Glass fuses are one of the easiest types of fuses to check because you can often see whether they’ve blown just by looking at them. A clear glass tube lets you inspect the thin metal filament inside, and if that filament is broken, discolored, or missing entirely, the fuse has blown. But visual inspection doesn’t catch every failure, so a quick test with a multimeter gives you a definitive answer in seconds.
Check the Filament Through the Glass
Start by removing the fuse from its holder. Always disconnect the device or turn off power to the circuit before pulling a fuse out. Federal electrical safety standards prohibit removing or replacing fuses by hand in an energized circuit, and even in low-voltage electronics, it’s a good habit.
Once the fuse is out, hold it up to a light source and look through the glass tube. You’re looking at the thin wire or metal strip running between the two metal end caps. A good fuse has an intact, continuous filament with no gaps. Here’s what a blown fuse typically looks like:
- Broken filament: The wire inside has a visible gap or has separated completely. This is the most obvious sign.
- Blackened or cloudy glass: A dark metallic residue coats the inside of the tube. This happens when the filament vaporizes from a large current surge, depositing metal on the glass.
- Melted or deformed filament: The wire looks warped, blistered, or partially melted rather than cleanly snapped. This usually means the fuse experienced a sustained overload rather than a sudden spike.
If the glass is clear and the filament looks intact, the fuse might still be good. But “looks intact” isn’t always reliable. A hairline break in the filament can be nearly invisible, and some fuses fail at the connection point where the wire meets the end cap, which is hard to see. When visual inspection leaves you unsure, a multimeter removes all doubt.
Test With a Multimeter for a Definitive Answer
A digital multimeter is the most reliable way to confirm whether any fuse is blown, and the test takes about 10 seconds. You’ll use either the continuity setting or the resistance (ohms) setting. Continuity mode is slightly easier because it gives you an audible beep for a good fuse instead of requiring you to read a number.
Using Continuity Mode
Turn on your multimeter and rotate the dial to the continuity symbol (it looks like a small sound wave or a dot with curved lines). Before testing the fuse, touch the two probe tips together. You should hear a beep, which confirms the meter is working. Then place one probe on each metal end cap of the fuse, pressing firmly for solid contact. If the meter beeps, the fuse is good. If there’s silence, the fuse is blown.
Using Resistance Mode
If your multimeter doesn’t have a continuity setting, switch it to the resistance or ohms (Ω) setting. Touch the probes together first to verify you get a reading at or very near zero. Then place the probes on each end of the fuse. A good fuse has extremely low resistance, typically a fraction of an ohm. A blown fuse reads “OL” (over limit), “infinity,” or displays a “1” on the left side of the screen, all of which mean the circuit is completely open. There’s no current path left.
The exact resistance of a working fuse depends on its current rating. Higher-amperage fuses use thicker filaments with lower resistance, sometimes as little as a hundredth of an ohm. Lower-amperage fuses read slightly higher but still well under one ohm. The key distinction is simple: near zero means good, infinite means blown.
Reading the Markings for Replacement
Once you’ve confirmed a fuse is blown, you need to replace it with the correct type. Glass fuses have markings printed or stamped on one or both metal end caps, and sometimes on the glass body itself. The two critical numbers are the amperage rating and the voltage rating.
The amperage rating (for example, 3A or 500mA) tells you the maximum current the fuse can carry before it blows. The voltage rating (such as 250V) indicates the maximum voltage the fuse can safely interrupt. You’ll also see a letter code indicating the fuse’s speed, which describes how quickly it responds to an overcurrent. Common codes include F for fast-blow and T for time-delay (slow-blow). A fast-blow fuse opens almost instantly during a surge, while a slow-blow fuse tolerates brief spikes, which is useful in circuits where motors or compressors draw a momentary burst of current at startup.
Physical size matters too. The most common glass fuse sizes are 5x20mm (standard in most electronics and European equipment) and 6.3x32mm (common in American appliances and automotive applications). Always match all of these specs: amperage, voltage, speed rating, and physical dimensions. Installing a fuse with a higher amperage rating defeats its protective purpose and can allow dangerous levels of current to flow through the circuit.
Why a Fuse Blows in the First Place
A blown fuse is a symptom, not the problem itself. The fuse sacrificed itself to protect the rest of the circuit from excessive current. Simply replacing it without understanding why it blew can lead to the new fuse blowing immediately.
The most common causes are a short circuit somewhere in the wiring or a connected device, an overloaded circuit drawing more current than it’s rated for, or a failing component like a motor or power supply that’s pulling too much power as it degrades. If a replacement fuse blows right away or within a short time, the underlying electrical issue needs to be found and fixed before you keep replacing fuses.
Occasionally a fuse blows from a one-time event, like a power surge during a storm or plugging in a device with an internal short. In those cases a single replacement solves it. But repeated blown fuses are always a signal that something else in the circuit needs attention.