Electric 13AMP Fused Spur Melted!!

[drJonesy63]

Hi
I have a 12sq m Electric underfloor heating mat rated at 200W per Sq Metre

Total of 2400W. A quick check online calculator indicates 10AMPs.

The feed to this spur is on 2.5mm electrical cable and is not part of a ring main. Its connected in the fuse box to a 32amp breaker. We also have an electric Hob connected to this 32amp breaker which runs on 6.0mm cable.

My issue is with the 13amp fused spur being used for the under floor heating. Its been installed for about 2years now, with no problems at all.

When I came home I noticed the floor was not on, and whats very very strange is the fused spur switch was turned off!!!, we never switch it off at the fused spur. (These aren't breakers so they shouldn't switch off on their own?!) I went to switch it on, and the switch mechanism didn't feel right, it was quite stiff. Once it was on, the floor thermostat came on. Within 5mins we noticed smoke appear from the socket and smelt.

We immediately switched it off from the fuse board. When I took the 13amp fused socket apart, it was hot, the fuse was hot and the Live Feed cable had melted into the socket. You could not unscrew it as I guess the plastic had melted around it. All other connections were fine (including the load cables) just the Live Feed. Once I removed the switch, I also noticed you could no longer operate the switch it was now stuck in the On position.

As it was in the kitchen, we have polished chrome switches, but I found a white 13amp fused spur In the shed, I connected it with this, and everything has been working (for the last 4hrs).

Please could I have your opinions? I don't think I have overloaded the fused spur as my calculations are 10AMPS. Do these sockets fail over time? (2years old!) We are worried now to ever rely on using it when not at home

many thanks

 

[Adam_151]

Its not uncommon for 13A spurs to overheat in this way. Bit of tarnishing on the fuse and the resistance (and therefore the heat produced in it) goes up.

What brand was the spur? cheap noname ones are probably more likely to suffer from this than decent brand ones.

I try and avoid situations where a plug top fuse is quite highly loaded for long lengths of time (so thats basically most heating loads). the best way of doing so is to use a B16 breaker back at the board and use a 20A DP for local isolation. In your case your circuit requires attention anyway as the 2.5mm radial should not be doubled up onto the cooker circuit... is there a spare way in the board? (if you post a picture, we will tell you if the breakers are readily available and whether it would be a simple job for an electrician to sort out)

 

[drJonesy63]

Hi
thank you so much for the quick reply.

The fused spur is a crabtree like this one on ebay:
http://www.ebay.co.uk/itm/270749960...49&var=570020407769&ssPageName=STRK:MEBIDX:IT

I could put the 2.5mm cable for the floor heating on its own B16breaker , I did ask an electrican at the time and he said it was ok to do double up with the hob....

Its a wylex fuse board and I do have a spare B16 NSB16 breaker. I think they are obsolete now but I bought a few as spares. The B16 breaker I have isn't installed in the board, but I could fit this if needed.

Does a 20A DP fit in a standard wall mounted socket , i.e to replace the 13amp fused spur I had? I cant remember what 2.5mm cable is rated at, is 20A DP ok and couldn't potentially melt the 2.5mm cable before tripping?

thanks
Martin

 

[drJonesy63]

I guess this is what I need:
http://www.ebay.co.uk/itm/Crabtree-...353158?hash=item3ab9442186:g:998AAOSwCQNWcqoc

Would this solve the problem without having to install this on its own 16A Breaker or should I definitely do both ?

 

[Adam_151]

Would this solve the problem without having to install this on its own 16A Breaker or should I definitely do both ?

You can't simply remove the over current protection in the local isolator unless the circuit is suitably protected upstream!

That is, you MUST NOT change the fused spur for an isolator without changing the breaker to 16A

 

[drJonesy63]

absolutely, thank you for the info. I will install this on its own B16breaker and buy a 20AMP DP.

Am I right in saying, the current installation is 'ok' but the Fused Spur sockets burn out if used with high loads for long periods?

 

[eveares]

The feed to this spur is on 2.5mm electrical cable and is not part of a ring main. Its connected in the fuse box to a 32amp breaker.

A spur fed by 2.5mm² cable is only rated to 27 Amps under the best case scenario conditions and should be protected by a over current protection breaker rated no larger than 25A. Your spur fed by 2.5mm² cable should not be protected the 32A breaker and is dangerous. This needs to be fixed.

(These aren't breakers so they shouldn't switch off on their own?!)

That's correct.

Once it was on, the floor thermostat came on. Within 5mins we noticed smoke appear from the socket and smelt.

It is possible that your underfloor heating may have developed a fault where its resistance has drooped low enough to exceed 13 amps but not low enough to blow the 13A fuse. If I recall correctly and due to the crude resistive nature of fuses, a 13 amp fuse needs nearly 21 Amps before it will blow; thus another reason to use high quality reputable branded fused spurs that likely will be able to handle current's greater than the 13 amps.

When I took the 13amp fused socket apart

I though it was a fused spur!? By Fused socket, do you mean a switched fused spur?, as sockets are the thing that you plug standard electrical plugs into.

As it was in the kitchen, we have polished chrome switches, but I found a white 13amp fused spur In the shed, I connected it with this, and everything has been working (for the last 4hrs).

You should definitely not of connect another fuse spur up without first investigating and then determining what lead to the fault in the first place. What you have done is potentially very dangerous and could cause the same thing to happen!

I don't think I have overloaded the fused spur as my calculations are 10AMPS.

I beg to disagree; your calculations of 10A are for a non faulty appliance and as I said your under floor heating may have developed a fault where it is drawing >13A but <20.8A.

Do these sockets fail over time? (2years old!)

While it's certainly possible, fused spurs (both switched and switched variates) failing in the way yours did is not a common occurrence, especially when it's only 2 years old. Yours likely failed due to a potential fault with your under floor heating as mentioned above or less likely due to a fault in the fused spur.

Please could I have your opinions?

I would get a competent and qualified electrician in ASAP, if not only for your own safety.

Good luck.

 

[Adam_151]

It should not be doubled up with the cooker circuit, end of!

The fused spurs *should* not burn out after carrying 10A for long perioids, but do from time to time, this is usually cheaper brands

Creating a new circuit:

a) Is notifiable work under part P
b) Requires that an EIC be issued

 

[drJonesy63]

I will install a B16 breaker for the floor only, and a 20AMP DP. If your theory is correct and the floor is faulty, it should trip the 16A Breaker correct?

 

[Adam_151]

Is the underfloor heating protected by RCD (hint: It should be)

If so (and if its not... that requires addressing asap) then I struggle to envisage how it could fail in the manner discribed by eveares without tripping the RCD

 

[drJonesy63]

Yes its covered by RCD. And I was thinking the same. You may think I was wrong to install a new 13amp fused spur but if their was a fault with the floor it would have blown within the last 5hours of me changing it. I'm not an expert and I really take others advice but my feeling is the 13amp fused spur cant handle the constant load and burnt out over time. I will do as you suggest and put it on its own 16A Breaker and 20AMP DP

 

[EFLImpudence]

It could have just been a loose connection which caused the overheating.

I doubt the UFH can cause an overload. Don't they have an earthed screen?
If that is the case then the circuit is not flawed regarding the protection.
Although it may not have been installed as per the instructions

 

[drJonesy63]

Hi, on removal nothing appeared to be loose. Yes the UFH does have an earth screening.

 

[EFLImpudence]

Hi, on removal nothing appeared to be loose.

You said it was melted into the plastic. Too late to tell.

Yes the UFH does have an earth screening.

Then there is no way the resistance could reduce and hence the current rise.

 

[Mikefromlondon]

I will go along with what EFl has said and though after a burn out you wouldn't know if it was lose connection and the other possibility is the switch contacts became a little resistive and overheated.