AM2S Insulation Resistance Test on RCBO

[MrKDilkington]

Hello,
I’ve got my AM2S coming up and I’ve been revising like mad but I’m stuck on something.

I’ve read that it’s a 3 phase board with just one RCBO for the ring circuit. Assuming they want me to do a global IR test, can I take the conductors out of the RCBO, switch it off, and then test the entire board without knackering the RCBO? It’s switched off but the supply side would obviously still be receiving 500v.

Any help would be greatly appreciated, I know it’s one mistake and game over with this exam so really don’t want to fail on something like this.

 

[littlespark]

I don’t know if they’ll accept a global IR, or if they want you do it circuit by circuit.

Whatever, just remember you’re trying to test the wiring inbetween devices and appliances, not the devices themselves.

 

[MrKDilkington]

I don’t know if they’ll accept a global IR, or if they want you do it circuit by circuit.

Whatever, just remember you’re trying to test the wiring inbetween devices and appliances, not the devices themselves.

Ok thank you. Doing each circuit on its own would actually be preferable to me, but I’ve heard it can be either.

Assuming I do take the conductors out of the terminals and test the ring on its own, as there is 2 line, 2 neutral and 2 CPCs is it acceptable to test with each one joined? As in croc onto both lives at the same time, and test to both neutrals? Then both CPCs to both neutrals etc. Do I even need to remove the conductors? Would having the RCBO off and testing on the load side at each terminal be ok? I’m not really sure where the sensitive components are in an RCBO.

I’ve tried to find guidance on this is GN3, on site guide, YouTube but there’s just nothing out there. I can see this being something that trips me up.

 

[Pretty Mouth]

Ok thank you. Doing each circuit on its own would actually be preferable to me, but I’ve heard it can be either.

Assuming I do take the conductors out of the terminals and test the ring on its own, as there is 2 line, 2 neutral and 2 CPCs is it acceptable to test with each one joined? As in croc onto both lives at the same time, and test to both neutrals? Then both CPCs to both neutrals etc. Do I even need to remove the conductors? Would having the RCBO off and testing on the load side at each terminal be ok? I’m not really sure where the sensitive components are in an RCBO.

I’ve tried to find guidance on this is GN3, on site guide, YouTube but there’s just nothing out there. I can see this being something that trips me up.

If you have already established end-to-end continuity on the conductors, then there should be no need to join both ends of the conductor as you described, when IR testing (although doing so won't harm).

When testing between live conductors and earth, remember that the CPCs must be connected to the earthing system, with all bonding, earth conductor etc in place.

Unless the RCBO has a note stating it's OK to IR test it, I would assume that it's not OK to do so, so would disconnect. Even if there is a note saying that it's okay to IR test it, I would still disconnect. Many RCBOs only switch the L, so the N would still be connected to the N bar and therefore the N of other circuits - this will skew the result of the test.

 

[MrKDilkington]

If you have already established end-to-end continuity on the conductors, then there should be no need to join both ends of the conductor as you described, when IR testing (although doing so won't harm).

When testing between live conductors and earth, remember that the CPCs must be connected to the earthing system, with all bonding, earth conductor etc in place.

Unless the RCBO has a note stating it's OK to IR test it, I would assume that it's not OK to do so, so would disconnect. Even if there is a note saying that it's okay to IR test it, I would still disconnect. Many RCBOs only switch the L, so the N would still be connected to the N bar and therefore the N of other circuits - this will skew the result of the test.

That’s a great point I hadn’t considered - I will (hopefully) have already established that they’re continuous so I can just IR test one leg of the ring.

I think I will just go ahead and remove the line and neutral conductors then, and test each one in turn to the earth bar with the CPC still connected.

Thanks very much for clearing that up for me.

 

[timhoward]

Don’t let the 3 phase board trouble you.
Just don’t fall at the first hurdle and remember all 10 safe isolation tests!
You’ve read up on PFC for 3 phase?

Let us know how it goes. All the best with it!

 

[MrKDilkington]

Don’t let the 3 phase board trouble you.
Just don’t fall at the first hurdle and remember all 10 safe isolation tests!
You’ve read up on PFC for 3 phase?

Let us know how it goes. All the best with it!

Thanks very much, yeah let me see if I’ve got this right from memory?

PEFC: 3 lead low test just in case there’s RCDs further back somewhere. L-PE setting. Board still safely isolated at main switch but incoming tails live.

Clip onto earth bar, probe onto neutral and L1, wait for reading. Then N-L2, take reading. Then N-L3, take reading. Remember the highest. This is the maximum current that can flow in a fault to earth.

PSCC: 3 lead test again. move meter to L-N setting. Leave earth clipped on and test between neutral and L1, then N-L2 and N-L3. Remember highest reading. If this is higher than PEFC (it should be on a 3 phase test) double it as a rule of thumb and record this figure as PFC on test paperwork. Ensure this figure is lower than the maximum rated current of the main cutout fuse, 30kA for example.

I’m a bit confused about whether I need to do them both as a 3 lead low test rather than 2 lead high, but i feel like it’s just safer doing them both 3 lead. I’ve heard people have tripped stuff before so didn’t want to take any chances.

 

[timhoward]

Thanks very much, yeah let me see if I’ve got this right from memory?

I think you have exactly the right idea.
Comments would be:
The double-it thing on the PSCC is to allow for a phase to phase dead short. Some MFT's can do loop tests between phases these days but stick with what you were taught.
The doubled reading will virtually always exceed the PEFC and I've never known it not be the one that is recorded on the certificate.

In my view it would be grossly unfair to tell you to test an install as if it were at the origin of a supply and not tell you about an upstream RCD. If you aren't told I wouldn't worry about asking - it would demonstrate understanding and in the real world you would have checked before proceeding. If there is one then you would indeed have to use a low current test for the PEFC.

I’m a bit confused about whether I need to do them both as a 3 lead low test rather than 2 lead high,

A low current test wouldn't be required for the PSCC tests since everything 'going out' through an upstream RCD is also 'coming back' so there is no imbalance. So if it were me I'd use 2 wire hi current for that test as it's easier.

 

[MrKDilkington]

I think you have exactly the right idea.
Comments would be:
The double-it thing on the PSCC is to allow for a phase to phase dead short. Some MFT's can do loop tests between phases these days but stick with what you were taught.
The doubled reading will virtually always exceed the PEFC and I've never known it not be the one that is recorded on the certificate.

In my view it would be grossly unfair to tell you to test an install as if it were at the origin of a supply and not tell you about an upstream RCD. If you aren't told I wouldn't worry about asking - it would demonstrate understanding and in the real world you would have checked before proceeding. If there is one then you would indeed have to use a low current test for the PEFC.


A low current test wouldn't be required for the PSCC tests since everything 'going out' through an upstream RCD is also 'coming back' so there is no imbalance. So if it were me I'd use 2 wire hi current for that test as it's easier.

Great, thanks. I’ll clarify with the assessor about upstream RCDs and if possible conduct both tests as 2 lead high, firstly Earth to each phase for PEFC, and Neutral to each phase for PSCC. That should make things a bit easier.

Really appreciate taking the time to reply, it’s been a nightmare trying to find some of this stuff out. College haven’t been much help unfortunately.

 

[Risteard]

Great, thanks. I’ll clarify with the assessor about upstream RCDs and if possible conduct both tests as 2 lead high, firstly Earth to each phase for PEFC, and Neutral to each phase for PSCC. That should make things a bit easier.

Really appreciate taking the time to reply, it’s been a nightmare trying to find some of this stuff out. College haven’t been much help unfortunately.

Don't forget as a rule-of-thumb which errs on the side of safety to double the highest phase to neutral measurement for a three-phase system.

 

[MrKDilkington]

Don't forget as a rule-of-thumb which errs on the side of safety to double the highest phase to neutral measurement for a three-phase system.

Yep, good advice. I’ve heard people have failed for this alone.

 

[Risteard]

Yep, good advice. I’ve heard people have failed for this alone.

No problem. Best of luck with it. I'm sure you'll be fine.