Troubleshooting Fuse Box Issues

Many beautiful, older homes still have their original Edison base fuse box. A fuse box is simply a box containing a bunch of screw-shell sockets that are quite similar to light bulb sockets. Like a light socket, they consist of a brass screw shell and a brass button contact located at the bottom of the socket. In a fuse box, the button contact connects to one of the panel's phase busses, either the A or B phase. The shell connects to the branch circuit hot wire through a screw terminal. Given how simply the fuse boxes are constructed, they are easy to troubleshoot as well. The only major problem with the Edison base fuse box is that they are no longer in production so finding replacement parts is next to impossible.

Burned or Dirty Fuse Holders

In many cases, the cause of flickering lights and low voltage at receptacles can be traced back to the fuse box and ultimately to dirty or burned fuse holders. Burned or pitted button contact creates a bad connection with the button contact on the Edison base fuse. It forms what is called a high-resistance connection. A voltage drop develops across it, which in turn reduces the amount of voltage available at the receptacle, light fixture or other load. Pitting results when a loose fuse causes arcing between the button on the fuse and the button in the fuse holder. If the pitting isn't too bad, you may be able to smooth it out with a piece of emery cloth secured to the end a 1/4-inch dowel rod. After smoothing out the pits, raise the button up carefully using a small screwdriver to increase its contact with the button contact on the fuse. Make sure you pull the main disconnect switch before attempting this repair.

Blown Main Fuses

With the Edison base fuse box, the main fuses were two cartridge fuses. As their name implies, they looked something like shotgun shells with brass ferrules at each end. Typically, these fuses measured around 60 amperes of apparent power. One fuse protected the A phase and the other protected the B phase. If the problem you are experiencing is that one or both of the main cartridges blew out as soon as the disconnect switch was closed, it is likely that one or more of the fuse holders have shorted or that there's a phase-to-phase short. You can troubleshoot to figure out the nature of the problem.

Step One: Pull the main service disconnect switch. Use a digital multimeter (DMM), set it to AC voltage function and check between each phase wire and the ground to make sure that both phases are actually dead. Remove all the screw-in fuses.

Step Two: Set your DMM to its Ohm range and touch one probe to each phase bus. If you receive a continuity reading, that indicates there is a phase-to-phase short. A reading of infinite resistance, or an open circuit, indicates no phase-to-phase short present.

Step Three: With the DMM still set to its Ohm function, check between each phase bus and the grounded metal box. A continuity reading here indicates a ground on that particular bus. You will have to locate the source of that ground and repair it.

Which is better: Repairing the fuse box or replacing it with a circuit breaker panel? If the fuse box is badly damaged, your better option is to replace the fuse box with a modern circuit breaker panel. If you take the latter route, you should upgrade your service drop to a 240-volt, 150-ampere service minimum.

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