How Plug Fuses Work
Plug fuses have a metal alloy ribbon inside that carries the current of the circuit. In the case of a short or overload that causes too much current to flow, the ribbon will melt (“blow”) and open the circuit. When this happens, no current can flow and the circuit is disconnected from the power supply. This protects against short circuits and overloads that may damage electrical wiring and cause house fires. When a plug fuse blows due to a circuit overload or short circuit, it is often quite evident because the metal fuse element inside the view panel will be obviously melted through, or the glass panel will be fogged or scorched. But if in doubt, it is possible to check the fuse with a multimeter.
Types of Plug Fuses
Screw-in plug fuses come in several types, and while all are tested the same way, it is important to know the difference between the types, since you will need to install replacement fuses that match.
Standard fuses have screw-in bases that thread into sockets exactly as a lightbulb does. They typically are available in 15-amp, 20-amp, 25-amp, and sometimes 30-amp sizes. This is an older style of fuse, and a somewhat dangerous one, since any of the circuit sockets can accept any fuse size. This makes it easy to put in a fuse that’s larger than the circuit rating, creating a notable danger of overloading the wires. If you have a fuse panel fitted with standard fuses, you will need to take care to use the proper size when replacing a blown fuse.Tamper-resistant fuses have a smaller, plastic screw-in base that is fitted into the fuse socket via a fuse adapter. These adapters fit permanently into the fuse panel socket, and they are sized so they will accept only one fuse size. This makes it impossible to make a mistake when replacing a fuse, since a 15-amp base will accept only a 15-amp fuse, a 20-amp base will accept only a 20-amp fuse, etc.Time-delay fuses are available both as standard fuses and as tamper-resistant fuses. These use a special design that allows brief surges of power to occur without burning out the fuse. They are usually used for circuits that provide power to motor-driven appliances, such as refrigerators, washing machines, or dishwashers, which create power surges during start-up that can cause standard fuses to blow.
Tools and Supplies You Will Need
Multimeter Eye protection Replacement fuse
Instructions
Carefully grip the fuse by the outer ceramic rim and twist counterclockwise to unthread it from the fuse socket. Take care not to touch any metal parts in the fuse panel as you remove the fuse. Safe practice for professional electricians often involves using only one hand while unscrewing a fuse, since this reduces the chance of deadly shock. Test the meter’s function by touching the metal tips of the testing leads together. The meter needle should move to show that little or no resistance is present. Power is simply flowing from one lead through the other, with no interruption. As you separate the leads, the needle should return to the 100% resistance reading, indicating no power is flowing. If the needle on the meter moves to show little or no resistance, this means that current is flowing through the fuse and that it is functioning correctly and is not blown. But if the multimeter shows an infinite (100%) resistance reading, it means no current is flowing and that the fuse is blown. If you have an older style fuse panel with standard plug fuses and want to add tamper-proof fuses, simply add plug fuse adapters. These adapters come in 15, 20, and 30-amp sizes. They allow the newer and safer S-type tamper-proof fuses to be screwed into the standard fuse sockets. Simply screw the new S-type fuse into the base and then screw the whole assembly into the fuse panel socket. In the future, if a tamper-proof fuse blows, the adapter will remain in place as you screw out the S-type fuse and screw in a new one.
