electure:
I think you'll find the cause of this failure to be the way this particular panel's bus bars are assembled. This one is the style where the breaker's tabs fit into the punched plate, which is mounted to the bus with an 8-32 screw and standoff. (Look closely at the bus to the right of the breakers.)
On the world-famous FPE "horror story" website they talk in detail about this bus style. (I'll find the link and edit it in.)
Here's a link: http://www.inspect-ny.com/fpe/fpepanel.htm I'm trying to find the link for a document that shows in detail the different styles of bus construction...edit 2: Here's the link to a detailed pic of bus types, scroll down to item 5:
http://www.inspect-ny.com/fpe/fpestlouis.htm I believe that the screw corroded/oxidized over the years and the high current of the feeder passing thru started an arc which grew to line-to line. (Pretty easy in these crappy panels as clearances are nil anyway.)
Was there a main breaker here? (The 100 amp one you refer to?) Or were there other breakers over this damaged part of the bus? Do you have pics of the damaged breakers, if any?
Inquiring minds want to know...
edited to add link and bold text items..
[This message has been edited by mxslick (edited 07-07-2006).
[This message has been edited by mxslick (edited 07-07-2006).]
[This message has been edited by mxslick (edited 07-07-2006).]