MCB melted

[catlad]

Hi all
Never posted in this forum before and know nothing about electrics
but would like some advice please.
The mrs noticed a burning smell under the stairs today which is were the fuse box
is located, anyway on inspection one of the mcb's has melted quite badly burned
but did not trip. I thought the new fuse boxes were supposed to trip well before
any chance of a fire. Can anyone give me a reason why this should happen?

The house was rewired about 6yrs ago

 

[Iggifer]

Loose connection. Is the melting at the top or bottom of the MCB?

Was it a power hungry circuit?

Post a photo.

 

[Taylortwocities]

What make of consumer unit is it?

About 6 years ago the Electrium Group (Wylex, Crabtree and Volex) had a bad batch of MCBs that were faulty and did stuff like that.

Info at
http://www.electricalsafetyfirst.or.../electrium-miniature-circuit-breakers-(mcbs)/

 

[PrenticeBoyofDerry]

Make of consumer unit and MCB? as they may be different1

 

[catlad]

Ok Thanks for the replies
The name on the front of the consumer unit is Domae
The mcb is burned at the top.
And from the top down it says in yellow square D
and under that MCB
dom20b6
b20
6000
240v
I did try and upload a picture but I have not been able to get the
photo's of my phone and onto the laptop for some reason.
The one that has gone runs the washing machine, fridge,and the kettle
oh and some led lights.

 

[Iggifer]

Sounds very much like a loose connection to me. If you're confident enough to replace it yourself, toolstation sell compatible breakers for a couple of quid, of you can get one on the internet that's exactly the same visually for a few more. You'll need to cut back the outgoing cable until the copper is bright again.

If you're not, it's time to get a sparky in.

 

[catlad]

Thanks Iggy
I am confident to replace like for like so to speak
but I have a sparky coming tomorrow
so I will let him deal with it.
The thing that is worrying me is why did it not trip out?

 

[Echo the husky]

I'm not sure, we still haven't seen the condition of the MCB yet!

 

[Taylortwocities]

The thing that is worrying me is why did it not trip out?

Because the MCB will trip if there is too much current going through the circuit. It does not trip because it has got too hot!
In fact, if it were a loose connection, this is more likely to reduce the amount of current going through the MCB.

 

[JohnW2]

Because the MCB will trip if there is too much current going through the circuit. It does not trip because it has got too hot!

Although they are certainly designed to trip when too much current goes through them, the thermal part of an MCB does trip "because it has got too hot" (usually due to too much current flowing) - so I'm at least a bit surprised that the whole MCB can get so hot as to melt without the thermal part of the device operating.

Kind Regards, John

 

[PrenticeBoyofDerry]

The problem is, that we need to rely on the fact the MCB is functioning correctly and is not faulty! And to do this means a very expensive bit of kit.
So really reliant on manufacture factory testing, maybe a faulty one has been manufactured.

But it does sound like a loose connection, but if the device is not performing as expected, a demand of kettle, washing mashing, F/F plus LED lighting, might be a tipper to overheating on a faulty 20 Amper, if load demand is at full.

Square D did have product warning notice a few years back, but if I recall it was related to some of their RCBOs and not MCBs!

 

[kai]

Did the breaker try to interrupt a fault current greater than its breaking capacity by any chance? Most household MCB's are rated at up to 6kA (M6) fault current at most. If a larger fault current flows, it may damage the MCB breaker beyond repair.

 

[JohnW2]

Did the breaker try to interrupt a fault current greater than its breaking capacity by any chance? Most household MCB's are rated at up to 6kA (M6) fault current at most. If a larger fault current flows, it may damage the MCB breaker beyond repair.

My experience is very limited but I have to wonder where these enormous PFCs in domestic premises come from, even in London - not that it means much, but I don't think I have ever seen one appreciably over 1kA. A PFC of 6kA means a total loop impedance of less than 0.04Ω - which would seemingly require one to have a substation in, or close to,one's garden, connected by pretty fat cable

I wonder how high are the 'high PFCs' which the electricians here see?

Kind Regards, John

 

[bernardgreen]

Most household MCB's are rated at up to 6kA (M6) fault current at most. If a larger fault current flows, it may damage the MCB breaker beyond repair.

A heavy current less than the rated safe breaking current ( 6 kA ) can damage the MCB. The rating is the current that the MCB is garanteed to break when it operates. A higher fault current may continue to flow because the MCB contacts have been welded closed by the current before the mechanism could fully open them to break the current.

 

[Echo the husky]

Highest I ever measured in a domestic installation is 8.1KA