Mystery Circuit Overload

[diy_dave]

Hi all

I have a combi-boiler recently wired up to a circuit that keeps tripping the MCB but I can't understand what it can be...

The only appliances on the circuit that I am aware of are the boiler and an extractor fan (small bathroom type).

The boiler has a fused switch with the extractor fan spurred off with a fused switch. I believe all fuses are 3A (the internal boiler fuse is either 2 or 3A) and the MCB is 15A.

I am unaware of any other appliances on this circuit (I will turn MCB off and test all my switches, sockets etc tonight to be 100% clear) and it is an intermittent problem.

On two occasions the MCB adjacent to the boiler MCB has tripped as well (a red herring?? Can the force of the trip knock an adjacent MCB out?)

I am a bit baffled as if the only appliances are the extractor and boiler, why aren't they blowing their lower rated fuses?

Cheers for any help.
Dave

 

[diy_dave]

hmm.. now both circuits are tripping their respective MCBs even with no load...

i had a brain wave last night well actually it's pretty obvious maybe.. i nailed some skirting board to a partition wall that is inbetween my CU and the boiler - not that the CU and boiler are actually on this wall but i am guessing there could be some cable in the wall as it is in the path between the two.

if i have pinned a wire could this exhibit the symptoms described?

i have pulled the skirting board off now (i only tacked it to hold it in place while my gripfill dried)

 

[Big_Spark]

Dave, if your nailing antics is the cause of your woes then you need to find the damaged cable. It certainly implies that there is a HEAT problem in the MCB.

If one MCB gets too hot, it can cause the operation of the bimental trip in an adjacent MCB.

I would suggest that you call in an electrician to do a proper test on those circuits so you an either establish or rule out the nail theory and identify the true cause of the fault.

 

[AnonymousImport]

I had a similar problem with a new ring installed by a so called electrian. Every hour or so the circuit would trip. I checked every socket for bad wiring & found a wire that had been arcing. Problem sorted (I think this was just one of those things, as the rest of sparkies work seems fine. So my guess is that there is a bad connection causing arcing.

 

[Igorian]

I think this was just one of those things, as the rest of sparkies work seems fine.

A bit off the subject, but I had a heating engineer in to service my boiler. About 20 minutes after he left, I noticed a) it was leaking and b) the boiler wasn't firing. The first was due to a leaky valve and the second was because one of the power cables had fallen out.

I wonder how many others have had 'just one of those things' happen?

 

[diy_dave]

Thanks for you comments. I intended to come home and cut the plasterboard back to investigate but i think a rogue nail might be a long shot...What are the chances of piercing wires from two circuits? I take it that the very purpose of the separate circuits is that they are isolated. Can a dodgy CU be a cause of my problems? It is an old fuse box (in a victorian house) with modern MCBs. Do MCBs have a life span?

The circuit that now has the boiler on it was previously used for an immersion heater that was rarely used. Now the circuit (and obviously the cabling) is in regular use, could it have induced a problem?

The intially faulty circuit at least allows me to turn the MCB on - the MCB from the other circuit doesn't come on at all now even with every appliance off plus there is a bit of scorching around this MCB on the CU that i don't recall being there before.

 

[securespark]

What are the chances of piercing wires from two circuits?

As FWL said, one hot mcb could cause another to trip incidentally.

 

[diy_dave]

Each of the two MCBs trip even if the other MCB is off. Might have to get an expert in...

Anyone know of any sites that have info re: troubleshooting consumer units / potential CU problems?

Thanks again
Dave

 

[diy_dave]

The originally faulty circuit had a cable problem. A circuit tester on live-earth and neutral-earth showed they were all connected.

Plus I have learned by taking the fuse box apart that each circuit isn't entirely isolated as they all share the neutral and earth blocks - news to me but probably not anyone else...

Replaced the wire and it's now working although the other circuit that was affected is fried with the cabling in partition walls and concrete floor - any ideas how to track down the location of the faulty wire? No idea how to start troubleshooting this one.

Getting there slowly...

 

[breezer]

you can often spend more time looking for a cable fault. it is best to change the whole cable*

when you find the fault what are you going to do? change the whole cable?


* unless economicaly unviable

 

[diy_dave]

Thing is, the whole cable either runs under a concrete floor or through a series of partition walls, some of which has been replastered a month ago.

If there are say 5 power sockets around the room, would you therefore have to run new cable to all 5?

I was hoping I might be able to replace a faulty stretch of wire to minimise disruption.

 

[breezer]

you only need to replace the damged cable. that way there are no joints

 

[diy_dave]

breezer - just checked out your useful reference post again and it makes much more sense now.

If I want to isolate and check each stretch of cable between power sockets on my ring circuit, what is the usual approach?

 

[securespark]

You need a megger to test insulation resistance.

If it is a ring, you could start by disconnecting all 6 conductors at the board (SWITCH OFF!) and putting a megger across L & N, L & E and N & E of one leg, while joining up the corrsponding conductors on the other leg.

This will show up an anomalies, then you could disconnect each outlet on the circuit, and seperate the conductors, and megger each leg, joining up at one end and attaching the meter to the other as before.