Why did I install MCBs?

[hedgert]

My house has been heavily extended and I'm sure any professional electrician would have had a fit had he looked under the stairs: 4 separate consumer units (all with old style fuse wire fuses) and wires everywhere.

I decided to replace the whole lot with a modern consumer unit with MCBs.
I was happy with my work, I now have a single modern consumer unit with nice resetable MCBs on each circuit.

The total number of fuses is the same, and I've made sure to keep the ratings of the MCBs the same/similar as the fuses they replaced.

All is fine except for one problem - one MCB keeps tripping. Alas it's the ring that includes my study with my computers in. Who knows whether I'll finish writing this post before it trips.

The tripping isn't predictable. The ring is a ring that supplies the whole of the back of the house and had a 45A fuse wire before and now has a 40A MCB.

With the power on and off like a yo-yo I've had an opportunity to identify each and every item on this ring - and the total consumption simply isn't 40A.

The ring covers the immersion heater and 4 rooms. I've turned off the appliances in all rooms except the study (including the immersion heater) and the items I have on in the study all go through a 13A fused wall switch. When the 40A MCB trips, the 13A fuse (through which the only stuff in use is going) does not blow. So I know I'm not using enough to "blow" a conventional fuse - and of course I didn't have a problem until I replaced the old style fuse wire fuses with MCBs.

I've tried another MCB (admittedly a 30A as I didn't have a spare 40A, but again from above, I can't be using more than 13A) and the same thing happens.

Any suggestions?
Could I have a "noisy" supply? If so how do I find out?
Can an appliance trip the MCB without running over the MCB rating?
Given that the only thing I've changed is the consumer unit arrangement - is sparking possible inside the consumer unit - there's a little bit of bare copper wire on the wire connected to this circuit - but the spark would have to jump in excess of a centimetre if it was to short. Is this likely?

 

[securespark]

A ring should be protected by a 30/32A device.

If you have an intermittent problem like a short, this would trip out the mcb.

An mcb protects against two scenarios - overcurrent and short circuit. They are separate conditions.

 

[supersparks]

first off, changing fuseboxs isnt for diyers.

getting mcbs of similar rating may not be good enough. you need to calculate.

and you obviously dident do the tests such as insulation which would indicate problems such as this, you would also have noticed that under normal conditions, domestic ring circuits are on 32 amp breakers.....

i suggest you get an electrician, a 40 amp breaker will not trip for nothing...

 

[hedgert]

I agree it sounds like I am going to have to get an electrician in.
However, on the specific points you've made:
1 - insulation testing - I haven't changed any of the wires coming out of the consumer unit - so any insulation issues were already there - I haven't introduced them
2 - on the rating the original fuse covers both a the ring and the immersion heater - which I guess is why it isn't the convention rating for a ring main.
Is it possible that there could have been an issue with the original wiring (as I haven't really changed any of that) that a conventional fuse wire fuse would tolerate but an MCB wouldn't?

 

[supersparks]

insulation resistance reading of the instalation, what you add to it dosn't mater, you should still do one as it highlights cable condition.
also should have done PFC and PSC readings, and r1+r2 to calculate ZS or measure it to make sure your mcbs are correct.

people don't realise what is involved in changing a unit, and wonder why the cost is high, because they don't realise the labour....(not nocking you btw!)

rewireable 3036 fuses are terrible. id never trust anything to them.

whats likley to have happened,is it was wired on a 30 amp fuse, something went wrong, previous owner dident want to pay, so just uprated fuse.... very common.

either that or next door is getting free showers/ electricity supply....

i have been to a place in the past where 3 houses were linked off eachother, all off one poor bastered

but that was thanks to conversions in the past not done properly rather than a quick drill through wall and hook a cable through

 

[delmel]

Sounds like an old house, prob original wiring as you would not have the immersion joined to any ring main. I would hazzard a guess that the immersions on, perhaps one of the other rooms has an appliance on it like a kettle, tumble dryer, freezer i.e kitchen and you are experiencing a surge in power, overloading and knocking it off.
Sounds like a rewire could be required. Perhaps not what you like to hear.

 

[Damocles]

Check the readings on your electricity meter. It is reading the current through your system. If you switch off everything else you should be able to see how much power is being used by your faulty circuit. 1 KWH is 1 unit on the clock in one hour, or about 4A being drawn. 40A is about 10 units used in an hour, one every six minutes. Looking at the meter should tell you if the circuit is really running under a heavy load or if it has an intermittent fault.

mcb are more sensitive to sudden surges than wire fuses. Got any lift motors plugged in in the extension? It could just possibly be a particular appliance causing the problem. Cooker or electric shower got onto it somehow?

 

[mapj1]

As a possibility -try disconecting the immersion, and feeding it from a separate supply if you can- it may be on its last legs. It is an MCB thats is tripping, not an RCBO- that is to say it couldn't be earth leakage could it?

 

[ban-all-sheds]

2 - on the rating the original fuse covers both a the ring and the immersion heater - which I guess is why it isn't the convention rating for a ring main.

That "convention" as you so interestingly call it is nothing to do with the expected load, and therefore available for change if your load is "unconventional", and everything to do with the capacity of the circuit cable.

 

[hedgert]

Following the advice given, I called in an electrician.
My CU wiring passed muster.
The issue turned out to be problem wiring burried in the walls/floor between two of the sockets in the ring - i.e. an existing problem that the previous wired fuses did not detect, but the new more sensitive MCBs did.
So problem understood - I just need to rewire the ring!