Not good Terry!

I don't know if you have HPS or metal halide. Metal halide bulbs are especially intolerant of operating outside their design wattage, and that includes at too low a power. The bulb will rapidly deteriorate and then fail. HPS bulbs are a little more tolerant but they too will fail if operated below their design wattage.

Perhaps more dangerous is that the ballast will be running at above its design wattage. The wiring and cooling is not designed for this and you are risking a fire.

Depending on whether the ballast is magnetic or electronic. Magnetic is much hotter. They both can be very hot. Are you sure it's a 400 w ballast and a 600w bulb?

Don't burn out your ballast. It's not cheap.

dcarch

Thanks for those answers as you said it is not looking good. I am sure the ballast is 400w as it is printed clearly on the side (maxibright compact power pack)the bulb is a Sylvania HPS GroLux 600w. The bulb was supposed to be 400w and hence my problem. I will try and contact the vendor today.
Terry

The Maxibright 400W does at least have a thermal cutout, so theoretically if it is still operating then it shouldn't be at a dangerous temperature. But it is also an earthless design and you don't want that melting on you ;)