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Very hot fused spur switch for underfloor heating


 
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tomtomato

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PostPosted: Wed Nov 21, 2012 6:31 am Reply with quote

Hi,

I have electric underfloor heating in my living room/kitchen (one cable, wattage unknown) and when switched on, the spur switch (with a 13A fuse) on the wall becomes very hot, particularly the fuse section, and there is a bitter/fishy smell (like something melting). The electric board for that room/cable has a 16A circuit breaker and a 20A contactor.

I also have underfloor heating in other rooms, and the spur switches become warm after a while but not hot.

I am thinking that maybe the heating cable in the living room/kitchen uses more current than other smaller rooms, and therefore could exceed 13A, or the heating is somehow getting transferred to the switch.

Maybe the spur switch should be replaced by a 20amp double pole switch?

Any ideas?
Thanks
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jj4091

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PostPosted: Wed Nov 21, 2012 7:14 am Reply with quote

Have you checked the terminations, it could be a slightly loose screw in the switch that is overheating.
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tomtomato (21 Nov 2012)
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Stevoon

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PostPosted: Wed Nov 21, 2012 7:47 am Reply with quote

From the description of the smell you will probably find the insulation / switch to have melted a little. Remake the connections to a new switch cutting out the dull copper first if this is the case.
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tomtomato (21 Nov 2012)
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tomtomato

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PostPosted: Wed Nov 21, 2012 10:14 pm Reply with quote

jj4091 wrote:
Have you checked the terminations, it could be a slightly loose screw in the switch that is overheating.


Thanks, I have checked and everything looks fine. The heat is mainly coming from the fuse, so I am wondering if it's not getting current close or above its rating.
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tomtomato

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PostPosted: Wed Nov 21, 2012 10:16 pm Reply with quote

Stevoon wrote:
From the description of the smell you will probably find the insulation / switch to have melted a little. Remake the connections to a new switch cutting out the dull copper first if this is the case.


I can't see any of the cables melting, so maybe it is the switch itself.

Would it be expected to put a 20amp double pole switch for a large UFH zone, rather than a fused spur switch? Why do I need a fuse anyway, when there is already a dedicated 16A circuit breaker?
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jj4091

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PostPosted: Thu Nov 22, 2012 7:58 am Reply with quote

[quote="tomtomato";p="2583296"]
Stevoon wrote:


Would it be expected to put a 20amp double pole switch for a large UFH zone, rather than a fused spur switch? Why do I need a fuse anyway, when there is already a dedicated 16A circuit breaker?
It is difficult to say when the rating of the mat is not known, but yes a 20A d/p isolator might be better if you are worried about overheating in the fcu. Do you not have access to a tong tester so that you can measure the current being drawn.
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bernardgreen

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PostPosted: Thu Nov 22, 2012 9:13 am Reply with quote

The wire inside fuses that are running near maximum rating currrent do get very hot and while this is not itself a problem it does cause the fuse to age in that the wire over months becomes thinner. in 13 amp plugs much of the heat is removed from the fuse via the fuse clips and dissipated by the live pin. In some FCU there is not enough metal to dissipate the heat and the fuse runs hotter than it would in a good quality 13 amp plug.


The thinner wire gets even hotter as a result of having to carry the same current. If a wired fuse is running on its maximum for long periods of time then there is some value in replacing it with a new fuse once a year. The aged fuse is still usable as a fuse in lower power applications where it is not running at maximum for long periods of time.


The fuse and the fuse holder may be the problem, poor contact between the holder and the fuse due to oxidisation ( tarnish )\ can lead to a resistive contact between holder and fuse which will heat up. Heat affect the springiness of the holder and makes the contact more resistive.
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tomtomato (22 Nov 2012)
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tomtomato

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PostPosted: Thu Nov 22, 2012 11:34 am Reply with quote

jj4091 wrote:
Do you not have access to a tong tester so that you can measure the current being drawn.

No, I don't have such tools. I am not an electrician!!!

I have contacted a proper electrician, so he will have a look, but just wanted to get some information first. I am not sure he will be able to measure the load since the UFH is only activated during off-peak hours (specific electric board) so after 22:30, so even if manually triggered on the thermostat, there is no current going through.

Are there any benefits in having a 13A fuse after a 16A circuit breaker, dedicated to that heating cable?


Thanks,
Tom
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Aragorn84

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PostPosted: Thu Nov 22, 2012 11:40 am Reply with quote

tomtomato wrote:


Are there any benefits in having a 13A fuse after a 16A circuit breaker, dedicated to that heating cable?


Thanks,
Tom


It would presumably depend on what fuse rating the manufacturer of the heating system specified?
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tomtomato

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PostPosted: Thu Nov 22, 2012 12:41 pm Reply with quote

I think the recommended installation for such heating cable in the screed would be 150w/200w per sq meter (I don't have any specific details for my installation).

The total room size would be around 22 sq meter, so the total output would be between 3300 watts and 4400 watts.

Therefore, the loading would be between 14 amps and 19 amps, so I think this is probably overloading the 13A fuse or very close, but should be under the 20A circuit breaker...

I can only assume that they used a 20A circuit breaker because this was within the maximum load expected.
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ricicle

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PostPosted: Thu Nov 22, 2012 12:50 pm Reply with quote

Unfortunately, in my experience, it is a failing with SFCUs or plug/sockets that they tend to eventually cook themselves when ran at currents at or approaching 13A for significant lengths of time.
I am about to start a complaint with MK about this very fact (with a constant load at 12.8A)

In the mean time I suggest you change the SFCU for a new one and keep an eye on it. (Assuming the load is 13A or below)
Any cable showing signs of heat damage will need cutting back.
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jj4091

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PostPosted: Fri Nov 23, 2012 9:13 am Reply with quote

tomtomato wrote:


Therefore, the loading would be between 14 amps and 19 amps, so I think this is probably overloading the 13A fuse or very close, but should be under the 20A circuit breaker...

I can only assume that they used a 20A circuit breaker because this was within the maximum load expected.
In that case I would go for the double pole switch, you only need to have a means of isolation & using an fcu is just introducing a potential for failure or overheat hazard that you do not need.
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