Seems that the legal deal with FPE's Stab-Lok breakers was on 2 Pole units failing to open automatically, during an Overcurrent or Overload situation.
For some reason, the 2 pole 50's come to mind as being the key players. Maybe from their use as Main breakers / disconnects for Residential panels, or used on Residential Kitchen Appliances, or Residential Heat Pumps.

I am not saying the FPE Stab Lok product line is only "Tarnished" by those certain frames, only that the loss of UL and the law stuff was directed at certain frames.

I have seen FPE Stab Lok 20/1 pole breakers trip from Overloads quite often [maybe too often - as they trip long before 20 amps has been exceeded!]. The same device would not trip during a Fault situation which fell into it's plotted range [well below 500 amps!].
This is true to many of the FPE Stab Lok frames I have seen subjected to low level Faults - either first hand or "To Fix Things When S**T Happened".

For Zinsco F / FM Frames, they sometimes trip from Overloads [gradual increased overloads - same as the FPE's] - most 20/1's will hold 25+ amps for more than 10 minutes, some for ever!

Faults of any intensity rarely trip these devices.

I have personally welded the contacts of two[2] different 20/1 units - at different locations - while throwing the handle into the "ON" position.

Each time, there was an error made in circuitry which was the problem.
One was a mistakenly terminated branch circuit [Line and Grounded Conductor terminated together], the other was a pinched ungrounded conductor, caught between a Receptacle's 6/32 screw and the box.
Fortunately, I only did the pinched wire, not the bad termination. [Linked Image]

Along with the above items, the higher costs of these devices makes them "Bad" in my mind.

Scott SET

Scott " 35 " Thompson
Just Say NO To Green Eggs And Ham!