Why Do Fuses Blow?

Breakers And Fuses

Electrical fuse box.
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You've heard the term blown fuses, but what is this exactly? First, you need to know what fuses are and just exactly what they are designed to do. You see, your home's electrical wiring must be protected from short circuits and circuit overloads. A fuse incorporates a fuse link that has a specific amperage rating, usually 15, 20, or  30 amps. Without a fuse or circuit breaker with this specific rating, there would be nothing to protect the wiring, devices, or the operator running things like power tools.

Just as the wiring is sized specifically for the load it can handle without breaking down and heating up, so works the fuse. This type of protection is in place to keep your home from having an electrical fire.

Your home is protected by either circuit breakers or fuses. Both circuit breakers and fuses are rated by the maximum amount of amps they are designed to trip or blow at. This predetermined amperage rating is clearly marked on both. Circuit breaker physically looks the same, except for the amperage rating posted on the trip handle. Fuses, however, may look similar, but the new style, Edison-based, only allows you to screw the appropriately sized fuse into the socket. This safety feature stops people from taking out a small fuse, like a 15-amp fuse, and replacing it with a 20- or 30-amp fuse. As you can see, placing a larger valued fuse on a circuit with lower rated wiring is a disaster in the making.

What is worse, I have seen the farmers replace the fuse in the socket with a penny under it to short the connection across, basically having no protection at this point. Like I told the man who said he did this, "Call the fire department now! You're going to need them."

Standard sizes include 15-, 20- and 30-amp fuses in the screw-in variety.

These type of fuses screw into a socket, shaped very much like a light bulb socket. Edison-based fuses (labeled with an "S", have a smaller screw-in base and it is called a rejection base. This means that only the right sized fuse can be replaced with this type of base.​

For instance, a 30-amp fuse will not make contact with a 20-amp Edison base, therefore it is foolproof. This is a great added safety measure to eliminate someone just putting an oversized fuse into a fuse socket to try and keep the fuse from blowing. The real answer is to eliminate some of the load on the circuit instead until the fuses no longer blow.

The 15-amp fuse protects #14 or larger wire, the 20-amp fuse protects #12 or larger wire, and the 30-amp fuse protects #10 or larger wire.

If a circuit is overloaded, meaning beyond the preset amount of amps labeled on the fuse, the fuse blows (or opens the circuit) to protect the wires from breaking down and burning. To cure this problem, lighten the electrical load on the circuit by removing things connected to the circuit until the fuse holds.

Electrical shorts also cause fuse failure. An electrical short can be from the hot wire to ground or a hot wire to neutral. In either case, the fuse will open and shut the circuit off.

To find this problem is a little more complicated, but start with the easiest solutions and work backward to the fuse connection. Start by finding the affected circuit that is off and what is connected to it. The appliances, cords, and lighting connected to the circuit may have a short causing all of the problems.

Check cords and connections to see if you can find the problem. Next, unplug each item, one at a time, to see which is the problem with the short. Not every problem will be solved this easily though. It may be a wire in the attic or basement that a mouse chewed that is causing a short. Maybe it is insulation on a wire that has crumbled and now the bare wire is touching a grounded box? If you cannot find the problem on your own, don't hesitate to call in a professional electrician to help you find the shorted circuit.