RCBO Tripping

[martinxxxxxx]

Right then, I had a problem yesterday that had me beat and it is annoying me a lot.
Fully tested an installation in preparation for a new fuse board. This included ir testing between the neutral, live and cpc conductors of all circuits and further testing between the above conductors to the same conductors in the other circuits to ensure all circuits were completely independent from each other no shared neutrals etc.
Plus all of the other tests in GN3. No abnormal results recorded apart from an o/c phase on the up circuit which was traced and rectified, loose connection in socket.

There are 3 ring circuits
Down
Up
Kitchen

Kitchen is fine

If any socket on the Up circuit is used - the RCBO for down trips, but the up circuit stays on and continues to work
If any socket on the down circuit is used the down RCBO trips.

Have checked that the conductors for each RCBO are the correct ones and that they have a near infinite IR from all of the other conductors in the neighbouring RCBO - and they do.
Have checked tightness of all connections to busbars and sequence of fly leads from RCBOs.
If I remove the conductors from the down rcbo and run UP circuits - all operates OK.
And (and here is a big clue that I have not been able to digest yet) if I remove the N and L from the outgoing of the UP the down now works fine.

Baffling that the there is interaction between the circuits when they megger out ok from each other.

Also the RCBOs have been tested with the outgoing circuits removed and go through the test sequence fine no trip at 1/2 both ways and 29ms and 30ms at 30 and 150ma respecttivly

 

[HandySparks]

Got to be something you've missed in testing. I'd put my money on a neutral to earth fault on one or both affected circuits. What figures do you get for (L+N) to E insulation resistance for each circuit, with the CPCs still in the earth bar and all other earths connected?

 

[martinxxxxxx]

Got to be something you've missed in testing. I'd put my money on a neutral to earth fault on one or both affected circuits. What figures do you get for (L+N) to E insulation resistance for each circuit, with the CPCs still in the earth bar and all other earths connected?

I removed all conductors of both circuits and checked that there was no DC connection at 500v between any of them (all combinations), but I stll have a feeling that you are correct and that neutral will be involved, possbly borrowed by a hidden device like an arial amp in the loft, but the L-N were high too so that should not be possible

 

[rich.250]

Sounds a lot like a borrowed neutral to me.....
Or a interconnection of the
RFCs.
As you say there is no connection between the two it can't be.

 

[Richard Burns]

If the RCBOs are next to each other I think I remember something on here where the tripping of one RCBO could cause the other to trip and it may be that in this case the RCBO is too sensitive to external magnetic fields and is tripping on current supply to an adjacent RCBO, try swapping the RCBOs around and see if the same one still trips, this would eliminate that possibility, or more easily swap the circuits from one to the other.
Try a 250V IR test between the circuits with the circuits connected and the RCBOs off, not 500V cos they don't tend to like this, this would eliminate a cross connection some where in the CU, (though not sure where that could occur with RCBOS).

 

[Guest123]

You need to inspect each socket outlet on both circuits to rule out the possibility of crossed polarity between N-E at some point on that circuit, the figure of 8 test would also identify this.

When anything is plugged in and switched on, this would give the problem you mention but would not show up under routine testing at the DB.

 

[martinxxxxxx]

Yep, all sorted nail through cable making partial connection, furniture moved round; making full connection.

Thanks every one, thing is about being one man band - you nedd others to bounce ideas off of so you can keep from going off on wild goose chases

Thank ya all

 

[polo1]

Sorry, a bit late to this post - but did one of your subsequent tests find the problem (if so, how) or was it literally found as furniture was moved around?

Regards.

 

[martinxxxxxx]

Yep, all sorted nail through cable making partial connection, furniture moved round; making full connection.

Thanks every one, thing is about being one man band - you nedd others to bounce ideas off of so you can keep from going off on wild goose chases

Thank ya all

When I tested before putting the new board on - all of the IR figures were in the hundreds of megs.
New board on do the live tests - go away - one hour later (on Friday afternoon) I get a call - it is tripping as described above.
I can only conclude that people moving about and furniture being shifted has put pressure on the damaged cable. Because when I measured it again on that Friday, the IR figures were still all good.
What I think was happening and giving me erratic results was there were people in the room. So that it would randomly trip during my faultfinding - driving me to such despair that I needed to come on here and be reminded of the laws of physics.
Returning the following Thursday with a cool head and a battle plan paid dividends and the fault was easy to find.
I am usually quite quick at fault finding so it was a low day for me on that Friday and one I did not want a repeat of. So for anyone reading this thread as a result of a search - Please just go back to basics and do the dead tests as per GN3