Circuit Breaker drops before Fuse

[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!