Firstly this might be covered by more specific standards than BS 7671 - which might differ in some details.

From a general/BS 7671 perspective though, there's probably a bit more to consider...

1. What sort of voltages for the 2ndary are we talking about? (anything from 12V to 400V are seen in various applications)

2. Are there any functional requirements about earthing?

3. Are there any requirements for resilience? (e.g. while some systems will happily disconnect on 1st fault, others (e.g. some safety critical) are intended to keep working safely on 1st fault)

4. What's the intended approach for shock protection? SELV, PELV, FELV, ADS, separation, double/reinforced insulation?

Once you've decided all that it'll probably drop out whether the secondary should be earthed or not, and then how many poles you need to provide protection for. Generally for overload protection you need only protect n-1 of the live conductors, and for ADS with an earthed neutral generally only the line conductors need disconnection - but there are exceptions in specific circumstances.

   - Andy.

What you do on the secondary side depends on the reason for the transformer, the transformer behaviour under overload, and the kind of load connected. In this case, a heater, the most likely fault would be one side of the middle of that heater faulting to ground. If you left it floating, neither secondary pole earthed, then there would be no problem at first fault, 

If the heater is happy with one side earthed then apart from wondering if the transformer is needed at all (voltage change perhaps) the you can treat it like a low current TN-s supply and fuse the non earthed secondary pole.

If the transformer already contains a thermal fuse or similar then you do not need  a secondary fuse to protect the transformer itself, and you may not need one at all unlessf you have multiple secondary loads and wish to protect the others from a failure of one.

A very small transformer, and you do not mention wattage, may not be able to muster enough secondary current to operate a trip or fuse sized to survive the inrush,- but a look at the secondary side voltage drop on load will tell you what the short circuit current would be.

I'm sorry if it is more questions than an answer, but with a bit more info, it may be possible to zoom in on a solution.

Mike

Hi

Thanks folks, really helpful..........Some more info:

- The Primary voltage is 230Vac and secondary for the motor heater is 110Vac.

- Happy to disconnect on first fault, its non-critical.

- Not sure what this means "functional requirements about earthing ].

- And ADS would be shock protection for the heater.

- Wattage would be between 50 and 100W for a small heater.

Thanks

Not sure what this means "functional requirements about earthing

Just the some applications require the 'supply' to be earthed - e.g. some older (ELV) data/signalling systems that use chassis return, or things like flame failure monitoring devices.

For a 110V circuit supplying  a small heater you'd have a fair bit of flexibility. Earthing one pole and having a single pole overcurrent device in the other may well be reasonable. If it came under BS 7671 you'd probably be looking at having to achieve 0.8s disconnection time for ADS (which might be a bit of a challenge, especially given the added impedance of the transformer) and possibly a bit OTT if it's essentially part of a overall bit of equipment that perhaps only has to achieve 5s disconnection times, but I strongly suspect it'll fall under another (equipment) standard, that'll have slightly different requirements.

I'm sure someone here will know which is likely to be an appropriate standard - I'd maybe guess BS EN 60204 - but that's only a guess and I don't know the detail of what it says.

    - Andy.