What Happens When a Fuse Blows

A fuse is a glass tube with a thin piece of wire (ribbon) inside that handles a specific amount of current. But when too much current flows through, it causes a blown fuse. The electrical circuits in an older home are protected and controlled by these fuses which are located in a central fuse box. 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.

In an older home's fuse box, these devices serve the same function as circuit breakers to protect against circuit overloads and short circuits. But rather than resetting the fuse box when fuses "trip," you must replace them when they burn out or blow. Learn more about what to do when you have a blown fuse.

What Is a Blown Fuse?

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 cloudiness in the glass.

Types of Fuses

Two different types of fuses control 120-volt circuits and 240-volt circuits in older electrical systems.

• Screw-in fuses for 120-volt circuits: Fuses are small ceramic screw-in devices that fit into threaded sockets in the fuse panel, much the way lightbulbs screw into lamp sockets. There is a metal ribbon inside through which all the current on the circuit passes. Screw-in fuses are typically 15-amp or 20-amp fuses, or occasionally 30-amp.
• Cartridge fuses for 240-volt circuits: These circuits control major appliances, such as an air conditioner or electric range. The fuses are small cartridge devices that fit between metal contacts and can be easily pulled out for replacement. 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 fuse is acceptable.
• For 12-gauge or larger wire, a 20-amp fuse is acceptable.
• For 10-gauge or larger wire, a 30-amp fuse is acceptable.

Disregard advice about replacing burned-out fuses with a copper penny inserted into the fuse socket. Besides the rarity of copper pennies, this old method also causes an immediate danger of fire since there is no longer any limitation to how much power can be drawn through the circuit wires.

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 common causes for blown fuses. As a result, the metal ribbon inside the fuse burns through and all lights, outlets, appliances, and fixtures powered by the circuit will go dead suddenly.

Common Causes of Blown Fuses

• Overload: A fuse blows when too many lights or plug-in appliances draw power from the same circuit. You can replace the fuse but you will need to move some appliances to other circuits to avoid another blown fuse. Appliances that heat (such as toasters or clothes irons) or those with motors (such as vacuum cleaners) draw a lot of power and are prone to causing overloads.
• Short circuit: A blown fuse occurs when loose wire connections, damaged wires, an appliance with an internal wire problem, or an incorrectly wired lamp causes a short circuit. Wires eaten through by rodents in walls can also cause a hot wire to touch the grounding path or a neutral wire, resulting in a short circuit.

Other Causes of Blown Fuses

• Ground faults: A ground fault is a short circuit where electrical current flows directly to the earth (ground) or a grounded part of the system (grounding wire or the electric box). A ground fault circuit interruptor (GFCI) will cut off the power flow before any potential shock occurs.
• Arc faults: Arc faults occur when wires and terminals don't work as they should such as when a screw is loose. If your home has arc fault circuit interrupters (AFCIs) that are up-to-date, they will stop any potential sparks caused by faulty wiring.
• Circuit issue: Besides a short or overloaded circuit, the fuse may have blown or circuit breaker tripped which cuts off the flow of electricity in case of a surge (fault) or other problem. Repeated blowing or tripping can be a sign of another issue and should be investigated by an electrician.
• Damaged outlet or wiring: If something is wrong with the wiring or any of the connected parts (outlet), it can create a power surge that can blow a fuse or trip a circuit. So, it's not the circuit breaker or the fuse that's the problem, but rather the faulty equipment causing the issue.
  

What Blown Fuses Look Like

When you examine a blown fuse, you will likely notice that the metal ribbon located behind the glass window is melted through or there is a metallic smear on the inside of the glass tube. You may also notice a fog or cloudiness in the window, indicating a very sudden melting of the ribbon. 

How to Fix a Blown Fuse

Diagnosing where the short circuit is can take patience. Because many short circuits occur in plug-in lamps or appliances, start by unplugging every lamp and appliance to determine the location. The fuse itself may have simply become degraded over time.

When you find a blown fuse, you can replace it to restore power. These same basic steps can be followed no matter what type of fuse it is:

1. Turn off the power at the fuse box.
2. Pull the main fuse block out of the panel to disconnect the power.
3. Twist the blown fuse out of the socket or use a fuse puller to remove it.
4. Screw or push in a new fuse of the same size, amperage and type.
   
5. Insert the main fuse block into the designated socket and apply firm pressure to fit securely.
6. Close the panel door and check if this has restored power to the problem area.

If the fuse keeps blowing right after replacing it, the problem exists in the circuit wiring itself. You can visually inspect each outlet, wall switch, and light fixture for loose connections, but you will likely need to call a professional electrician to locate and fix the problem.

When to Call a Professional

Faulty electrical wiring and other electrical failures and malfunctions can be extremely dangerous, putting people at risk of fire and electrocution so erring on the safe side is always best. If you have fuses that continually blow, have a licensed electrician inspect your home to identify the underlying issue. If you have any doubts at all about the safety of your system or what may be the culprit, it's best not to take any risks by investigating further on your own. If your house is more than 50 years old, it is especially important to have the wiring inspected by an expert to make sure it's secure and capable of meeting the needs of modern homeowners.