Step-by-Step

By examining a fuse you usually can tell what made it blow -- an overload or a short. A short circuit usually explodes the strip, blackening the fuse window. An overload usually melts it, leaving the window clear.

A tamperproof fuse is an important safety device that makes it impossible to install a fuse with a higher amperage rating than the circuit is designed for. It comes with a threaded adapter that fits permanently into the box. The adapter accepts only a fuse of the proper rating. When an electric motor on a washing machine or refrigerator starts up, it causes a momentary overload, which can blow fuses unnecessarily. A time-delay fuse avoids this problem by not blowing during the surge. Only a sustained overload will blow the fuse.

Fuses for 30- to 60-amp circuits typically are the ferrule-contact cartridge type. Knife-blade-contact fuses carry 70 amps or more. Handle both with extreme caution. Touching either with your bare hand could fatally shock you.

For safety, keep a plastic fuse puller with your spare fuses, and use it as shown. Note, too, that the ends of a cartridge fuse get hot, so don't touch them immediately after you've pulled the fuse.

To see if a cartridge fuse has blown, check it with a continuity tester. Clamp or hold the clip on one end, and touch the probe to the other. The bulb will light if it is not blown.