Fusebox dilemma

[clueless26]

I dont know anything about electrics and was wondering if someone can help?

About 18 months ago we had an electric shower installed and it was working fine. About a week ago the fuse blew for the shower (we have the old style fuse box with the wires - not switches) and we replaced the wire. Soon after, we smelt burning and on opening the cupboard containing the fuse box, smoke came out and the fuse unit had melted and burnt and also the cover to the fuse box had melted slightly . On turning off the electricity and taking the fuse box cover off, the mains wire for the fuse had burnt through too (although the actual fuse wire was still in tact).

The fuse was then changed to an updated switch one and the mains wire rejoined.

Now the fuse switch keeps tripping out and the same fuse unit gets hot when the shower is used, which is really worrying.

Does anyone have a clue why is this happening and what would an electrician have to do to rectify it?

 

[Adam_151]

How powerful is the shower?
What size is the cabling?
What size is the fuse/breaker?

Who did you have to fit the shower?


I expect you are probably looking at a separate shower CU next to the existing CU (fuse box), or the fusebox replaced with a modern split load unit

 

[clueless26]

Off the top of my head....its a gainsborogh 9.5W shower.....and the fuse is a 30amp. I'm not sure what size cabling was used.....the shower was installed by my other halfs father.

Its hard to understand why we have had no problems with it until now, 18 months later......what would have changed? I thought the fuse trips when there is too much current travelling through the wires? But then if the wiring was wrong, wouldnt we have had this problem from the beginning? (Nothing else is wired into that fuse except the shower and there is also an isolation switch for the shower placed before the fuse, which we've switched off)
When I call an electrician in I'd really like to have some kind of incling what the reason might be, so i dont feel totally .
thanks!

 

[Adam_151]

Off the top of my head....its a gainsborogh 9.5W shower.....and the fuse is a 30amp. I'm not sure what size cabling was used.....the shower was installed by my other halfs father.

9500/240 = 39.5A

Its hard to understand why we have had no problems with it until now, 18 months later......what would have changed?

I expect its a slow overheating problem, there is a high resistance, heat is generated, this oxidises the connection, the resistance increases, and so does the power disapated in the faulty connection, and the process runs away, such a high resiastance joint could be where the fusewire is screwed dowm, where the fuse plugs into the CU, where the outgoing cable is clamped. And I'd probably guess that your cabling is 6mm² which is border line for that load anyway (ok sometimes depending on how its installed, but in the way cables are usually run in a house, you generally need 10mm²), remember also that your fusebox won't be designed for more than 30A through any single 'way'

I thought the fuse trips when there is too much current travelling through the wires?

Re-wireable fuses are particulaly bad for sustained overloads, no device trips is you exceed its rating by 0.1A, they'll take double the design current for longer than you'd expect (and put out quite a bit of heat while it is doing so)

EDIT: A 32A breaker will never trip at 1.15 x 32 (= 36.8A) but at 1.45 x 32 (= 46.4A) must trip within an hour, most devices will be faster than this though

But then if the wiring was wrong, wouldnt we have had this problem from the beginning?

As above, there will have been a slow breakdown of joints and terminations, a good analogy is driving a car at the red line all the time, it'll work, but it'll be stressful for the engine and will break down a lot sooner than a car driven in a more sensible manner

(Nothing else is wired into that fuse except the shower and there is also an isolation switch for the shower placed before the fuse, which we've switched off)

Its correct that nothing else should be on the same circuit, but how do you put an isolation switch before the fuse? is it a separate fuseboard to the rest of the house and an isolator in the tails just before that board? or possibly an RCD? (does it have a test button?), A picture might be usful here [/hint]

When I call an electrician in I'd really like to have some kind of incling what the reason might be, so i dont feel totally .
thanks!

A commendable approach

 

[clueless26]

Thank you for that!

I tried to attach a picture...but it didnt work....Its an isolation switch that even when the electricity mains is on, when switched into the off position, no electricity is supplied to the shower. It doesnt have a test button just a flick switch.

So how can the resistance for that fuse be lowered so it doesnt over heat again?


[/img]

 

[Adam_151]

I tried to attach a picture...but it didnt work....

You need to upload it to a free imagehost, google for imageshack

So how can the resistance for that fuse be lowered so it doesnt over heat again?

Well oxisdised contacts can be cleaned up, but the whole thing will just happen again as they aren't properly rated for the load in question, the correct way to fix it is to replace it with a device thats actually designed for the load it is being asked to carry (which will come down to the contacts in the new gear having a lower design resistance)

 

[plugwash]

given that you claimed in your original post that there was heat damage to the CU itself i would strongly advise a new CU.

 

[daviegarden]

As you are clearly not electrically minded and probably miss not having the shower, if it's 9.5Kw and you like to stay in it for a while. e.g. 20/30 minutes, you need a 10.0mm cable, a 45 amp double pole switch to control the shower and although RC protection is not required by the regs, you should fit a 45 amp 30mA RCBO, BETTER TO BE SAFE THAN SORRY. The RCBO will trip BEFORE you know you are being electricuted and you will live to shower another day. If the cable you have is a 6.0mm, then get a 8Kw shower, and reduce the current rating of all the accessories I have mentioned to 30 amps. Make sure all your bathroom pipes are bonded.

 

[JohnD]

If it's an old rewirable fusebox, there probably isn't an RCBO to fit in it (Mr Clueless, can you tell us the brandname and colour ofthe old unit please?).

Really, aren't we talking about calling in a self-certificating pro to fit (at least) a new small CU for the shower, or (at best but more expense) a new split-load CU for the whole house?

 

[ban-all-sheds]

Yes.