Plug fuses that have blown may often be considered to be out of service, but this is not always the case. Old fuse boxes may have out-of-date, missing, or illegible directories, so it’s necessary to find the open fuse. Many times, old fuses are stuck in place, so it is difficult to unscrew them. Rather than removing each fuse and checking it with an ohmmeter, it is better to measure the voltage across each fuse. A good fuse will always measure 0V, whereas an open fuse will read some voltage, because the load side is hot. The output terminal will have a ground potential through the load, if it is connected. A neon tester or solenoid voltmeter works well for this operation.

Sometimes, a branch circuit will exhibit intermittent operation, causing flickering lights. If you gently move the breaker handle from side to side, the flicker will respond. You may hear a frying sound or see light coming from behind the breaker. This means the contacts — where they draw current from the bus bar — have become corroded and are making a less-than-perfect joint. Once this process begins, it will accelerate as less good metal remains available to perform its function.

A missing panel cover can also create problems. One of the functions of the cover is to hold the breakers firmly in place.

If the damage at a single-pole breaker is severe, most electricians replace the box. This is actually not necessary, because it is possible to replace the offending bus bar at a fraction of the cost, using a bus bar kit available from the manufacturer. Be sure to get the catalog number off the box, due to the number of variations.

Breakers should be exercised (turned off and on) periodically. Trade practice is for breakers to be exercised once per year, regardless of size. Where there is no excessive dust or humidity, this interval may be extended to once every three years, but the procedure is so easy that you might as well do it once a year. This is particularly true in the larger sizes, and it should be part of the preventive maintenance program in any large facility. If properly maintained, breakers last a long time. However, some workers intentionally shunt a branch circuit in order to trip the breaker. This is a very poor maintenance practice, because it stresses the contacts and shortens the life of the breaker.

If large aluminum conductors are terminated at a circuit breaker, then the lugs need to be retorqued periodically. A loose connection where heavy current is present will inevitably heat up.