Call out to a melted fuse box

[RF Lighting]

Got a call out this morning to a customer with a "melted fuse box"

I wonder if anyone can see what caused the immediate surprise on my arrival, and also what might have caused this to happen.

(I took the FCU in bits just to have a look out of curiosity)

 

[333rocky333]

silver foil round the fuse

 

[ban-all-sheds]

What was it supplying which necessitated the use of metal foil?

 

[RF Lighting]

It was supplying a 3kW immersion heater.

 

[ban-all-sheds]

Had it been working?

Was it on an RCD?

 

[RF Lighting]

Yep apparently it was working fine until last night.

The tennant said she has been in the flat for 2 years and never had any problem at all with the immersion. It was when she went to investigate the lack of hot water this morning that she found the switch welded closed, and the fuse drawer stuck in solid.

It was a split load CU on the non RCD side. Installation was about 6 years old at a guess.

 

[Lectrician]

Immersion faulty and reverse polarity on supply?

 

[RF Lighting]

Nope everything was healthy.

Polarity was correct, and after cutting the wirig back and replacing the FCU, the immersion was back up and running.

 

[Lectrician]

Dog ****ed on it?

 

[kai]

Why did the fuse blow in the first place? - the foil round the fuse implies that it must have blown sometime back...

 

[DetlefSchmitz]

Fuses just blow sometimes. If you don't have another fuse you use foil (well I wouldn't, but...). It only takes a slightly high resistance connection between the foil and the holder to create a lot of heat.

 

[RF Lighting]

Don't know what caused the fuse to blow in the first place. The over temperature cutout in the immersion was a bit burned due to what looked like a bad connection, but it didn't appear to be short circuit, and according to the tennant the circuit breaker had never tripped.

 

[conny]

I would suggest cracked front is caused by over tightening of faceplate screws due to back of switch compressing cables. This may cause the the contacts to distort resulting in poor contact. It also looks as though the cord grip has not been used, the holes don't look as though they have ever had securing screws fitted.

 

[Peet]

Looking where the crack runs, it looks like the immersion cord has been too fat for the FCU, and like conny says its broken its back when it's been tightened up, or got hot.

Some fuses when continuously operating near their limit seem to prematurely age and fail, despite not actually being over the limit. I've see it very occasionally in powerful kettles.

How much of an inductive load is an immersion heater? Is it that that's caused the fuse to maybe expire more frequently than usual, and somebody has eventually bridged it with foil?

 

[JohnW2]

I would suggest cracked front is caused by over tightening of faceplate screws due to back of switch compressing cables. This may cause the the contacts to distort resulting in poor contact. It also looks as though the cord grip has not been used, the holes don't look as though they have ever had securing screws fitted.

The crack might also have occurred when the user tried to force the 'welded' fuse carrier out.

It certainly looks as if the problem probably arose from a poor connection either in the switch or the fuse contacts - there is heat damage all along the L path from the switch contact to the load terminal, but with both the supply and load L terminals seemingly largely spared (and no real heat damage on the N side). That seems to rule out a poor connection at a terminal despite (as you say) the apparent lack of use of the cord grip. Given the foil, poor contact with the fuse contacts presumably has to be a prime suspect, as has already been observed. Mind you, if the foil has been there for a long time, one has to wonder 'why now?'. I suppose that gradual oxidation of the surface of aluminium foil could be the answer.

Kind Regards, John.