Overload protection

before the timer, after the fuse & isolating switch

Cable ampacity needs to be at least 6A. Resistances need to allow a quick enough 16A MCB trip.

NT

Given this is a dedicated hard wired installation by the sounds of it, then you while you *must* have fault protection, you could argue you don't have to have overload protection at all since there is no way an unskilled user can plug in extra load to make overload a realistic possibility[1]. (and you can assume that an installer changing a hard wired heater would do the necessary checks to make sure they will not overload the supply cables and switchgear etc).

To answer the specific question, overload protection needs to be set at a level higher than the nominal circuit demand, but low enough to protect the circuit (inc cables and switches etc). Generally if the appliance itself needs tighter protection it will include it internally itself[2], or its installation manual will include explicit instructions on supply protection.

Can't be done without adding another FSU, unfortunately, as they are all one unit, without an "always on" output.

That's what I hoped.

Apparently 1.5mm is ok for 10A if it is going through insulation (which it might be, I am pulling the new cable fthrough an existing route which might be in cavity wall insulation if there is any). So that seems ok to me subject to your footnotes

Using an oil filled radiator, I assume any realistic high current (but below MCB rapid trip levels) fault would be an earth fault and trip the RCD.

I've seen fan heater elements droop and short circuit part of the element, creating a short-lived IR emitter.

The reason for my question is that I don't want to use two 2.5mm T & E cables, as some of this has to be surface wiring through nearly full mini-trunking.

Is there any scenario where the unit could need to dissipate the whole lot and more to the point will you fuse the lower current bit accordingly. Brian

That will only switch on when the frost stat switches on. Then the timer will run and the heater may not switch on at all if the timer is set to on. It needs to be wired in parallel across the timer so either can turn it on.

Generally "short lived" is ok since this will not result in a sustained overload...

Do you need an extra cable? i.e. the frost stat only needs to connect to the switched live output of the existing timer.

The RCD will provide additional protection.

A failure of the element would result in a LE short.

However in your case I suspect that this will trip the MCB or the RCD in no particular order or both.

If the machine requires an FCU, then you have a problem, you can't simply bypass it with the thermostat. That might be solvable by reducing the MCB/RCBO to 10A, if the mfr says that's ok. But you still need an effective isolating switch.

NT

Quit talking rubbish.

For historical reasons (not unconnected with Economy 7) the heater is on its own circuit not available on the side of the room the thermostat needs to go. And the thermostat needs a neutral for the anti-hysteresis heater. The sockets in the room and a much used outside socket are on another radial circuit and I hesitate to put the heater on this circuit for maximum load reasons. Though that would be by far the easiest solution!

It would be annoying to put another FSU on the wall and it would rely on a future supply of 10A fuses plus the discipline to use them!

It's the only load connected to the cirtuit. But if the makers say it needs lower current protection than 15A (its an old CU, so not actually

I do believe that is what the OP intends to do.

Roger, you do know that without an extra fused spur or DP switch in front of the frost stat and the heater you are bending the electrical regs by about 90 deg.

Try thinking and then apologise for being wrong again.

Then why a frost stat? Does he only set the timer when frost is forecast?

You are missing the obvious.

But it would be fine if I used the timer and 'stat provided by the heater and plugged it all in to a socket. I was slowly beginning to recognise this. Without the frost thermostat the timer does the isolation and fusing job! Does anyone make a timer with a 'live when not isolated' output? I am fairly sure it would be a bad plan to modify the itmer to get at the switch o/p. An additional FSU does solve all my other problems though! It just looks messy on the wall. I will have to put it below the timer and loop back, for neatness.