random mcb trips

[onzadog]

Hi folks

Wonder if anybody can help shed some light on a little problem I'm having?

House built mid 1980s, consumer unit replaced about 3 years ago. Upstairs lighting circuit supports only 7 lamps, therefore worst case even if these were all 60w which they're not, then we'd be drawing less than 2 amps. However that's academic as the MCB is tripping whether there's lights on or not.

Now for the really weird bit.

We can go months without an issue and then we seem to have a spate of 3 or 4 days where it will trip several times a day sometimes it will switch back on, sometimes it won't. However it normally settles down after a few days and then nothing for months.

It's a Wylex 6amp mini circuit breaker in a Wylex consumer unit. None of the bulbs have blown and there's no obvious signs of damage/shorts to anywhere in the circuit.

Possibly a red herring but we've just suffered a series of short power failures in the area we live, not sure if these are connected or not and can't remember whether this happened last time or not, but we do seem to get more than our fair share of power cuts in this neighbourhood.

I'm sure MCBs don't trip for no reason, I just can't for the life of me work out what the reason is for this one. Can anyone help?

 

[ban-all-sheds]

Damp? Do the faults happen during or after periods of heavy rain?

Are they associated with people moving about in any particular place(s)?

Has it been happening since the CU was replaced? What are the insulation resistance figures for that circuit on the EIC?

Did it first happen not long after something else was done which involved work on floors/walls/ceilings?

Have you checked the loft for rodent damage?

 

[holmslaw]

..

 

[onzadog]

Cheers guys. Swapped the MCBs from the up and downstairs lighting circuits earlier and the upstairs has just gone pop again so I guess it's not a faulty MCB. Wish it was, that would have been an easy one. I've just done a quick visual inspection of pendants and switches and I can't see any obvious faults or signs of arcing. Test wise, I've just got standard multi-metre so a bit limited. I'm now thinking of getting another MCB and splitting the circuit until I can narrow down the area of the fault. It doesn't seem to be weather dependant either with regard to heat or water ingress. Most of the trips happen when the lights are NOT on. I just hope I don't have to go routing around in the insulation trying to find a faulty bit of T&E.

 

[davelx]

OK, so somewhere in that circuit, if this is not an intermittent low resistance short, but a higher resistance short, potentially something taking in excess of 6A for long enough to pop the MCB, and you don't know where.

A 6A type B MCB will pass 7A indefinitely - so that means that something is putting out 1600watts+ of heat, possibly for quite a long time - and you don't know where.

Doesn't that worry you? It would worry me. I'd suggest you get this investigated urgently, it's just possible you have a fire waiting to start somewhere in your house. On the other hand, perhaps you have some little visitors gnawing at your cables and every now and then they cause a short.

Either way I think you need to get this sorted sooner rather than later.

 

[onzadog]

Well, I can't see any sign of that sort of out put. We have an energy monitor in the house as well. Right now, even running the laptop, the TV and all the usual background stuff like fridge freezers etc, we're not even using 600W so I guess that points to an intermittent low resistance short.

 

[dannyboi2003]

Extractor fan in the bathroom, outside light or loft socket for aerial booster connected to the upstairs circuit? Things like this i would suspect and check/disconnect first.

 

[maltaron]

Do you have any 12v downlighters running off inductive transformers? The surge current, particularly on Switch off, can be extremely high. I have cured this problem with a type C MCB instead of type B.

 

[JohnD]

does it happen more on rainy days?

 

[onzadog]

Last time it happened was in the summer when it was really warm. There's just two strip lights in the loft and 6 normal pendant type lamps, most of those have energy saving bulbs in. It's stopped doing it now but I don't know why. That's the bit that makes it really hard to diagnose.

Thanks for keeping the ideas coming though.

 

[sparkwright]

Check the loft for any damaged cables.

A sheet of chipboard or floorboard can easily damage a cable laid over a rafter, then walked on.

Check inside any light fittings or switches that have been replaced recently for 'snagging' of wires.

Check for any DIY that may have caused some damage.