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.

 

[Octopus]

I would suggest you get a spark to test the resistance of the mut and the IR too ............ to begin with

Upping the MCB is not recommended ....

 

[Wilko]

I've seen these 3kW items eat up the fused connection unit over time, but not so much the MCB. Inspection and test, and if no fault is found I'd replace the 16A MCB with a new one (exactly the same) as they're cheap and it may have just gone soft. It should never trip at 16A, let alone 13A.

 

[marconi]

Does your Consumer Unit have residual current detection -there will be a test button on it? Someone asked this question earlier and I think you may have missed it. If it does is the underfloor heating circuit breaker fed from it?

When you call in the electrician ask him to measure and record the current flow through the mat and the voltage of your mains supply at the same time.

Also ask him to check the tightness of all screws which connect the circuit breakers to the busbar - it may be that one or more adjacent mcbs are getting warm and the copper busbar being a good conductor of heat is allowing heat to flow to the UFH 16A mcb warming it up and making its thermal trip readier to trip at a lower current/after a shorter time interval.

Heat transfer can also occur between mcbs placed side by side - out of interest what mcbs are to the left and right of the UFH 16A mcb? I have come across a problem when a 6A mcb lighting circuit (passing 4-5 A) would trip in the evening - it was next to a 16Amcb for the immersion heater which was was on and young children were having their night time baths one after the other in fresh hot water. Separating these mcbs with a blank solved the problem.

 

[Gil Williamson]

(re: documentation We are not the kind of people who chuck stuff away, but no sign of documentation. It was part of a much larger re-building project. May have got lost in the rubble.

 

[Gil Williamson]

Wilco: Just replacing the B16 MCB was my first impulse.
Marconi: Excellent ideas. The MCB is at the end of a row. To its left, the B32 MCB kitchen ring main (which carries a kettle, microwave and dishwasher as its max load) I'll talk to the electrician when he comes and ask him what you suggest.

 

[Octopus]

I wonder if the buzz bar is the wrong side of the clamp ....

 

[davesparks]

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.

I'd recommend you don't do experiments but rather have this tested properly.
Its not uncommon for a fault in a heating element to only appear when it is hot and expansion has occurred.
I would probably start with measuring the cold resistance, cold and hot insulation resistance, running current and leakage current.

12 years doesn't sound like an unreasonable lifespan

 

[Andy-1960]

It just needs 14A to blow the 13A fuse, leaving the 16 intact.

I’m afraid I do not agree with this, What many people may not realise is that a 13A rated fuse is not designed to actually blow at 13A. In fact a 13A fuse will allow a current of 20A to pass indefinitely without blowing. If you look at the attached graph, it shows the operating characteristics for both 3A and 13A BS 1362 fuses. The grey shaded area is the point where the fuse is designed to operate. So for example, a 13A fuse will blow within 0.01 - 0.3 seconds for a fault current of 100A; shown in red on the graph. For a current of 20A, shown in blue on the graph, a 13A fuse will not blow!

So that is the reason the 16A MCB will trip way before the fuse does.

I would disconnect the heater mat and measure the resistance as a starting point, this should be somewhere between 17 & 18 ohms for a 3kw mat rated at 230v.

Then carry out an IR test on the mat.

The heater mat will pull a little more current when first switched on (usually around 10%) so this is getting a little close to the 16A rating of the breaker.

If the mat tests out ok, I think I would change the breaker as it has probably fatigued over the time it has been installed

 

[Risteard]

It just needs 14A to blow the 13A fuse

You certain about that?

 

[littlespark]

#23 and #24
If you read the thread on a bit, I say I misread the OP, so my initial reply was mostly in error.

The part about 13A, 14A and 16A... was an attempt to explain electrical loads and overcurrent protection in laymans terms to non professionals that might be reading.

Everybody is quite right in saying the 16A breaker will trip before the 13A fuse knows there's a problem

 

[Gil Williamson]

Look. This has become so complex that I think my next move is to get an electrician. I'll put all the salient points raised here to him, and see how he jumps!