I have a mains-powered (240V AC) bathroom ceiling heater light / extractor fan. (This kind of thing: https://www.bunnings.com.au/ixl-tastic-original-3-in-1-bathroom-heat-fan-light_p4442438) At the moment the extractor fan and the heating lights are connected to separate switches so they can be turned on independently of each other. I am going to put a Draft Stoppa (http://www.draftstoppa.com.au/draftstoppa) on the heater/fan unit. However, when a Draft Stoppa is installed on a combined heating-light/fan unit the fan is required to be wired so that it runs whenever the heating lights are turned on (to avoid overheating or worse).
What I want is that everything is set up so that if the heating is on then the fan is also turned on, but I can also turn on the fan without the heating turned on. I had an electrician look at this but was advised that it wasn’t possible without pulling extra cables through the wall (which is not really possible in this case). All the electrician could do with the existing wiring was to reconnect the wire from the fan switch to the heater switch, so that the heater switch controlled the heater and the fan simultaneously (both on or both off).
Having thought about it, I am positive I can do what I want by installing a box in the ceiling between the current wires and the heater/fan unit. I think that the simplest and most reliable approach would be to use a relay that is driven by the wire to the heater to switch a line from the heater input across to the fan input. This would require a relay with a 240V coil driving contacts rated at 240V and a much higher current than the fan needs. It only relies on basic physics. The coil only draws an extra ~0.5W (~2mA current). If the contacts ever fail permanently open or permanently closed there’s no safety problem. As far as I can tell, the worst that could happen would be if the coil insulation broke down and caused a short between the windings. I could put in the lowest power fuse I could find to guard against short circuits. So I think I could easily build this, that it should be very reliable and safe to leave unattended in the sealing (appropriately boxed to keep out rats or whatever).
Ms question is: Is there a readily available guide to laying out and enclosing mains power circuits so that I can be confident that I haven’t done anything dumb from a safety point of view. Basically I want to be confident that if I handed it to an electrician to connect the wires they wouldn’t refuse to do it.