How To Test Screw-In Plug Fuses

Plug fuse test
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In older electrical service panels, the circuits are protected with fuses rather than circuit breakers. Larger, 240-volt circuits will be protected by cylindrical cartridge fuses, while standard 120-volt household circuits use screw-in fuses that fit into threaded sockets. These are known as plug fuses or Edison-base fuses. The outer threaded part is one contact point for the fuse and the bottom of the fuse is the other.

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

Instructions

Identify the Plug Fuse

Open the door on the fuse panel and look for the fuse you want to test. If the panel is properly indexed, the circuit that has lost power can be identified by the circuit label. You may notice that the glass view window on the fuse is fogged or scorched.

Safety Tip

Always make sure the floor around the fuse panel is perfectly dry before you open the panel or do any work. Any moisture on the floor increases the chance of shock.

Remove the Plug Fuse

Put on eye protection to guard against the possibility of flying sparks, which sometimes occur if there is a short circuit or other problem in the panel.

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.

Prepare the Multimeter

Connect the black wire lead on the multimeter to the COMMON socket. Connect the red lead to the Ω (OHMS) socket. Move the dial on the face of the meter to the lowest range of the Ohms scale. With most meters, this action turns the multimeter on, but if there is a separate ON switch, turn the meter to the ON position.

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 now power if flowing.

Test the Fuse

Place the fuse on a non-conductive surface, such as a laminate or wood tabletop. Touch the tip of one multimeter lead to the metal threads on the base of the fuse, then touch the tip of the other lead to the metal tip on the end of the fuse.

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.

Install a New Fuse

If the fuse is blown, install a replacement that is is exactly the same style and same amperage rating as the blown fuse. Under no circumstances should you install a fuse with a larger amp rating, as this could allow the circuit wires to carry more current than they can safely handle.

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.

Consider an Electrical Service Upgrade

An electrical service that is protected by fuses rather than circuit breakers is by definition an old service, since circuit breakers became the norm in the late 1960s and 1970s. Most fuse panels carry only 30 or 60 amps of power, and this is woefully inadequate for modern power needs. It is, therefore. a good idea to consider having your electrical service panel upgraded to a more modern larger service with circuit breakers. While there is nothing inherently dangerous about a fuse panel that is functioning properly, upgrading the electrical service may make it easier (and cheaper) to obtain homeowner's insurance, and it will be a decided advantage when you try to sell your home.