Two brands of residential panel, installed widely in the mid-twentieth century, are treated by the trade as replacement items regardless of apparent condition.
Federal Pacific Stab-Lok
Installed in enormous numbers from the 1950s into the early 1980s.
The concern is that a meaningful proportion of Stab-Lok breakers fail to trip on an overload or a short circuit. Testing conducted from the late 1970s onward reported failure rates far above what any protective device should exhibit, and the company's UL listings were the subject of a federal investigation.
A breaker that does not open leaves the circuit conductors to carry the fault, and the fault is what the breaker exists to interrupt.
Zinsco and Sylvania-Zinsco
Installed from the 1950s into the 1970s.
Two failure modes are documented: breakers that fail to trip, and a bus bar design in which the breaker's connection to the bus overheats and, in some cases, the breaker welds to it.
A welded breaker cannot trip and cannot be switched off, which removes the disconnect as well as the protection.
How to identify them
Federal Pacific: the panel cover usually carries the FPE name, and the breakers are marked Stab-Lok, typically with a red or orange stripe on the handle.
Zinsco: the name appears on the panel or the breakers, and later ones are branded Sylvania-Zinsco or GTE-Sylvania. The breakers are distinctive, often with a colourful handle.
Photographs of both are widely available, and a home inspector identifies them immediately.
What is not a fix
Replacing individual breakers. New-manufacture replacements exist and the concern is with the design and the bus connection, not with an individual worn part.
Testing the breakers. A breaker that trips on a test today may not on a fault tomorrow, and the failure rate is statistical.
Leaving it because nothing has happened. The panel is a protective device that has not yet been called upon.
What replacement involves
A new panel in the same location, with the existing circuits reconnected.
Current requirements apply to the circuits as they are landed, which usually means AFCI or GFCI breakers for many of them — and that is a substantial part of the cost, often more than the panel itself.
Where the service conductors and the meter base are sound and the load calculation supports the existing service size, this is a panel replacement rather than a service upgrade, and the two should be priced separately.
Grounding and bonding are brought up to current requirements at the same time, which frequently means adding a grounding electrode conductor and a second ground rod.
Insurance and sale
Some insurers decline to write, or decline to renew, on a house with one of these panels, and some require replacement as a condition.
They also appear in essentially every home inspection report, which makes them a negotiating item on a sale. Replacing before listing removes both problems and is usually cheaper than the concession asked for.
The other panels worth attention
Split-bus panels with no single main disconnect — obsolete rather than dangerous, and awkward.
Any panel with evidence of overheating: discoloured bus bars, melted insulation at a breaker, a burning smell.
Panels in prohibited locations, such as a clothes closet or a bathroom.
None of these is as clear-cut as the two brands above, and each is a reason to have the panel looked at rather than a reason to replace it on sight.
What to do if you have one
There is no urgency in the sense of evacuating the house, and there is no reason to leave it indefinitely either.
Get a quote for a panel replacement, keep the load calculation separate from it so a service upgrade is not bundled in without justification, and have the grounding and bonding brought current as part of the work.
In the meantime, the ordinary precautions matter more than usual: working smoke alarms, no extension cords carrying heavy loads, and nothing left running unattended on a circuit that has ever tripped or warmed a cover plate.