Tripping RCBO

[Knotty]

After failing to find why one of my ring circuits tripped the RCD for the whole house, apparently at random, I replaced the CU with a new MK high integrity box. This meant that I could fit RCBOs to each of the ring circuits and (lights plus fire alarms) circuit. The rest go via an RCD, as before. That way only the faulty circuit trips out.

The main house ring works perfectly with all appliances while the ring is broken, but as soon as I complete the ring it trips, either immediately, or after a few minutes

The voltage across the break is zero for line, neutral and earth and an ammeter shows minimal current flow. I have isolated and Meggered both parts of the ring and the insulation seems ok.

The fault is not related to any appliances, as it does the same with nothing plugged in, nor any fused spurs switched on.

The house works fine using the ring as twin spurs- (no problems in two weeks). However, I feel that I must identify the fault and connect the ring properly.

Any advice gratefully received.

 

[holmslaw]

..

 

[Knotty]

How do you mean "Mixed up"?
There is only one ring involved in this problem and if I had crossed the Phase and Neutral ther would have been volts between them at the ring break.

I have not rewired any of the ring circuit, only its supply.
Please clarify what you think I may have miswired. Thanks

 

[holmslaw]

..

 

[JohnD]

each ring has two legs. So for each, the two L conductors must come back to the same MCB or RCBO, and the two N conductors must come back to the same Neutral terminal (which for an RCBO must be in the non-rcd block of a split board)

It is not unusual for the conductors of adjacent rings to get mixed up in a board.

Try more continuity tests incudng the conductors from other circuits.

 

[Knotty]

You are right, there are two rings, but the other one seems to be working fine and it is a complete ring. That one was added when we had major extensions a few years ago. The wires for that go off in another direction, so I do not think they are involved.

The ring that I cannot close is basically the original house wiring, which has been fine for the past 30 years.

Something was tripping the main RCD previously, but I have just discovered that it is on what I call the House Ring, not on the one I call the Back Ring.

 

[EFLImpudence]

You do not mention continuity tests at your 'break' - L-L, N-N, E-E.

Have you done this test?

 

[Knotty]

I cannot remember if I did those continuity tests, so I will do them again.

I rater assumed that if there were no volts between them they were joined together, but that may not be so. Thanks

Unfortunately, my computer is on the circuit in question, so it will take some time to respond.

 

[Knotty]

That was interesting!

L-L Dead Short
N-N Deat Short
E-E 100 ohms

So I checked both the eart wires to a water pipe

Clockwise Dead Short
Anticlockwise 100 ohms

This means that I have some resistance in the earth wire for that part of the circuit. I suppose it could be a loose or even burnt connector.

I supose the Megger would have seen that as a short circuit, but i an a bit surprised that my Sucket testr did not show any problems. On the other hand 100 Ohms is not a lot. However the RCBO seems to think so, but I am a bit surprised that it does not trip when the circuita are isolated.

I am grateful that you have all pointed me in the right direction.
What I am not looking forward to, is breaking the ring in umpteen places to track down the culprit.

When/if I find it I will report back.

Thanks

 

[RF Lighting]

What are the insulation resistance test results for the circuit?

Did you really swap out the board rather than doing some basic tests to try and sort this fault?

 

[Knotty]

Yes, I did try to find the fault, but the trips were intermittent and did not show up immediately.
To be honest, I was convinced that I wiould find it on the "Back" ring, which has things like waterfall pumps and outdoor sockets, rather than the "House " ring, which has been installed and stable for many years.

I also did not want to be without power while I carried out an extensive exercise. The idea was to isolate the offending circuit and that has been achieved.

Hindsight is a wonderful thing!

 

[flameport]

i an a bit surprised that my Sucket testr did not show any problems.

If you mean one of those plug things with 3 lights in it, then it won't show any problem - they only give a warning when certain types of really obvious faults exist. Generally they are useless. Even when they show a problem exists, they can be wrong.

On the other hand 100 Ohms is not a lot.

Considering what it should be, 100 ohms is massive and indicates a serious fault.

You don't need to break the ring or dismantle anything to locate the fault.
Further basic tests will quickly locate it.

 

[EFLImpudence]

This is my thinking:

Untitled

• EFLImpudence
• 26 Sep 2011
• 1

I don't know what kind of earthing.

As above - Socket testers cannot detect N-E faults

 

[Knotty]

Thanks EFLImpudence, that would indeed explain the situation.
I can close the L-L break or the N-N break and the RCBO does not trip, but as soon as I join the earth wires it trips.

However, your diagram means that I have now two faults: a N-E resistive fault that could be anywhere plus a break in the cpc between the fault and the CU.

I feel sure I can isolate them eventually, but I do not understand how to do it without finding out exactly where on the "ring" it is.

The last time I had a problem like this it was a skirting board nail that had grazed the neutral conductor and that took some finding.

What are these "further basic tests that will quickly locate it" referred to by flameport? I must admit that I am more familiar with 400kV circuits than malevolent 240 volt ones.

I take your point about the 3 light socket tester, which says on it that it will not find N-E faults. However, it has in the past helped me to locate a broken neutral.

I will make a few more measurements tomorrow, but this evening I decided to go Morris Dancing to preserve my sanity!

 

[bernardgreen]

The fault might be on a different ring to the one that appears to have the fault. The trip being caused by some of the current from a load on the ring you suspect is the faulty ring finding an alternative route that bypasses the sensor in the RCD that is tripping.

Neutral_Earth_trip

• bernardgreen
• 16 Jan 2011
• 48