sparkync,
Most of my work these days is wiring these cold-rooms, so if you need any further advice after I have submitted this, just ask away!, A drain heater, as a rule, should not require an isolator on it, provided that it is supplied from the same circuit as the refrigeration circuit that it protects, remember, this heater, is only rated at 20-40Watts and it must be continuously energised, as if it fails, the drain will block.
Everything associated in a Cool-room or freezer, must be waterproof, using Silicone sealant, where it is required, will help, but using IP Rated (NEMA) fittings from the start, is a mandatory thing,with these types of environments, especially where condensation is concerned. [Linked Image]