Okay, I know that this same question has been asked a million times but i just cant seem to find an answer that im entirely happy with. I have a 400w metal halide fitting, today i bought a 400w hps bulb, i asked the guy at the shop if it would be okay to using with my ballast, he said if the hps bulb didnt fire up then it wouldnt work but if it did it would be fine to use with the mh ballast. I think its a pulse start ballast, if that changes anything. Another thing i wouldnt mind knowing just to kill my curiousity, doesnt a hsp ballast have an extra part that it allows the hp bulb to fire up, if so how does the bulb i bought work, because when i turned it on it worked just fine, had it on for about 4 hours and didnt have a problem with it. Am I right in thinking that if it fired up and works fine and the ballast and the bulb are both 400w then i should be pretty safe?