Circuit Breaker drops before Fuse

[Gil Williamson]

I have underfloor heating rated at 3kW. The circuit goes through a fused switch with a 13A fuse in it. That is supplied directly from the main fuse box with a B16 circuit breaker dedicated to that circuit alone. Every time we use the system, it warms up for a bit, then the circuit breaker goes. The 13A fuse doesn't, nor does it seem to warm up. It seems to me the circuit breaker is at fault. It's been in place for 12 years without any trouble before. Is this likely?

 

[littlespark]

It’s likely the is a fault with the MCB, but more likely a fault with the under floor heating.
It just needs 14A to blow the 13A fuse, leaving the 16 intact.
An electrician will be able to tell if there’s an element gone in the heating.

Can you give us a make and model no of both the heating and the mcb? or Photographs? We like photographs.

 

[Pete999]

3KW is on the border line for a 13 Amp fuse, at either voltage 230 or 240, could be a tired Breaker, had any testing done of the UFH ?

 

[ruston]

I know you are familiar with electrical equipment. Just to clarify , is it the MCB or RCD that is tripping?

Sorry for the delay in this reply . There were no replies when I posted.

 

[Spoon]

Can you give us a make and model no of both the heating and the mcb? or Photographs? We like photographs.

Pictures and drawings are good as some of us are unable to follow the complexities of the written regs.

 

[ruston]

Pictures and drawings are good as some of us are unable to follow the complexities of the written regs.

Our fault or the authors ?

 

[Spoon]

Our fault or the authors ?

Good question mate.

 

[shaun1]

It would be worth measuring tbe resistance of the mats to check for short circuits, and also the current they are drawing whilst they are on. I would have thought the MCB would be the least likely component to fail in that setup. There might be an intermittant short on a mat which is triggering the magnetic trip in the breaker before the fuse burns out.

 

[static zap]

Any other symptoms , flickering lights , recent dead lamps ,(Smart Meter fitted )?
(Just fishing for the " Unusual")

 

[littlespark]

I misread the OP. I thought it said the fuse went first.

If the MCB is tripping so quickly (before the fuse) suggests a short circuit, but happening a time after the heating is turned on?

Have to agree with other members... an intermittent fault.

Does it happen when someone stands on a particular area of the floor, or just anytime?

 

[Gil Williamson]

So what you're saying, shaun1, is that a circuit breaker might blow while a lower rated fuse was making its mind up...

Well, I'm hoping that it's the circuit breaker, not the mats. I saw the mats being laid, and I'd believe open circuit, but the way they are laid and separated, I don't see how a short could develop. If it's the mats, it would involve an enormous upheaval, and I'm afraid we'd just isolate them and never use them. But if there's any possibility it's a weak circuit breaker - I take it they do exist - I'll get an electrician to fit a fresh one and see if it still flips. The other possibility - is there a circuit breaker rating between b16 and b32?

I've had a look at the mats brochure. They laid about 12 sq m, which would rate at 2400W, so well within the 3kW they told me. "Just a 3 bar electric fire" was the expression.

I'm trying to attach an image of the brochure...

 

[Gil Williamson]

littlespark said:Does it happen when someone stands on a particular area of the floor, or just anytime?

That's a good question. I'll get someone to tramp around the room and see if there's a consistent fail.​

 

[shaun1]

Yes, well possibly anyway. Fuse wire, and the thermal trip element of an MCB (a bimetalic strip) have a time/current curve so as current increases the time to trip decreases. MCBs also have a magnetic trip, which for a B type breaker will operate in less than 0.4s with a current of 5x its rated current.

I still think you should start with some tests on the mat.

20A MCBs are available, but increasing the size of the MCB would potentially be masking the problem instaid of fixing it. The cable sizing and instalation method would have to be suitable for the increased current rating, as would the timer/thermostat

 

[Gil Williamson]

Thanks for all your help, guys. I won't be able to do do any experiments this week or next, but I'll get back here when I can.

 

[ruston]

If you still have the instruction /installation book ; and it has been filed in , the resistance of the mats will be recorded in it.