• IMPORTANT: Please note that nobody on this forum should be seeking from or providing advice to those who are not competent and / or trained and qualified in their field (local laws permitting). There is a discussion thread on this global industry-wide matter HERE. This also has more information about the warning with regards to sharing electrical advice in some countries. By using this forum you do so in agreement to this.

Discuss Circuit Breaker drops before Fuse in the The Welcome Forum area at ElectriciansForums.net

Welcome to ElectriciansForums.net - The American Electrical Advice Forum
Head straight to the main forums to chat by click here:   American Electrical Advice Forum

Reaction score
4
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?
 
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.:)
 
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 ?
 
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.
 
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.
 
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?
 
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...
Circuit Breaker drops before Fuse thermolay - EletriciansForums.net Circuit Breaker drops before Fuse thermolay - EletriciansForums.net
 
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
 
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.
 
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.
 
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.
 

Reply to Circuit Breaker drops before Fuse in the The Welcome Forum area at ElectriciansForums.net

This website was designed, optimised and is hosted by Untold Media. Operating under the name Untold Media since 2001.
Back
Top