Insulation Resistance Test with RCBO | on ElectriciansForums

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J

JuniorSparky

Hi guys,

I was going to carry out a IR test on a CU with a few RCBOs, but I wasn't quite sure how to proceed. Which neutral do I use - the one on the N-bar or the one going to the RCBO? Because if I use the one from the N bar I will actually test just that short piece of cable and through the RCBO!

If I touch the test leads to the L and N screws at the RCBO and carry out IR test, is this going to damage the RCBO or I should disconnect them first?

Thanks
 
Last edited by a moderator:
Junior Sparky, an edited reply to clarifie.

Disconect the circuit from the RCBO and test the out going side.

I'll add, some brands will stand an IR test, I've found they end up giving low readings. Circa 4Mohm.
 
Last edited by a moderator:
This is a very good question, and something I was very unsure of for quite a long time myself.

Most RCBOs have what is called a solid neutral, what this means is that the neutral flylead is permanently connected to the outgoing terminal, so any test you apply to the neutral bar will be applied to the outgoing circuit neutral regardless of whether the switch contacts are open or closed.
If the RCBO is of the type which has a functional earth connection then this should be disconnected for the IR test to be carried out, otherwise you will be putting the test voltage across the electronics of the RCBO.
An RCBO without a functional earth wouldn't normally present any problems when carrying out IR testing.
But this is one of those situations where if you are unsure then a quick read of the manufacturers information could help.
 
Junior Sparky, test on the outgoing side of the RCBO. The same way you would an RCD.

Testing through an RCBO may give spurious results dependent on brand.

I assume you mean disconnect the circuit from the RCBO and test only the circuit as an IR test can damage the RCBO ... I'm sure you meant this Rob but it ambiguous in how you wrote it?
 
This is a very good question, and something I was very unsure of for quite a long time myself.

Most RCBOs have what is called a solid neutral, what this means is that the neutral flylead is permanently connected to the outgoing terminal, so any test you apply to the neutral bar will be applied to the outgoing circuit neutral regardless of whether the switch contacts are open or closed.
If the RCBO is of the type which has a functional earth connection then this should be disconnected for the IR test to be carried out, otherwise you will be putting the test voltage across the electronics of the RCBO.
An RCBO without a functional earth wouldn't normally present any problems when carrying out IR testing.
But this is one of those situations where if you are unsure then a quick read of the manufacturers information could help.

Lets just say you have switched the main switch off and you clip to the N bar then disconnect the L wire of the circuit from the rcbo.. you may get a feedback voltage if a fault existed to say another circuits L wire ...if its associated OCPD was still in the on position this may track the busbar into the rcbo and damage its delicate circuitry.
 
Thank you all for the answers. I will check then first what type of RCBO it is in there, before starting IR test. I guess I won't be able to test the whole installation and will have to do it circuit by circuit.
If I don't know I guess best I can do is to disconnect before carrying out IR test.

What darkwood says seems logical to me! Has it ever happened to you?
 
Last edited by a moderator:
No because I don't have RCBO's in line for any testing - they cost money to replace and you cannot charge the customer for your own stupidity when its stated in all the manufacturers guidelines not to IR test them. Its quite possible you may not see any issues if you did accidently meggar a rcbo but one day it may be required to save someones life so I'll stick to adhering to the common sense of not zapping 500v through them.
 

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