How to Test a Fuse Using a Multimeter

It is important to understand what a fuse is. A fuse is essentially a piece of wire designed not to extend a circuit but to prevent damage to electrical devices and reduce fire risks (especially in homes) caused by sudden voltage surges. When excessive voltage passes through, the fuse will 'blow' (literally), creating an open circuit, thus preventing current from continuing to flow. There are various types of fuses with different shapes. The two main types commonly seen are:

• The cylindrical fuse, which has been widely used for many years in household appliances and small electronic devices. It consists of a cylindrical shape with a wire inside and metal contacts at both ends of the tube.
• The flat fuse, commonly found in automotive electrical systems in the past 20-30 years, replacing the smaller glass tube fuses. It has a similar structure to a plug, with metal prongs extending from a plastic casing that houses the wire. Flat fuses are convenient for installation and help save space when multiple fuses need to be connected.

Learn How a Multimeter Works. A multimeter measures AC and DC voltage, resistance, and current. To check a fuse, you can use the multimeter to measure continuity (when the circuit is closed) or the resistance (ohm value).

• A multimeter comes with both positive and negative probes. When testing the resistance within a circuit, a small current from the multimeter's battery is passed through to measure the voltage across the circuit or device.

Understand Why It's Necessary to Test a Fuse. Checking a fuse is the simplest way to diagnose problems in a car or household electrical system, making it an essential skill to master.

• Testing a fuse is simpler than checking other electrical devices. Other components in a car or home include complex wiring systems. Furthermore, most car parts can only be checked at a garage, which can be costly. The process of checking a fuse with a multimeter is straightforward, and the tools are affordable and easy to use.
• Many fuses have a transparent design that allows users to quickly assess their condition by sight. They are made of clear materials, so you can see if the wire inside is intact. If the transparent material is clouded or blackened, it generally indicates that the fuse has blown. However, some fuses may turn black after becoming slightly overheated, which could happen if the fuse has not been checked for a few weeks or months. If an electrical device stops working, it's wise to check the fuse. If the fuse appears fine, a more serious issue may be at play, and it might be time to call an expert.

Turn Off the Power and Remove the Fuse. Make sure all electrical devices or vehicles are powered off before removing the fuse. Simply pull the fuse out of its socket to begin the test.

Turn On the Multimeter and Set It to Continuity Mode. Rotate the dial on the multimeter to the continuity setting, which is marked with a symbol resembling five vertical curved lines. Before testing the fuse, touch the positive and negative probes together to check if the multimeter emits a 'beep' sound. If it does, the multimeter is ready to measure.

• For resistance testing, you can set the multimeter to the omega (Ω) mode.

Place the Probes on Both Ends of the Fuse and Check the Display. A fuse is as simple as a piece of wire, so there is no need to worry about the polarity of the probes.

Test the Fuse. Pay attention to whether the multimeter continuously emits a 'beep beep' sound when you connect the probes to both ends of the fuse. If no sound is heard from the multimeter, it means the fuse has blown and needs replacement.

• For a digital multimeter in resistance mode, touch the probes together to read the initial measurement. Then, connect the probes to the fuse ends and see if the reading changes. If it remains the same, the fuse is working properly. If the screen shows no reading or displays 'OL,' the fuse has blown.
• Additionally, if the multimeter shows an 'Open' or 'Not complete' status, it indicates the fuse is damaged.

• Nowadays, home electrical systems are no longer solely protected by fuses. Modern protective devices and circuit breakers are more commonly used and are considered safer. You might want to consider upgrading old setups to meet modern standards.
• Most vehicles use color-coded flat fuses. By inspecting the transparent fuse head in the fuse box, you can easily assess whether the metal strip inside the fuse is intact (good condition) or damaged (due to burning/explosion).

• Never replace a blown or damaged fuse with one that has a higher rating. The rating ensures that the current flowing through the wires is safe. Always replace a blown fuse with one of the same rating (or lower).
• Never test a fuse while the device is powered on.