You can only measure heat radiation properly when the ballast is loaded, i.e. the bulb is connected. Otherwise the ballast draws minimal power, therefore minimal heat radiation. Most ballasts are safe to run unloaded.
houdini Reviewed by houdini on . is it safe to run a ballast with no bulb i wanted to see how much heat it gives off i just assembled my 1000watt mh ballast so i could see if i should fan the enclosure. Rating: 5