Does the Megger MFT RCD test mimic an electric shock

[Jupiter01]

Bear in mind that I am not, and never have been, an electrician but, for what it's worth, that is what I would usually do, at least in the first instance. Indeed, if it were a sockets circuit and I was absolutely certain that no loads were connected to the circuit, I might well test the RCD 'from a socket' (with the MCB 'on'). In either of those cases, I think the only situation in which one would get misleading results would be if there were an N-E fault on the circuit - and one should have detected that (by IR testing) long before one got to testing the RCD.

Some real electricians may well disagree with such practices.

Again, for what the view of a non-electrician may be worth, I do all I can to minimise the number of things that are disconnected and then re-connected in the name of testing - since every time one does that one invokes a finite (hopefully very small!) risk that the 'testing' may create new faults/problems!

Kind Regards, John

Thanks John.
For completeness, can I also understand how you prefer testing RCBO's? Based on the above, do you disconnect the loads (L&N) from RCBO before testing?

 

[JohnW2]

Thanks John. For completeness, can I also understand how you prefer testing RCBO's? Based on the above, do you disconnect the loads (L&N) from RCBO before testing?

Again with the caveat that is 'just me', I would deal with an RCBO in exactly the same way as an RCD. The fact that it also has OPD behaviour is irrelevant to testing the RCD functionality, so that the only difference is that the RCBO will (usually) only do SP switching (i.e. will not switch neutral) - but even that does not appreciably affect anything I've said.

So, as with an RCD, my usual initial testing would be with the RCBO switched 'off' (as above, only in terms of L) but without 'disconnecting' anything (or, if it were a sockets circuits which I was certain had no loads connected, test from a socket with the RCBO 'on').

As before, others may disagree.

Kind Regards, John

 

[Jupiter01]

So, as with an RCD, my usual initial testing would be with the RCBO switched 'off' (as above, only in terms of L) but without 'disconnecting' anything (or, if it were a sockets circuits which I was certain had no loads connected, test from a socket with the RCBO 'on').

If the RCBO is set to off, how can the RCD test be executed? Does this not rely on the RCD (part of the RCBO) being in the on position and then measuring the trip time? That's why I assumed that with the RCBO, you would have to disconnect the loads..

 

[JohnW2]

If the RCBO is set to off, how can the RCD test be executed? Does this not rely on the RCD (part of the RCBO) being in the on position and then measuring the trip time? That's why I assumed that with the RCBO, you would have to disconnect the loads..

Sorry, what I wrote was obviously nonsense (now corrected)! I should have agreed with you that, to just test the RCD alone, one must leave it switched 'on' and disconnect the loads (ideally both L and N, although there's theoretically no need to disconnect N if you have already established the absence of any N-E fault).

If you wanted to follow eric's suggestion (or if you were certain that no loads were actually connected to the circuit), then you could leave the circuit connected to the RCD (and even, if you wanted, 'test from a socket').

Exactly the same applies with an RCBO.

Kind Regards, John

 

[Jupiter01]

Sorry, what I wrote was obviously nonsense (now corrected)! I should have agreed with you that, to just test the RCD alone, one must leave it switched 'on' and disconnect the loads (ideally both L and N, although there's theoretically no need to disconnect N if you have already established the absence of any N-E fault).

If you wanted to follow eric's suggestion (or if you were certain that no loads were actually connected to the circuit), then you could leave the circuit connected to the RCD (and even, if you wanted, 'test from a socket').

Exactly the same applies with an RCBO.

Kind Regards, John

Slightly confused. Easily done...

Is this your approach?

RCD alone: disconnect busbar and N for the RCD. Main Switch and RCD is on. Test
RCBO: disconnect N and L of the appropriate circuit. Main Switch and RCBO is on. Test

Assuming the above is correct, could the "RCD alone" test not be done without removing the busbar and N (whilst MCB's are set to off)? I thought this would avoid the risk of disconnecting/connecting cables and introducing faults. I appreciate that this is unavoidable with the RCBO.

 

[EFLImpudence]

although there's theoretically no need to disconnect N if you have already established the absence of any N-E fault).

Yes, but if your purpose is solely to test the proper working of the RCD, why would you want to have to go to the bother of IR testing ( and all that entails) rather than just disconnect the neutral conductor.

 

[JohnW2]

Slightly confused. Easily done...

Yes, apologies, I wrote in haste without paying enough attention to what I was writing!

Is this your approach? RCD alone: disconnect busbar and N for the RCD. Main Switch and RCD is on. Test

That depends upon what you mean by 'disconnect busbar' (and N). If you want to definitely test 'just the RCD', the supply side must remain connected but the L and N on the load side (the L of which will usually be a {RCD-protected'} busbar) should be disconnected (with, as you say, the RCD and Main Switch both 'on'.

RCBO: disconnect N and L of the appropriate circuit. Main Switch and RCBO is on.

Yep - i.e. exactly equivalent to the situation with a RCD.

Assuming the above is correct, could the "RCD alone" test not be done without removing the busbar and N (whilst MCB's are set to off)? I thought this would avoid the risk of disconnecting/connecting cables and introducing faults.

It could and, given that MCBs are nearly always SP, as I said that is fine so long as you are certain that there are no N-E fault on the circuit.

Kind Regards, John

 

[JohnW2]

Yes, but if your purpose is solely to test the proper working of the RCD, why would you want to have to go to the bother of IR testing ( and all that entails) rather than just disconnect the neutral conductor.

As you say, "if your purpose is solely to test the proper working of the RCD" then it would be more convenient ('less work') to "just disconnect the RCD", but if one adopted that 'more convenient' approach, it would invoke the risk of creating faults/problems.

I was thinking more of a situation in which testing the RCD was part of a more extensive/comprehensive testing exercise - in which case, as I said, one would expect IR testing to precede RCD testing.

Kind Regards, John

 

[EFLImpudence]

I was thinking more of a situation in which testing the RCD was part of a more extensive/comprehensive testing exercise - in which case, as I said, one would expect IR testing to precede RCD testing.

Mmmm.

In such a situation, would you not want to make sure the RCD was functioning properly before hunting for possible circuit (or appliance) faults?

 

[JohnW2]

Mmmm. In such a situation, would you not want to make sure the RCD was functioning properly before hunting for possible circuit (or appliance) faults?

I wasn't thinking/talking about 'fault-finding' (in which case I would agree with what you say). Rather, I was thinking about the 'routine' testing of a circuit (e.g. for an EIC or EICR), in which case IR testing (indeed, all 'dead' tests) would usually precede RCD testing wouldn't it?

Kind Regards, John

 

[EFLImpudence]

Ok. fair enough. Depends what one is doing.

 

[JohnW2]

Ok. fair enough. Depends what one is doing.

Exactly -and, for whatever reason, you and I were 'thinking' of different things - you about 'fault-finding' and me about 'routine I&T'.

Kind Regards, John

 

[Jupiter01]

That depends upon what you mean by 'disconnect busbar' (and N). If you want to definitely test 'just the RCD', the supply side must remain connected but the L and N on the load side (the L of which will usually be a {RCD-protected'} busbar) should be disconnected (with, as you say, the RCD and Main Switch both 'on'.

By disconnecting the busbar and N on load side I am referring to completely disengaging these. Unscrewing them from the terminals of the RCD. In practical terms, the busbar will then need to be unscrewed from most of the neighboring MCB's. A lot of work and potential faults that are being introduced...
You have suggested in your subsequent response that this is not necessary when an IR test has been successfully completed. In that situation, we can leave the busbar and N (load side) connected to the RCD whilst ensuring that all MCB's (fed by this RCD) are set to off.

To complete the puzzle, no such option is available with an RCBO given the nature of what it does. We have to unscrew L and N on the load side even if IR test has been done.

Is that a fair summary of the situation @JohnW2?
Really appreciate your help with this.

 

[JohnW2]

By disconnecting the busbar and N on load side I am referring to completely disengaging these. Unscrewing them from the terminals of the RCD. In practical terms, the busbar will then need to be unscrewed from most of the neighboring MCB's. A lot of work and potential faults that are being introduced...

That's what I assumed you meant and, as you say, it's potentially quite a lot of work, and work that has the potential to introduce all sorts of 'faults' when one re-connects things.

You have suggested in your subsequent response that this is not necessary when an IR test has been successfully completed. In that situation, we can leave the busbar and N (load side) connected to the RCD whilst ensuring that all MCB's (fed by this RCD) are set to off.

That is my personal view (but others may disagree). If IR testing shows a very high N-E resistance then, in electrical terms, the neutral is effectively 'disconnected'.

To complete the puzzle, no such option is available with an RCBO given the nature of what it does. We have to unscrew L and N on the load side even if IR test has been done.

Yes, one would have to disconnect the outgoing L (unless one was absolutely certain that no loads were connected to the circuit ) - but, as far as the neutral is concerned, it's no different from an MCB.

Is that a fair summary of the situation @JohnW2?

Yep.

Really appreciate your help with this.

You're welcome, but remember that I am not, never have been (and presumably never will be!) an electrician!

Kind Regards, John

 

[EFLImpudence]

By disconnecting the busbar

You do not need to unscrew the busbar from the RCD and all the MCBs; switching off the MCBs will disconnect the circuits.