FPE seems to be universally despised. Most of the time, the breakers fall out when you take the cover off the box.
My experiance has been that some FPE breakers will not trip even when you tkae a wire from the load terminal to the neutral bar & turn the breaker on.
Chances are, once you replace this mess with another panelboard, there should be no more overheating problems.
If you haven't priced FPE replacements lately, you're going to be in for a shock. One breaker will cost you about 25% of what a cheap 200 amp panel costs & almost 50% of the cost of a cheap 100 amp panel.
Tom
