
Imagine a simple 2 circuit installation drawn in terms of the parallel networks its made up of.
Obviously for all circuits (2 in this case) theres 3 combinations you have to insulation resistance test.
Ph to N
Ph to CPC
N to CPC
Well look at the Ph to N first for the first appliance 1.
Say you left N1 in the neutral bar and tapped a probe onto it.Say you then take the other probe and tap it onto P1.Nothing is going to go through P2 as when all the breakers are off, theyre not connected through the live bar, and nothings going to go through N2 as its an open circuit.The end result is that even though you havent bothered to remove the N1 and P1 wires to test them, youve still tested the insulation resistance between them by just tapping onto them.
Exactly the same principle applies when you move the probes over one place to test between N2 and P2.
But hang on a bit When youre moving from N1 to N2, youre just moving along the same bit of metal (the neutral bar), so you may as well not move at all, so in practice to Ph N test a distribution board just clamp somewhere onto the Neutral bar with a crocodile clip, leave it there, take the other probe and tap along the live breakers.As you go along you test the insulation resistance of that phase and its corresponding Neutral.
The situation is pretty much exactly the same when you want to test all the Ph ( CPC connections.Crocodile clip onto the earthing bar and tap along the various breakers.Here you are testing between all the earths and bonding conductors against the phases
LASTLY WE LOOK AT THE N ( CPC TESTING, THIS IS THE ONE POINT WE HAVE TO SHOW A BIT OF CARE
When we were doing anything with the phases, we were ok as when the breakers are off, each phase is just a separate wire.
However, with the neutrals CPCs, all the neutrals are connected into one at the neutral bar, and likewise all the CPCs are connected into one at the MET. Look at point (1) on the diagram and imagine the N CPC are broken and in contact at this point.
Say youve crocodile clipped onto the MET youre tapping along the earth bar.Youd spot a fault on every circuit Why ?
Well, youd obviously spot the fault between E1 N2, but because N2 is connected to N1, N3, etc through the neutral bar, youd register the same fault on every circuit !
In fact, you wouldnt even need a fault to be present to get some weird readings.The N CPC are both connected through the star point of the local supply transformer for TN-S and TN-C-S systems, so if you get a zero reading, it could be measuring through here !
To prevent either of these 2 situations, when testing between N CPC, clamp onto the MET with a crocodile clip.When testing each neutral, remove it from the neutral bar, test it then replace it before moving onto the next, alternatively you could disconnect the main neutral.
HOW INSULATION REISTANCE TESTING WORKS
Please note the explanation here is if youre testing the individual circuits in an installation WITH NO VOLTAGE SENSITIVE EQUIPMENT LEFT IN OR NOT BYPASSED.If you’re doing the initial verification on the whole installation, then you have to use the testing at the meter tails method.
Imagine a simple 2 circuit installation drawn in terms of the parallel networks its made up of.
Obviously for all circuits (2 in this case) theres 3 combinations you have to insulation resistance test.
Ph to N
Ph to CPC
N to CPC
Well look at the Ph to N first for the first appliance 1.
Say you left N1 in the neutral bar and tapped a probe onto it.Say you then take the other probe and tap it onto P1.Nothing is going to go through P2 as when all the breakers are off, theyre not connected through the live bar, and nothings going to go through N2 as its an open circuit.The end result is that even though you havent bothered to remove the N1 and P1 wires to test them, youve still tested the insulation resistance between them by just tapping onto them.
Exactly the same principle applies when you move the probes over one place to test between N2 and P2.
But hang on a bit When youre moving from N1 to N2, youre just moving along the same bit of metal (the neutral bar), so you may as well not move at all, so in practice to Ph N test a distribution board just clamp somewhere onto the Neutral bar with a crocodile clip, leave it there, take the other probe and tap along the live breakers.As you go along you test the insulation resistance of that phase and its corresponding Neutral.
The situation is pretty much exactly the same when you want to test all the Ph ( CPC connections.Crocodile clip onto the earthing bar and tap along the various breakers.Here you are testing between all the earths and bonding conductors against the phases
LASTLY WE LOOK AT THE N ( CPC TESTING, THIS IS THE ONE POINT WE HAVE TO SHOW A BIT OF CARE
When we were doing anything with the phases, we were ok as when the breakers are off, each phase is just a separate wire.
However, with the neutrals CPCs, all the neutrals are connected into one at the neutral bar, and likewise all the CPCs are connected into one at the MET. Look at point (1) on the diagram and imagine the N CPC are broken and in contact at this point.
Say youve crocodile clipped onto the MET youre tapping along the earth bar.Youd spot a fault on every circuit Why ?
Well, youd obviously spot the fault between E1 N2, but because N2 is connected to N1, N3, etc through the neutral bar, youd register the same fault on every circuit !
In fact, you wouldnt even need a fault to be present to get some weird readings.The N CPC are both connected through the star point of the local supply transformer for TN-S and TN-C-S systems, so if you get a zero reading, it could be measuring through here !
To prevent either of these 2 situations, when testing between N CPC, clamp onto the MET with a crocodile clip.When testing each neutral, remove it from the neutral bar, test it then replace it before moving onto the next, alternatively you could disconnect the main neutral.
