Lets see how good this forum is ?

[Desmond.22]

Apologies for headline trying grab attention

So can someone help on this scenario.

Plant room prob 7Mx7M square ish. Loads of HVAC, pumps etc as you would expect and a ring final circuit.

Socket been damaged somehow and went to replace socket.

After replacing socket just for peace of mind we test that circuit. IR ok elip ok polarity etc.

Test RCBO and with cables disconnected as per GN3 all ok.

Just testing further and left loop tester on high current and the RCBO did not trip unusual so we got out RCD tester and couldn't get the RCBO to trip from socket. Yet with cables disconnected works as should.

So what going on and ae replaced RCBO and same ?

But circuit looks ok all in tact all tests on circuit as expected.

So unplug everything thinking maybe some item plugged in causeing problem but NO joy still.

Anyone ?

 

[marconi]

'We' think you have a 'double-grounded neutral' problem when the N is linked to E before and after the RCBO. Because disconnecting the ring from RCBO enables it to detect earth faults, the ring is likely to have an N_E wiring fault which your tests or way of testing has not yet discovered.

 

[Lucien Nunes]

In theory there could be something still connected that you have overlooked, that has a high DC resistance N-E hence does not affect the IR reading, but a low enough AC impedance to affect the test. But that would require a very large capacitance so is not very likely.

What type of wiring system is used in the RFC? In a plant room, there may be heavy, noisy loads on cables run along with the RFC, that affect the operation of the tester by induction when the cables are connected to the RCD.

 

[James]

'We' think you have a 'double-grounded neutral' problem when the N is linked to E before and after the RCBO. Because disconnecting the ring from RCBO enables it to detect earth faults, the ring is likely to have an N_E wiring fault which your tests or way of testing has not yet discovered.
[/Q

In theory there could be something still connected that you have overlooked, that has a high DC resistance N-E hence does not affect the IR reading, but a low enough AC impedance to affect the test. But that would require a very large capacitance so is not very likely.

What type of wiring system is used in the RFC? In a plant room, there may be heavy, noisy loads on cables run along with the RFC, that affect the operation of the tester by induction when the cables are connected to the RCD.

Low resistance,
Very big capacitor
Power factor correction device is bouncing around my head?

 

[Desmond.22]

Ok to clarify few points made.

Socket front only damaged knocked. So replaced.

IR tests all ok checked and re checked single pole mcb from MG isobar 4c tp&n DB.

Turned off everything in board now except this circuit and still got the fault. 10 outlets across room nothing in any of them.

All bar anyone we try the switches in off position. No fused spurs etc on ang of them.

N- E IR CLEAR as this was our best guess.

Now dont know if this has always been like this as we stumbled across it.

 

[Desmond.22]

Sorry should say single pole rcbo not mcb and b32

 

[gaz_sparky]

Interesting one Desmond, double pole though, surely?

 

[curly]

Is there another rcbo in the board you could try the circuit on?

 

[Wilko]

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.

 

[Desmond.22]

Is there another rcbo in the board you could try the circuit on?

Tried swapping for new one ?

 

[marconi]

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.....

 

[Dan]

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.

 

[marconi]

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

 

[marconi]

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.

 

[Desmond.22]

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.

Not read all but questioning me ?

 

[Desmond.22]

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