ring main fault

[Radon]

A ring main in my flat, wired in 2001, has started tripping the mcb. It has never been overloaded. It is excessively (?) large (21 sockets, mostly doubles) and serves several rooms. Several sockets are served by only one input cable, but only one has three incoming cables. So there must be joint boxes somewhere under the floor, which is all laminated and would be hard to uplift. Is there any alternative way of detecting the location of the fault than splitting the ring at one of the sockets and at the consumer unit, and then linking the non-live section to the live one a bit at a time until it trips again, and then doing the same process on the other half of the circuit? I am a competent amateur and don't have any specialist equipment. Would the process be simpler if I got some?

 

[Taylortwocities]

Is it the MCB that is tripping? Or the RCD? Or is it an RCBO?


What is written on the device that is tripping. A picture muight help.
?

Fault finding on rings is always entertaining. An approach is to split the ring in the middle and test in both directions back to the board. You may need specialist test gear. But lets find out what's tripping first.

 

[Mursal]

Is it the MCB that's tripping (little narrow things) or the RCD (the big one with the test button)?

We had a MCB trip and it was a fault in a double socket between live and neutral. Granted the socket was in high use running a space heater.

Edit: Sorry, a bit slow at the writing, didn't see above post. ...

 

[wingcoax]

With only one socket having three cables and most having only one? that should be fun locating the fault. Have you unplugged all appliances? If not, do so and try resetting. If it stays in then plug an appliance in one at a time and see what happens.

 

[RMS]

An approach is to split the ring in the middle and test in both directions back to the board. You may need specialist test gear.

Assuming you've separated the rfc conductors at the consumer unit of course and we are attempting to locate low insulation resistance.

 

[Radon]

Thank you for your responses, everyone. The MCB tripped once, but the thing that trips every time is the RCD/RCBO - I don'tknow the difference. The one in place when I started was a Contactum one marked 7132B (B32). I replaced it with a new one (just in case) which has a slightly wider switch but the same serial number. The MCB tripped while the circuit was off at the RCD/RCBO while I was chipping out a metal box on which I saw signs of rust. I wondered (since it was on an outside wall) if the recent heavy rain had penetrated, but although the cement and plaster around it had deteriorated, it didn't seem damp now. I have left the space open for 2 days, and warmed the cable ends with a hair dryer, but when I restore the power, it still cuts out. Usually it cuts out straight away, but sometimes it stays for 20 seconds or so, and then cuts out. I don't have any special test equipment, which is why I thought of splitting the ring at every socket (heaven knows where the centre is), detaching one end of the circuit from the consumer unit and then powering the other one up, and restoring one socket connection at a time until the RCD tripped again. Then I could do the same to the other half of the circuit till it tripped. Presumably what was left between (what tripped the RCD both times) would be the faulty part. But this will be a long process ! I suppose it will be satisfying if it works out (possibly even entertaining!!), after which I can tackle the laminate floor (or install surface trunking, ugh! everywhere). An added irritation, but not perhaps relevant, is that the walls are very hard and the cables have been plastered in with no conduit or trunking, and the tails in the boxes were left so short that the installer had often to crimp bits on the ends in order to fit sockets and switches. And it pased inspection ...

 

[PrenticeBoyofDerry]

Did your problem start before you started to remove metal back box or after?
I assume when you say RCD/RCBO you mean RCD? as your ring final circuit would not be protected by a RCBO and a MCB.
Either protected by a RCD and a MCB or just a RCBO as this serves both as a RCD and a MCB combo.
You will find that when working on a circuit protect by a RCD even with MCB open it may still trip.
Surprised that the MCB tripped though when circuit was isolated at RCD, are you sure of this?
The RCD will have both a test button and a dolly(switch) on it , where the MCB will just have a dolly.
How long since the installer installed?
If the RCD is tripping does it happen when nothing is on load (all appliances unplugged from circuit)?
If you have no test equipment, I would suggest a little investigation if trips occur off load, check all terminals are tight in accessories and no insulation of either neutral or phase conductors are trapped or bare within accessories. Do isolate the circuit and prove the circuit is dead, before carrying out any investigative work within covers of accessories.

 

[Radon]

Thank you for this response. It's now clear tome that my system has and RCD and an MCB. Yes, I am sure that the MCB tripped when the RCD was open. The circuit was originally installed in 2001, but may have been modified subsequently before I bought the flat. Yes, the RCD was tripping when there was no load. I opened all the sockets and checked the connections for tightness. The tripping started before I removed the metal box. In fact I was out of the house when it first happened. The good news is that I was out all Sunday and did nothing on the work. When I switched on the current this morning, nothing tripped. So I concluded that the cables serving the previously damp box (which I had removed) had now dried out. I removed some skirting board, pulled the wires forward as much as I could and stripped them back a bit. The copper was slightly corroded. I rejoined them in a 30A junction box, which I hope to move to underneath the floor. The circuit has worked fine all day.

 

[PrenticeBoyofDerry]

Moisture in the cable terminals will cause tripping and could well have been your problem. As for junction being placed under floors! Part of the requirements state that all joints should be easily accessible for inspection and testing, so a standard 30A junction box would not be allowed to be hidden under floors, unless a trap/hatch for access was made.
There are other methods of jointing cables that conforms to requirements and can placed in ceiling/floor voids. Some do need special tools but using a MF/ Maintenance Free Junction Box will be the easiest way to conform.

 

[ban-all-sheds]

Yes, I am sure that the MCB tripped when the RCD was open.

Then it's not on that RCD - simples.