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Ok, here’s another (hopefully) rare one (done it to myself :) ) - rcd/rcbo internal switch mechanism is able to be jammed by rough handling. I discovered this with 25mm not properly lined up. It looked fine, tightened fine but I had twisted the body of the rcd sufficiently to prevent normal trip operation. Remove the cable, pressure comes off the body, rcd works fine. Cable back in and it wouldn't trip again.
 
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In the plant room is the final ring the only single phase circuit?

Are the other final circuits 3 phase 4 wire or 3 phase 3 wire or both?

At Plant Room DB:

Could you check the phase voltages wrt to N and then to E and compare.

Also check the line voltages and compare.

Measure N-E voltage with RCBO connected to final ring and closed and then open.

At Main Board which feeds Plant Room DB measure N-E voltage.

Is functional earth of RCBO for FRC connected to earth bar?

Did you remove the FE when doing IR and ELI tests? Some RCBO manufacturers advise this to avoid damage to RCBO electronics and did you use 250V or 500V for IR test? The MG board you mentioned I think is using voltage dependent RCBO electronics and uses the L and N on the output side to power the trip detection and trip circuitry. The FE is connected to electronics used to detect loss of input neutral and for some filtering.

I am wondering if you have a floating neutral (derived from 3 phase 4 wire loads) at the Plant Room DB which is 'upsetting' the FRC's RCBO electronics and 'things change enough' when the FRC is disconnected for the RCBO to function.

Or maybe one of the motors has a phase-earth winding fault which is elevating the potential of the Plant Room DB's Earth wiring which is upsetting the normal working of the RCBO.....
 
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Can I ask; are you a qualified electrician?

(Just to get this out of the way)

I haven't read the full thread. Only because I wouldn't understand it. But if you're DIY we have a user group for that which I will need to apply to your account.

Nothing personal. :cool:
 
Desmond.22: May I clarify something please? Forgive me if you have said it but I have not correctly understood you. When the FRC is connected to the RCBO, does your MEGGER 1721 actually carry out the test and then report 'FAIL'? What do you see on its display from the moment just before you press the test button to the end of the test?

As an experiment could you try doing the RCD tests with the FRC's neutrals disconnected from the RCBO?

Also, did you try 0 and 180? And what is the earthing system? Is the FRC wired in metal conduit, SWA or MICC to metal-clad sockets?

Could you move the RCBO to one of the other two phases and see what happens?

:)

M
 
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So, some further musings. I mentioned the RCBO is most likely a voltage dependent type. There is an internal power supply which drives the detection and trip circuits. The minimum L-N voltage for the power supply to deliver enough voltage to these circuits is about 50V. I think you have a loose, floating or disconnected neutral to the RCBO which has never been discovered because the sockets have never been used for anything.

With the FRC disconnected the RCBO tests out ok. Why? Because the FE provides a lowish impedance path to neutral through the E sufficient to provide over 50V to the internal power supply.

When you connect the LNE of the FRC, and bearing in mind the RCBO is SP, the N rail of the RCBO is now 'pulled' upwards in voltage by inter L-N capacitance so much that the available voltage to energise the power supply is less than 50V so it is unable to detect and trip.

The same sort of thing could happen with a high impedance L input but I think not because the ELI was satisfactory - this only used the L and E conductors.

I would use my voltmeter to measure the L-N voltage at the output of the RCBO with the FRC connected and then disconnected. I'd also do the measurement at the input to the RCBO, again with FRC connected and disconnected. Does the voltage measurement drop below phase voltage and below 50V?

If you have a voltmeter which can capture min and max it would be helpful like some FLUKEs do.
 
Can I ask; are you a qualified electrician?

(Just to get this out of the way)

I haven't read the full thread. Only because I wouldn't understand it. But if you're DIY we have a user group for that which I will need to apply to your account.

Nothing personal. :cool:

Not read all but questioning me ?
 
UPDATE _ RESOLUTION

so we traced all cables through trunking all singles so ----ing nightmare and found someone has tapped off the cables within trunking not labelled or marked up and nearly impossible to trace all the way to item.

So we disconnected this unknown item and all sorted.

Left the item disconnected until someone complains etc something isnt working and will then know what it is for.

Thanks guys enjoying reading some stuff on here
 
Yeah never helped me find fault specifically but good all the same.

Ppl ask about loads connected so good call by them

Yeah impressed
That's what we like to hear. :)

I'm not a sparky so reading threads on here just baffles me. Sounds like one of those situations where somebody needs to be there to look.
 
You tried turning it off and on again?

(Sorry I'm not a sparky :()

Top comment made me laugh!
I only come for the comments!
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That's what we like to hear. :)

I'm not a sparky so reading threads on here just baffles me. Sounds like one of those situations where somebody needs to be there to look.

Its all these acrnyms that get me its actually worse than being in IT.
lol

RCBO, FE, FRC???
 
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