How to Troubleshoot a Switch Box Fuse

An electrical switch box contains fuses for your older home or apartment.

You will usually find fuses in older homes and apartments located in metal, electric service panels or switch boxes. Modern homes and apartments use circuit breakers instead of fuses. Just like a circuit breaker, a fuse protects a circuit and will blow to protect the electrical system from overloading and overheating; otherwise, you may have a fire. Knowing how to troubleshoot a switch box fuse takes only a few minutes, and your electrical system will be protected once more.

Things You'll Need

• Fuses

Instructions

1. Check to see what caused a blown fuse before replacing it. Look for damaged cords on the appliances you had plugged in, then disconnect them. Do not use the appliances until you replace the cords. A short circuit can happen when two bare wires touch, which can lead to a fire. Short circuits commonly occur when wire insulation deteriorates. Identify a short circuited fuse by looking at the front window of the suspected fuse. The usually clear window will appear brown and discolored from the vaporized metal strip behind it, vaporized from an extremely high temperature. Replace the short circuited fuse with a new one of the same amp or lower. Fuses come with ratings of 13, 20 and 30 amps. Unscrew the round fuse counterclockwise, just like unscrewing a light bulb. Screw in the new fuse clockwise.

2. Check to see what appliance or lighting caused an overloaded blown fuse by disconnecting the appliance or lighting and checking its wattage. For instance, standard 120-volt systems generally use a 15-amp current, and when you multiply 120 volts by 15 amps, you have 1,800 watts, the most wattage or electrical power you can load on this appliance or lighting circuit. An overloaded blown fuse happens from an overloaded circuit, such as when you have too many appliances or lights connected to the same electrical circuit. This overload causes the circuit to overheat, which can cause an electrical fire from burning wire insulation. Recognize an overloaded blown fuse by the clear window on the front of the fuse and the melted metal strip behind the window, then replace the overloaded blown fuse with a new fuse of a matching amp rating or lower. If you replace the fuse with a higher amp, you will have a fire hazard.

3. Replace a fuse if it has not blown if you see pitting and severe discoloration along the bottom of the fuse. This means the fuse was never screwed in properly, resulting in a loose connection.

Tips & Warnings

• Purchase new fuses at home improvement stores and online. Bring your old fuse in with you to make certain you get the correct match and amp rating.

• Never insert anything into an empty fuse socket -- this can cause an electrical short, resulting in injury or death.

• Remove any pennies you may see under a fuse. Some may place a penny under a fuse to bypass the function of the fuse, an illegal activity.

• Remove fuses cautiously if you still have the electrical power on.

References

• Center for Disease Control and Prevention: Healthy Housing Reference Manual
• AFB Senior Site: When the Lights Go Out: Replacing Fuses and Resetting Circuit Breakers
• City of Chula Vista: Electrical Safety

Resources

• Surplus Sales of Nebraska: Electrical Fuses: Household Screw-In