Techniphase's information is correct, but it goes deeper. First, a good rule of thumb is 10 times the breaker trip rating for "instantaneous trip". Per Mil spec, instantaneaous is 5 cycles or less. Armed with this information, you need to start troubleshooting. You are referring to the breaker tripping when utilizing certain pieces of equipment. 10x the rating of the 200A breaker is going to be FAR more amperage than the draw of your building, let alone the main's rating. You have either a direct ground fault or a direct phase to phase short. If your 800A is always the one that is tripping, I would expect the short to be on the line side of the 200A breaker, but that is not science, it goes back to the random nature of instant trip that Jim cited. At least on occasion the 200A should trip before the 800A. I would start with some megger readings though. Basically, I am indicating that overload, which you are implying in your initial comments isn't the cause of either breaker tripping.
Originally Posted by techniphase
I know what I don't know, and I know where to go to find it!