Why fuses blow; replacing a blown fuse

In older electric systems, incoming power is divided into branch circuits at a fuse box. Each circuit is protected by one or more fuses containing a metal strip that melts and breaks when the flow of current exceeds the circuit’s capacity. In a plug fuse, the metal strip is visible through a plastic window. An overload breaks the strip; a short circuit discolors the window. A blown cartridge fuse shows no sign of damage; test it with a continuity tester. Keep spare fuses handy near the fuse box. Always replace a fuse with one of the same amperage.

Plug fuses of 15 or 20 amperes protect circuits for basic lighting, kitchen and laundry areas, and individual large appliances. Ferrule-type cartridges, rated up to 60 amperes, control 240-volt circuits for heavy-load appliances such as a clothes dryer. Knife-edge fuses in the large pullout main fuse block protect the main power line. Removing the pullout block or, in some fuse boxes, turning off a main power switch, cuts off all power in the house.

Caution: Before working at the fuse box, pull out the main fuse block or turn off the main switch. Wear shoes with rubber soles, do not stand on a wet spot. Keep one hand free when removing or re placing a fuse.

If the power to a circuit goes off, locate and correct the problem. Then, holding the blown fuse by its glass rim, turn it counterclockwise to remove it. Replace it with a new one; restore the power.

To test a cartridge fuse, pull out its fuse block and remove the fuse with a fuse puller. Touch the probes of a continuity tester to the ends of the fuse. If the tester doesn’t light, the fuse is blown. Insert a new fuse into the spring clips by hand.