There are plenty of single pole FPE breakers that cannot be made to trip, even if you ground out the wire to the neutral bus of the panel (don't ask).
Add to this the following experiance, about half the time, many of the breakers fall out when you remove the box cover. Many other times, the breakers are held in place by the stab-loc welding itself to the busbar from so much arcing. This is a high resistance connection & the breakers run hot.
The only good FPE breaker is one that is headed to the landfill.
