Help! Mcb Tripping...

[Larches]

I've been having a problem with the MCB tripping on one of my downstairs socket circuits.  It has been happening for many months now, but just occasionally and sporadically, and I can usually reset the switch straight away and it's fine for a while.  

It's concerning as obviously it's indicative of a problem somewhere, and it takes out both the fridge freezer and a separate freezer, so we're always worried if we go away that we'll come back and find a waterlogged floor and ruined food.  Trouble is, I don't know how we can figure out the problem when it happens so infrequently - it's not like we can just unplug stuff in turn and then wait weeks on end to see if it has made a difference.  I could of course call in the professionals but would rather not waste someone's time wave goodbye to a load of cash if there are things I can check myself.  So if anyone has any thoughts/suggestions, I'd be grateful to hear them.

It's a Hager consumer unit with RCD-protected circuits, and sometimes both the RCD and MCB have flipped, other times just the MCB.

Any help much appreciated.

 

[NozSpark]

During testing a fault might show itself but not at a level that will trip a RCD or MCB (on a "good day")

The last time I went to a fault like this I traced it to the fridge freezer...it wasn't an old one IIRC

 

[ProDave]

What you have diagnosed so far helps.  If it's sometimes just the MCB tripping, then it's an over current issue. (if it were always the rcd tripping it would probably be a leakage issue)

Fridge or freezer at start up are likely candidates.  

A good test, would be to find a long extension lead, and temporarily power the fridge or the freezer from a different circuit, perhaps upstairs sockets.  If the same mcb trips, then that item is not the culprit, if the mcb for the other circuit trips, you have found the faulty item.

 

[Blue Duck]

Got mice?

 

[steptoe]

what size is the MCB?

could someone have changed it, or put the wrong size in originally???

 

[apprentice87]

Hi Larches,

You are obviously not daft!!!! Anyway, on average what sort of current does you downstairs socket circuit draw?? What size MCB is it?? Any particular time of day the thing trips out??

john..

 

[Murdoch]

Do you have any test equipment?

 

[Windpasser]

It does sound like an appliance problem.

Fridges and freezers are easier to check as stated ealier, unplug them and place on a separate circuit.

The other appliances to check for are any hard wired ones such as a dish washer, washing machine, CH system if its on the rfc.

While your delving under the work surfaces you might like to check the state of the sockets.

 

[NozSpark]

Also make sure that all the card legends off the sockets have been removed, they soak up and hold moisture

 

[Doc Hudson]

If you have another MCB of the same rating in your fuse box it may be worth while swapping the two over just to verify it's not simply an oversensitive MCB.

Doc H.

 

[Larches]

Thanks everyone for your replies, really appreciated.

@NozSpark, leghr, ProDave (top profile picture btw :-D) ... yes it might have started when we got the new fridge freezer, come to think of it.  It's not even a year old but as per NozSpark I don't suppose that means anything.  It's a mid-range Bosch thing though, not any old cheap crap.  :/  Made in Turkey rather than Germany though, so for all I know it could be a Beko with a posh Bosch badge.

Anyway, the circuit swap is a good idea (probably need to get a longer ext lead to try that).

@NozSpark, good point about the card legends (I'd never have thought of that) but yes they're all removed.

@Blue Duck, no mice to my knowledge, but maybe we should let the neighbour's cat in for a bit...

@steptoe, apprentice87; it's a 32A MCB (Hager B32 - got 3 of them on the board).  Not sure about the total current offhand, I should probably try and figure that out.  No particular time of day.  First time it tripped was early morning (on the one day of the year we had a flight to catch, of course).  Last time was late afternoon and we were all out at the time.

@Murdoch: no, no test equipment :/ (yet...)
 

If you have another MCB of the same rating in your fuse box it may be worth while swapping the two over just to verify it's not simply an oversensitive MCB.

Doc H.

Good plan, I'll give that a whirl first.
 

 

[Marvo]

If you have a poor termination on the MCB itself that could cause it to nuisance trip. Check for terminations that show signs of heat and check the busbar is actually inside the terminal clamp of the MCB and not mistakenly at the back of it. Obviously isolate the entire CU when you're checking these things.

 

[Larches]

If you have a poor termination on the MCB itself that could cause it to nuisance trip. Check for terminations that show signs of heat and check the busbar is actually inside the terminal clamp of the MCB and not mistakenly at the back of it. Obviously isolate the entire CU when you're checking these things.

Will do - thank you

 

[Murdoch]

So we are suggesting a DIYer messes around inside his fuse board - madness.

 

[apprentice87]

I know what you are saying, but no law against it, and the chap does sound sensible, probably a lot better than some of the 5WW....

john...

 

[binky]

Random tripping is nearly always fridge freezers, but what about weather conditions, like does it coincide with rainy days?

 

[ProDave]

Have you done that long extension lead from a different circuit test yet?  Do that before you start taking stuff apart.

 

[Larches]

Random tripping is nearly always fridge freezers, but what about weather conditions, like does it coincide with rainy days?

I think the last one was a cold rainy day, but we also had it when the weather was hot (and the neighbour reckoned it was the heat that'd done it...).

Don't know where we stand if it is the fridge freezer... I mean assuming the f-f is faulty in some way, we'll have a job on proving that (it's still under warranty).

 

[NozSpark]

Is shouldn't think that you'd have a problem, if its faulty then its faulty and it will then fall under the terms of your warranty

 

[Larches]

Have you done that long extension lead from a different circuit test yet?  Do that before you start taking stuff apart.

Not yet - need to get a longer lead.  I'll be working through all the suggestions starting with the easiest first.

Is shouldn't think that you'd have a problem, if its faulty then its faulty and it will then fall under the terms of your warranty

Yeah, maybe you're right, I just know what swine some of these companies can be if there's any wriggle room and of course sporadic electrical faults can be hard to replicate if they don't feel like taking our word for it.  But then it's Bosch and I think their service is normally pretty good.