When I was involved (in UK) with fire appliances, (40 years ago) they were plugged into the mains, so they'd be ready to go at full chat as soon as the engine was started. The mains was for an immersion heater in the cooling system so the engine was near enough up to temperature immediately it was started. That could be useful on cold mornings for the car!
A similar system has been available for smaller vehicles for many years. The Kenlowe Hotstart. I first fitted one about 20 years ago and they had been around for quite a while then. It's a 3kW heater with a pump that fits in the heater loop, it warms up the engine block in about 15-20 minutes and thermostatically controls it to prevent boiling. Plus if you leave it on a bit longer and arrange for a thermal switch to turn on the existing heater fan it warms up the interior and melts all the ice off. Instant engine starts with a relatively warm and dry interior with fully deiced windows are a huge plus on cold winter mornings.
Anyway back to Dave's original question. The Kenlowe Hotstart uses a 3 pin version of the conventional lawnmower extension connectors. See RS
531-431. I don't think it carries any official IP rating but with a tight fitting mains lead it's stood up to many years use. For 9 months of the year it just sits tucked inside the radiator grill. Despite being moved between many vehicles I think I've replaced maybe 3 connectors in 20 years - all because I forgot to stow the connector and it dragged on the road.
Thanks for that. Thinking on it that type of connector is likely to be at least semi moisture proof since it's likely to be dragged through wet grass, etc.
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