smoke from fuse box - fuse didnt trip

[andrew55]

Hi,

I’m hoping someone can give me any information regarding my shower. (Sorry in advance for the long post!)

Basically we moved into our house about 15 month ago, and everything has been fine until a couple of months ago. My partner had a shower, and when she came down stairs there was smoke coming from the fuse box . I pulled the shower fuse out immediately and am now trying to work out what happened.

I’ve hosted some pics:

http://homepage.ntlworld.com/andrew.cenic2/wires/DSC01771.JPG
http://homepage.ntlworld.com/andrew.cenic2/wires/DSC01773.JPG
http://homepage.ntlworld.com/andrew.cenic2/wires/DSC01775.JPG

Anyone know what could of happened? It had been fine for over a year and I can’t see why this would suddenly happen? Why didn’t the fuse blow? I think the cable is 6mm, but it may be a bit thinner (I’ve found an off cut in the attic and will nip to homebase and compare with 6mm cable) – the shower is 7.5kw, and have a 30amp fuse

Any help appreciated.

Thanks

Andrew

 

[plugwash]

the shower fuse is the one whose base is removed in that picture right?

if so it looks like a bad contact on the top connection of the fuseholder to its socket.

you probablly could clean it up and bend the contact tight again but to be honest its probablly better to get the whole CU replaced with a modern one equipped with mcbs and rcds.

you appear to have a spare way in that second CU, i wouln't put the shower on it because of incomer rating issues but you could possiblly move one of the lighting cuircuits to it freeding up a space for a new fuse/breaker for your whower. The trobule with doing that is that someone may re-use the dodgy way in your main CU later.

 

[Steve]

looks like a bad connection. Similar thing happened to our shower, but at the isolator switch. By the time we noticed, the wire had completely melted and turned green (copper oxidising) and the backbox was melted to the ceiling. Check the screws that are holding that wire in, though the heat may have seized them up. Chances are, these have worked slightly loose. In any case, check the other end of this cable for similar damage. If there is none, reterminate this cable here. Looking at your figures, this is the only possibility i can think of, a bad connection. Though looking at the install i think its time for a new consumer unit.

Can you open the fuse box and post a picture of the rest of it so we can judge what cable size this is? Careful when doing this - you are exposing permenantly live parts

 

[Nickthedentist]

I'd agree with the other lads - a bad joint causing arcing (sparks) and a lot of heat. Lucky someone noticed it

A new consumer unit would be a good idea, especially as yours has cartridge fuses and no RCD that I can see, so even when it's working properly, it's considerably less safe than a more modern one.

Nick.

 

[HDRW]

Andrew,

Others have suspected the cable connection, but 7500W at 230V is 32A (and change) and I reckon this will cause the 30A fuse to run hot, and although it won't blow it will cause heat damage over time, resulting in loss of elasticity and thus grip of the plug-in blades, which will then increase the resistance of the connection, causing heat... and it goes on until something gives, which it just did! You can see where the heat has spread from the top plug area towards the left. It may have been exacerbated by the cable connection getting loose too, again due to heat-cycling, and the cable has obviously been very hot!

Whatever the cause you can't use that fuseway again, and when you have established that the cable size is big enough you'll have to find another way to connect it, with a fuse (or preferably MCB) that's able to handle the current, but not exceeding the cable's capabilities. A seperate "shower unit" is a possibility, a box with an RCD and an MCB in it. As a matter of interest, what's the seperate 45A MCB on the left for?

Cheers,

Howard

 

[plugwash]

1: shower ratings are generally given at 240V

2: all the damage appears to be concentrated towards one end. this is what makes me suspect a bad contact.

 

[HDRW]

1: shower ratings are generally given at 240V

Even though the harmonisation at 230V happened ten years ago? Anyway, if so it's 31.25A - still OTT!

2: all the damage appears to be concentrated towards one end. this is what makes me suspect a bad contact.

Well perhaps - the cable leaving the terminal has been well heated but the strands poking out the bottom look fine - I know heat rises but that does seem a tad odd. Looking again, the top screwhead looks like it may be bottomed, so not clamping the cable properly, but that may be a trick of the light.

Cheers,

Howard

 

[andrew55]

Cheers guys – super fast response! I think what you’ve said make sense, as elsewhere the cable looks fine.

Here’s a pic of the cable: (at the pullcord end)
http://homepage.ntlworld.com/andrew.cenic2/wires/DSC01805.JPG

The blue cable on the right is solid core 2.5mm mains cable.

What should I be looking at getting in terms of an rcd consumer unit? Would this be any good?

http://www.screwfix.com/app/sfd/cat/pro.jsp?cId=101863&ts=08821&id=82204

If I was to get a sparky round to install it, what should I expect to pay?

Thanks

Andrew

 

[andrew55]

As a matter of interest, what's the separate 45A MCB on the left for?

we have a summer house at the bottom of the garden. and like a small plunge pool - with a 5kw heater. Don’t get excited its nothing special just a Argos pool that’s be built under the decking - previous owners.

What worries me is the guy that owned the house before was a right cowboy! As when we first moved in that was connected so it bypassed the meter. - no wonder they had the pool heated on all year round!

Andrew

 

[kai]

The order of the fuses are wrong, the largest fuses should be next to the main switch, then the rest in descending order, in such a way as the five amp ones are furthest from the main switch.

 

[Spark123]

The blue cable on the right is solid core 2.5mm mains cable.

Doesn't quite sound right? Your best bet is to have an electrician carry out a PIR (periodic inspection report) which should highlight any problems you have with the installation.

 

[andrew55]

The blue cable on the right is solid core 2.5mm mains cable.

Doesn't quite sound right? Your best bet is to have an electrician carry out a PIR (periodic inspection report) which should highlight any problems you have with the installation.

ah, should have said. The blue cable is not part of the install. I just held it there for comparison.

Andrew

 

[ban-all-sheds]

The order of the fuses are wrong, the largest fuses should be next to the main switch, then the rest in descending order, in such a way as the five amp ones are furthest from the main switch.

Also, the max load for that box is 60A.

I know we've got diversity and all that, but there are 3 x 30A circuits, 1 x 15A and 2 x 5A.

And with the shower running, half the capacity of the box is gone.

But please don't buy a Volex...

 

[hughesey]

i havent read all this page cuz i dont have chance, at work u see....

but thought i'd write a quickie.... our shower was wired up REALLY dodgy, it had the 30A fuse in the consumer unit, controlling the shower, the lights and the fan, last week my Mrs was having a shower and it went off on her, 1st thing i checked was the fuse, it was RED hot, couldnt touch it, managed to get it out (bout 15mins later) still piping hot, checked and it had gone, since then I have had my Dad up this weekend helping me with wiring the lights/fan up to another circuit.

all seems fine now with ours, thanks to a little help from sparky123 (from this site)....

PS...thanks again sparky123...!!!

 

[andrew55]

Right thanks for all the advice so far.

Took my off cut of cable to diy store yesterday and compared it with 6mm cable and its thinner, so I reckon its actually 4mm cable. My house isn't particularly big, so the cable runs from the CU, straight up into the attic (maybe 6m?) and then across the floor of the attic (maybe 5m?).

So where do I stand, as this cable is right on the limit with 30amps? (although it seems to have managed up until now).

Suppose I should swap it for thicker cable but how much work will be involved in this? My house is a 15year old Barrett style one so has an outer single block wall and then plasterboard walls sorta an inch or two off this wall (if that makes sense) - would the cable be fixed to the wall as it runs up into the attic making swapping it with thicker cable a really big job?

Thanks

Andrew