If you own a home or business built between 1950 and 1985, it is possible that you have a Federal Pacific Electric Stab-Lok breaker panel. According to a blog post from The Hartford Steam Boiler Inspection & Insurance Co., the Stab-Lok panels are associated with several problems related to the breakers not tripping and issues with internal connections on the busbars.
John Weber, principal electrical engineer, writes in his blog, "The hidden danger lurking in your electrical box," that a quick check of your circuit breaker panel can shed new light on electrical hazard exposure. The Consumer Product Safety Commission looked into many problems in 1982 with these breakers not tripping properly, according to UL testing requirements.
Tests performed by the Consumer Product Safety Commission, and independent consulting engineers, concluded that certain Stab-Lok breakers do not trip according to requirements and in some cases can jam in the “on” position. This condition was most pronounced in the Stab-Lok two-pole version of the breakers.
For a more detailed explanation and additional photos, see the original blog post.