A blown fuse is a common cause of equipment that suddenly stops working. Identifying a blown fuse correctly saves unnecessary replacement of components that were not at fault. This guide covers how to find fuses in common audio and electronics equipment and how to test them safely.
What a fuse does
A fuse protects a circuit by providing a deliberate weak point. If current exceeds the fuse's rating, the fuse element melts and opens the circuit, stopping current flow before it can damage more expensive components. A fuse that has blown has done its job — but replacing it with an identical fuse without understanding why it blew may result in the replacement fuse blowing too, or in the underlying fault causing further damage.
Finding the fuse
Equipment fuses are typically found in one of three places:
- Mains inlet: a small compartment at the rear of the equipment, usually near or integrated with the IEC mains socket. Many equipment use a fuseholder that requires a flat-head screwdriver or a coin to open. Some have a slide-out tray that also contains a spare fuse.
- Fuse holder on the chassis: a cylindrical holder with a screw cap, typically mounted on the rear panel.
- On a PCB: internal fuses on circuit boards are not accessible without opening the equipment. Diagnosis and replacement of these requires technical skills. If you are not confident working inside electronics, take the equipment to a service technician.
Before opening any fuse compartment, disconnect the equipment from the mains supply.
Visual inspection
Glass-bodied fuses can often be checked visually. Hold the fuse up to a light source. A blown fuse may show:
- A visible break in the wire element inside the glass
- Blackening or discolouration of the glass, especially towards one end
- Evaporation or melting of material inside the glass body
Ceramic-bodied fuses cannot be checked visually. Test them with a multimeter.
Testing with a multimeter
Set the multimeter to continuity mode (usually indicated by a diode symbol or a sound wave icon — it beeps when continuity is present). Touch one probe to each end of the fuse. An intact fuse completes the circuit and the meter beeps or shows near-zero resistance. A blown fuse shows no continuity (open circuit).
This test can be performed with the fuse removed from the equipment. Always remove the fuse before testing it — testing in-circuit can give misleading results because other paths in the circuit may complete the test.
Replacing a fuse
Replace a blown fuse only with one of the same rating and type. The rating is printed on the fuse itself — for example, “T500mA 250V” (Time-delay, 500 milliamps, 250 volt rated) or “F2A 250V” (Fast-blow, 2 ampere, 250 volt rated). The fuse rating is also noted in the equipment's manual. Never fit a fuse of a higher current rating to “stop it blowing” — this removes protection from the circuit.
When a replacement fuse blows immediately
If a correctly-rated replacement fuse blows immediately or soon after installation, there is a fault in the equipment causing excess current draw. This requires diagnosis by a qualified service technician. Do not continue replacing fuses — doing so risks damage to components or a fire.
Frequently asked questions
My equipment has a blown fuse but I am not sure whether it is a fault or a power surge. What should I do?
Replace the fuse once with an identical fuse. If it blows again, treat this as evidence of a fault and have the equipment inspected. If it does not blow and the equipment functions correctly, the original blow may have been due to a transient event. Consider whether the equipment is adequately protected from mains transients — a surge-protected power strip provides a degree of protection.