Tripping RCBO

All true.

However, I had that same scenario - except the actual break (which must be there and could be at the same place as the fault).

Just thinking Mr. Householder fits a spur badly which Knotty doesn't realise is on the same ring.

Do you not agree that an open circuit caused by the break is more likely than an exactly 100Ω fault. What could that be?

Oh well, we shall see.
 
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All true. However, I had that same scenario - except the actual break (which must be there and could be at the same place as the fault). Just thinking Mr. Householder fits a spur badly which Knotty doesn't realise is on the same ring.
Indeed, not impossible, although I continue to think that the probability of that 'treble' must be extremely small (in fact, it's almost a 'quadruple', since the overlooked load would not only have to be plugged in to a mis-wired socket, but would also need to be switched on).

Do you not agree that an open circuit caused by the break is more likely than an exactly 100Ω fault. What could that be? Oh well, we shall see.
Yes, as you say, an open circuit would seem much more likely than a resistance of exactly (or even approximately) 100Ω. I suppose that 100Ω would not be impossible if the CPC had somehow been wired through a load - but that would be an even more bizarre/unlikely scenario than the one you're postulating, and to get 100Ω from a damaged CPC or poor connection would be quite difficult.

As you say,we'll see!

Kind Regards, John.
 
Back again with some answers.

Fortunately, granddaughter also comes with son able to assist in this project, which made matters much easier. None of you gave me the obvious advice that any faults would be found in the places most difficult to access! But so it transpired.

The first post from EFLImpudence gave the correct diagnosis; namely that there were two faults: a break in the earth wire and a N-E Fault, (in different places on the system). However, finding them both proved far from easy.

The broken earth was in a twin and earth cable that has bee untouched for many years. Probably this fault would have remained undetected if I had not installed the RCBOs and then encountered the strange symptoms I described originally. I suspect that the cable could have been damaged by a spike from a carpet gripper, which may even have been the source of the high resistance to earth reported earlier.

This cable went through a wall to a JB which was located behind a glass fronted display cabinet containing all of the wife’s valuables and carefully secured to the wall. Moving the cable off the carpet spike restored the earth continuity, but I replaced that length of cable. This solved problem one, but meant that the RCBO would trip on both halves of the ring, so I simply had to find the N-E fault.

Despite suggestions to the contrary on this forum, I could think of no way of finding it other than breaking the ring in various places and using a long wire on the multi-meter back to the CU. The final point that needed to be isolated was a JB with two spurs at the back of a low attic. Fortunately, son, who is into caving, managed that after we had removed a few hundred books.

The culprit turned out to be a relatively recently replaced twin socket, the fault disappeared when the fixing screws were loosened. On inspection, it seems that the earth wire, (multi-strand, as used in the original house wiring), had bent in such a way that the sleeving slid back and permitted the earth conductor to touch the neutral. You could not do that if you tried!

We found no incorrect wiring. After another couple of hours reassembling the ring, reinstating cabinets, books etc. the system was retested for insulation and put back into service, hopefully safer than before.

Many thanks to all contributors who helped solve this odd problem.

This has shown me the value of RCBOs over RCDs and MCBs, but it has also shown me that this forum offers a lot of expertise to the DIYer, even if he is a Chartered Electrical Engineer who thought he understood domestic wiring!
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Incidentally, when I said 100 ohms; that was meant to imply approximately. If I had meant anything else I would have put 100.00 ohms, as per my NAMAS training! As it was measured on several ranges of the ohm-meter, the digital restrictions did not apply.

I apologise fro my casual use of the term Megger. It should have read 1000v 2000MΩ YEF YF509 digital insulation tester. I think these are now banned, because you could turn them into TAZERs – such is life.
 
After another couple of hours reassembling the ring, reinstating cabinets, books etc. the system was retested for insulation and put back into service, hopefully safer than before.
A brave man to put all the cabinets, books and ornaments back before the testing :)

Anyway, I'm very pleased to hear that the problem has been sorted, and congrats to EFLI for having got the diagnosis. I have to say that I'm always a bit suspicious of theories which require two or more simultaneous separate faults - but, as this case illustrates, even 'the unlikely' does, indeed, sometimes happen! People even win the Lottery, and that's very unlikely :)

Kind Regards, John.
 
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Can't claim to be that brave, as all the individual bits had been tested beforehand and I was not allowed to actually replace the ornaments. I also have to admit that as the attic really needed sorting most of the books are in a big pile in my office!

The more worrying part is that the lack of earth could have presented a safety hazard without me knowing. Fortuitously, the sockets on that part of the ring are used mainly for lights.

I suppose that very few houses are fully tested unless they are being sold.
 
I suppose that very few houses are fully tested unless they are being sold.
Yes and a lot of questioners on the forum become quite upset at being told they should.

I'm glad you have it sorted.


John,

Thank you, unfortunately I'm still waiting to win the lottery.
I think it must be next week.
 
The more worrying part is that the lack of earth could have presented a safety hazard without me knowing. Fortuitously, the sockets on that part of the ring are used mainly for lights.
In fact, that in itself probably wasn't a significant hazard. Assuming that the break was between two sockets, by virtue of it being a ring, every socket should have had a satisfactory earth (round one side or other of the ring) - in fact, that redundancy of the CPCs (earths) is one of the 'pros' (and there are not that many of them!) for ring final circuits.

Kind Regards, John.
 
What did your BCO say when you told them you had changed the board and showed the tests results from your multimeter :rolleyes:

I suppose it was just a case of "I'm not selling so i don't need to follow the rules".
 

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