I have actually had to replace a main because water wicked BETWEEN the strands of the main incoming hot conductors and right into the main breaker! Far fetched I know but it happened!
Just curious, the newer FPE that burned up, It had the newer Square-D main? " 1C-100" I think the part # is and the older FPE mains that went were the older plug on style "NA-2P100"?
A few years ago I remember Cutler Hammer had a big problem... Their "BQL" series main breakers used to have the sides burn right off, That or the breaker-buss connection burned up!
A.D