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

 

[sellers]

you have something borrowed from each circuit.

Switch your testers to ohms and measure accross the live and neutrals with your item plugged in. And see where your load lies.

 

[martinxxxxxx]

The house was unoccupied and all devices were removed for IR testing. As I sa above.

I also IR tested between:-

rn of UP and rn of Down

r1 of UP and r1 of Down

r2 of UP and r2 of Down

All were over 200MOhms

Plus each circuit was IR tested in the ususal way between conductors.

There is no DC link between any of the circuits...

 

[Sidewinder]

OK, if you are sure the cables themselves are independent.

Rebuild the board without the RCBO's, use mcb's for TEST purposes ONLY.

Then test again between all conductors and let us know what you get.

Oh I don't just mean the up & down rings, but all circuits, I trust you have already done all individual circuits, all cores to each other all permutations?

 

[sellers]

The house was unoccupied and all devices were removed for IR testing. As I sa above. I also IR tested between:-

rn of UP and rn of Down

r1 of UP and r1 of Down

r2 of UP and r2 of Down

All were over 200MOhms

Plus each circuit was IR tested in the ususal way between conductors.

There is no DC link between any of the circuits...

This wouldn't show a borrowed neutral, unless you had items plugged into both circuits at the time.

 

[Andy™]

with the cables out of the RCBO's, did any of them have a voltage when the other circuit was in use?

 

[unphased]

Hello Martin. I've have just had a thought. I think there is reverse polarity of earh and neutral on one (or more) of the downstairs sockets. I would take all downstairs plates off and look. Reasoning for my thoughts. Only the downstairs circuit trips. It holds until you plug something in when current will leak to earth through the cross conected neutral. Upstairs I am not too sure about but when plugging something in only the down RCBO trips. All the testing you have done so far would not detect this fault you would need to do figure 8 test at all sockets to find it, or just look and see. Good luck!

 

[martinxxxxxx]

[quote name='Andy

 

[ProDave]

Disconnect ALL circuits from CU.

Connect JUST upstairs ring final. Does it work? does anything trip?

If all okay, connect downstairs ring final. Does it work? does anything trip?

continue re connecting each circuit one at a time, until your trip problem them comes back. When you find which circuit causes the problem, you know where to look for the fault.

 

[sb_89]

Probably a long shot, but when u did your ir test, did you test with the earths still connected in the earth terminal (with main bonding ect all still connected) my reasoning for saying this is that you could have a neutral to earth fault ( say a screw through a metal stud wall) that would show as clear until reconnected and turned on. My first port of call would be to open the sockets up and look at the connections behind though,

 

[martinxxxxxx]

Thanks everyone. I think more testing will uncover the problem. Solution will get posted

 

[SLIPSHOD & SLAPDASH]

As unphased mentioned i reckon its a reversed N & E in a socket.

 

[kme]

Thanks everyone. I think more testing will uncover the problem. Solution will get posted

Always the case. At the risk of repeating myself - if you haven`t found the fault, you haven`t done enough testing.

 

[martinxxxxxx]

If it is a n - e reversal in a socket surely there would be a low IR on the Neutral to Earth or it would have no effect unless there was something plugged into that particular socket

 

[unphased]

No affect. Think about it. If you have a socket with the n and e reversed all that is happening inside the socket is that the terminals are providing continuity to the conductors. There is no connection to earth at the socket itself. The neutral on the earth terminal cannot earth. The earth on the neutral terminal cannot earth. However, when energised and something is plugged in the fault will manifest itself as the reversed n and e will be connected in live conditions through the appliance and will cause the RCD to trip. If there were no RCD the circuit would have no disconnection mechanism. Fuses do not operate when n and e are reversed. Trust me, the fault will be a crossed n and e in the socket.

 

[Doc Hudson]

I did try to test this but I had to start buttoning things down as there was a viewing of the property due at any moment and they wanted me off site.

This has got to be one of the main underlying reason for drifting away from a logical testing process and not resolving a fault whilst on site. Rushing because someone is pressuring you, is never a good way to evaluate a problem. Keeping a clear head working systematically and steadily through a logical process, preferably with a pen and paper to note you results will always bring dividends. A process such as ProDaves example would be a good example.

Doc H.

 

[Steve3948]

Probably a long shot, but when u did your ir test, did you test with the earths still connected in the earth terminal (with main bonding ect all still connected) my reasoning for saying this is that you could have a neutral to earth fault ( say a screw through a metal stud wall) that would show as clear until reconnected and turned on. My first port of call would be to open the sockets up and look at the connections behind though,

Had this fault last year, in a void area above the fuseboard wood butchers had used 6in screws on 3in stud.

 

[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

 

[ProDave]

Speaking from experience of todays job. ASK as many questions as you can to locate the fault.

I was faced with a rental property where the RCD was tripping. Narrowed it down to the downstairs lighting circuit so disconnected that pending further investigation to get the power back on.

Upon looking at the lighting circuit it was obvious it was wired spider fashion back to a big junction box, so the first thing I needed to do was locate that. Of course the tennant didn't have a clue. All upstairs rooms carpeted so where do you start.

So I left it and when and phoned the landlord. They told me "ah yes this happens when it rains a lot. I think there's a problem in the garage."

sure enough the light switch cable in the garage had very poor IR. Replaced that run of cable and all was well.

So much time saved, just by asking the right questions to the right person.

 

[martinxxxxxx]

That is so true, I am in like columbo - er before I go, just one last question....