552.1.2 motor overload requirements

The fan oven assembly is, I presume, "an item of current using equipment complying as a whole with an appropriate British or harmonised standard"

Assuming so (look for the CE mark or a makers plate to see which standard), then as such it will already have been tested to make sure it can contain the flying sharp bits and any smoke and flames that occur should the fan motor do it's worst. In practice I'd expect there will be a thermal fuse-link inside the oven somewhere or maybe embedded inside the motor body, but exactly where and if it is replaceable or gets thrown out with the burnt out motor, the oven makers will have decided for you.

You need to do no more than meet the oven maker's recommendations for external fuse/circuit breaker or whatever they ask for, and then you will also meet BS7671 automatically.

If the oven is  home made, then it gets more complex, but i bet it isn't...

Mike.

The oven (Or warmer) is made by an assembler, who has purchased the fan heater, ductwork, storage container etc and assembled all together, the concern also comes into effect because this is for use within an ATEX area, and I imagine a collapsed bearing would generate a sufficient amount of heat prior to the overload eventually going, the manufacturer has thankfully said they will try and provide some clarification

So, it may not be suitable for use in that environment. Is it officially  specified for such use.

Z.

Ah, then it is more complex, and will indeed depend on what  the design authority of the equipment intended (I presume they did not CE mark it..). Basically something needs to shut the motor off before it catches fire, and if it is not something inside  the motor, then it needs to be a starter with a time lag overload protection built in, as any normal fuse, when set hihg enough to not blow as a false alarm on the start up inrush, is probably not fine enough to reliably blow if the motor overheats.

Large motors have an unfavorable inrush to overload ratio.

mike.