yes. as long as everything is wired correctly, and it's not touching anything that will burn, you're money. move your hand to different spots around the fixture and if its comfortable your in the clear.
All these answers are right, but to answer your question, if nothing is touching your bulb or excessively close, yes you can run a 400w hps 24-7 without any damage to the plants. If u have bad wiring in your house, run your hand close to your breaker and you`ll know if something is overheating. Hope this answers your question, good luck:48: