What Happens When a Fuse Blows

Overloads and Short Circuits Are the Most Common Causes

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Most homes built after 1965, as well as older homes that have updated electrical services, have circuit breakers that control the electrical circuits in their homes. But in older homes that haven't been updated, the electrical circuits are protected and controlled by fuses located in a central fuse box. These devices serve the same function as circuit breakers to protect against circuit overloads and short circuits, but rather than resetting them when they "trip," you must replace fuses when they burn out ("blow").

The Anatomy of a Fuse

Two different types of fuses control 120-volt circuits and 240-volt circuits in older electrical systems. For 120-volt circuits, the fuses are small ceramic screw-in devices that fit into threaded sockets in the fuse panel, much the way lightbulbs screw into lamp sockets. Inside the fuse, there is a metal ribbon through which all the current on the circuit passes. The ribbon is sized to match the circuit wire gauge, and if too much current passes through the ribbon, it melts through, or "blows," and the circuit goes dead. The face of the fuse has a small glass window through which you can see the metal ribbon, and when a fuse blows, you will see the metal ribbon melted through, or a cloudiness in the glass. Screw-in fuses are typically 15-amp or 20-amp fuses, or occasionally 30-amp.

For 240-volt circuits that control major appliance circuits, such as an air conditioner or electric range, the fuses are small cartridge devices that fit between metal contacts, usually fitted into a fuse block that can be pulled out from the fuse panel in order to change the fuses. Cartridge fuses are usually used for 240-volt appliance circuits that draw 30, 40, or 50 amps.

Sizing of Fuses

Like circuit breakers, fuses are sized to match the gauge of the circuit wires. This prevents the circuit wires from drawing more power than they can handle. Using correct fuse sizes is, therefore, a crucial safety feature that can prevent fires due to circuit overloads.

  • For 14-gauge or larger circuit wires, a 15-amp circuit breaker is acceptable.
  • For 12-gauge or larger wire, a 20-amp circuit breaker is acceptable.
  • For 10-gauge or larger wire, a 30-amp circuit breaker is acceptable.

Tales are told of people who replaced burned-out fuses with a copper penny inserted into the fuse socket—a solution that did restore power to the circuit but also created an immediate danger of fire since there was no longer any limitation to how much power was drawn through the circuit wires.

  • Warning: Never replace a burned-out fuse with one of larger amperage rating.

A newer type of fuse called an Edison-base, has a specially shaped base that prevents the wrong-sized fuse from being inserted into the socket. Once the bases are fitted into the fuse sockets, only fuses of the proper size can be fitted into them. If your fuse panel does not have Edison bases, it is a good idea to install them.

What Happens When a Fuse Blows

There are two conditions that can cause a fuse to blow. First, and most commonly, when too many lights or plug-in appliances draw power from the circuit, it can overload the capacity of the fuse and cause the metal ribbon inside the fuse to melt through. The result is that all lights, outlets, and appliances powered by the circuit will go dead suddenly. When you examine the fuse, you will likely notice that the metal ribbon located behind the glass window is melted through, or you will notice a fog or cloudiness in the window, indicating a very sudden melting of the ribbon. The immediate solution here is to replace the fuse with one of the same size. Longer term, though, you will need to move some plug-in appliances into other circuits to avoid another overload and another blown fuse. Appliances that heat (such as toasters or clothes irons) or those with motors (such as vacuum cleaners) are especially prone to causing overloads, since their power draw is fairly large, especially when they first startup.

Another cause of a blown fuse occurs when a hot wire somewhere in the system touches either the grounding pathway or a neutral wire. This is what is known as a short circuit, and it typically occurs because of loose wire connections, damaged wires somewhere along the circuit, or an internal wiring problem in some appliance plugged into the circuit. A miswired lamp, for example, can cause a short circuit and blown fuse if it is plugged into an outlet. Or wires that have been eaten through by rodents in walls can cause a hot wire to touch the grounding path or a neutral wire. The immediate symptom is the same as for an overload—the metal ribbon inside the fuse burns through and all lights and fixtures along the circuit go dead. But in the case of a short circuit, merely replacing the fuse will likely cause the new one to blow immediately—unless the short circuit has been fixed.

Diagnosing the location of a short circuit can take patience. Because many short circuits occur in plug-in lamps or appliances, start by unplugging every lamp and appliance, then replace the burned out fuse. If the new fuse holds, it is likely that the wiring problem was in one of the lamps or appliances you unplugged. If not, then the problem exists somewhere in the circuit wiring itself. You can visually inspect each outlet, wall switch, and light fixture for loose connections, but there is a good chance that you will need to call in a professional electrician to locate and fix the problem.