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# What Makes a Fuse Blow?

Article Details
• Written By: Michael Pollick
• Edited By: Niki Foster
2003-2018
Conjecture Corporation
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For those who still enjoy the charms of glass or ceramic fuses, there's rarely a good time to blow a fuse. It usually happens at the end of an exciting football game or just before the killer in a mystery is revealed. When you blow a fuse, the search for a replacement overrides everything, but what causes a fuse to blow in the first place? The short answer is heat from an overloaded electrical circuit, but there's more to it than that. That blown fuse may have saved the rest of your house from burning down.

Electricity enters the average home at a certain strength, which electricians measure as voltage. For best results, that electrical current should flow through your house and back to the outside line without unusual high resistance. When you plug all of your electrical appliances into the sockets, however, you create a certain amount of resistance on the wires. How much actual electricity a particular device uses is measured in terms of amperage. A clothes dryer with a large electric motor and heating element requires more amperage than a toaster, for example.

Electricians want to prevent the electrical wires from becoming overheated, so they install safety devices called fuses in a centralized box. Each fuse is designed to withstand a certain amount of amperage, but the wire inside the fuse melts when it becomes overheated. When you blow a fuse, the wire usually snaps in two, and the power running through that circuit is immediately cut off. The excess heat buildup occurs whenever appliances draw more amperage than the circuit can handle. If the fuse is rated for 25 amps, for example, and a user plugs in a 75 amp clothes dryer, the excessive amperage will blow the fuse.

Sometimes, a series of temporary overages can weaken the fuse's filament, which means you could still blow a fuse without exceeding the amperage rating. Some fuses are designed to withstand a number of brief overloads before blowing, but others snap quickly after one sustained power surge. When you blow a fuse, it is important to unplug all of the appliances and devices from that circuit before installing a new fuse. The energy required to restart those devices may cause yet another blown fuse.

If you continually blow a fuse when using a high-amperage appliance, you may need to change the rating of the fuse itself. The fuse rating should exceed the amperage demands of the appliances it protects, so you may have to buy bigger fuses to avoid future problems. While getting a more robust fuse can solve your problem, it won't always necessarily be the case. The circuitry connecting the power to the fuse still has to be able to handle that amperage. Increasing the fuse rating to something higher than the wiring can accommodate can cause you bigger problems than just a blown fuse. In the US, a normal house outlet generally has anywhere from a 15 to 30 amp fuse rating (i.e., 15 amps in bedrooms, 20 amps in kitchens, and 30 amps in laundry rooms), so increasing your fuse rating to something above this could cause problems. When in doubt, play it safe and call a professional.