Like circuit breakers, fuses are another form of overcurrent protection. There are two main types found in the home: cartridge fuses, and plug fuses. Some old houses may have cartridge fuses, the oldest type still in use. If so, inspect the fuse holders on a regular basis, as they can loosen, and lead to overheating. Also, check the cartridge, by pulling it out, and testing with a continuity tester. Replace if necessary. Modern plug fuses are more advanced, blowing almost instantly, thereby limiting damage. When replacing a blown fuse, look at the metal strip called the 'element.' If it's burned in the middle, with the ends in place, the fuse was blown by an overload. If the element seems to have exploded, and the window is blackened, a short circuit occurred. Most plug fuses are the same size, even when the amps are different... So be careful to get the right one. Never replace the original fuse with a higher-rated one, or modify a fuse with a penny. This creates a serious fire hazard. If fuses blow on a regular basis, your system needs an upgrade. For more details on fuse maintenance and replacement, contact a qualified electrician.