Ring main breaker tripping - can't identify cable fault

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craig1234

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Looking for some help I have with a ring main. The 32A breaker serving this circuit tripped and wouldn't reset so by a process of elimination, I established (I think) that there is an issue with a cable running between two adjacent sockets (around 2m apart). If the live from either end of this cable is in circuit then the breaker trips - removed and all appears ok (although with the ends removed the breaker did trip once a few hours later but did then reset). What is baffling me is the fact that I can find no shorts (to neutral, earth) on the cable so don't know why it causes the breaker to trip (unfortunately the cable runs under a wooden floor so I can't see what may be going on). Any ideas?
 
We have three trips which can be in one, a thermal trip and a magnetic trip set to 32A and 160A (Type B) and also often an earth leakage trip at 30 mA. For the first two to trip there needs to be a real fault, but for the third simply having too much cable can cause trips.

Rats have been know to damage cables, there are a number of testing tools, the 500 volt insulation tester, uses DC, but the clamp-on ammeter uses AC so the former can show clear but the latter shows some leakage due to inductive and capacitive linking. In theory design should allow 9 mA leakage with nothing plugged in, but in practice I have not got a meter to measure it, so I use the RCBO which is a RCD and MCB combined so the amount of cable on each RCD is no more that 106 meters (max for ring final) but many consumer units have multi-circuits on on RCD so not unknown for three times that length.

My old house would have a bout of tripping then a period of few years with no trips, with 2 RCD's, which is why new house has 14 RCBO's so every circuit is independent.

So exactly what has tripped, and how have you tested it? A multi-meter is unlikely to find any faults.
 
Whilst the RCD does trip ( there is one RCD (I believe it to be an RCD) serving the consumer unit which has 4 circuits (probably no more than 80m total length)), the MCB trips as well so doesn't this indicate a short somewhere? I only have a multimeter so testing capability is limited. To eliminate the MCB as the problem I swapped circuits in the consumer unit with the same result. The property is a rental (which means I cannot look easily at the moment to confirm the type of RCD) but I am returning there tomorrow for a different issue (will check then) and had the thought of running a temporary cable between the two sockets to see what happens.
 
Might be a nail through the cable. When this is in a floor, seemingly random trips can be caused by the weight of people walking or standing on it.
 
I agree but what I can't understand is why I don't detect any issues with my meter - doesn't make sense.
 
You might need a meter with a higher voltage insulation test. A normal multimeter isn't going to cut it, a proper insulation tester will test at 250V or even 500V
 
I'm pretty certain the cable is damaged even though I can't detect the fault (what else could it be to cause the trip?) and at the moment I have effectively created two radial circuits that appear to be working. Whilst there are lots of sockets, reality is that only around 6 are being used and with the difficulty of replacing the cable (t&g oak floor), I am inclined to leave it this way. As previously mentioned, if I get the opportunity tomorrow (tenant works from home in that room) I will sling a temporary cable across between the two sockets to see what happens - hopefully no trip otherwise I really would be stumped.
 
You better order a 20A or 25 MCB then.

The fact you have already had one trip may indicate more to come
 
True - will have to see if it holds up over the next few days. The cable has been in for about 12 years and where it runs I doubt very much whether mice have got to it so perhaps I was a little careless with a nail all those years ago!
 
I had a fault with last house, and I fitted a 20A MCB as a temporary measure, when I did find the fault, a lose wire, I did question what I had done as a temporary measure. I have when in a hurry used a multi-meter and found not fault, but returned to car and got the proper insulation tester using 500 volt and the same motor showed nearly short circuit. And fact it would rupture a 13A fuse clearly showed there was a fault. Yes we have all tried using multi-meters, but really not the tool for the job.
 
Thanks Eric. Will see how I get on tomorrow with my temporary cable and will update.
 
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