How To Tell if a Glass Fuse Is Blown? (With and Without Multimeter)

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Electronic and electrical devices and circuits use fuses as their safety barrier from overcurrent and heavy loads conditions; the fuse acts as an alarm that warns that there is something wrong in the circuit by blowing up. The fuses have many types, they can be classified according to current type to AC or DC fuses, and they can be classified according to their reaction speed to fast-blow and slow-blow.

How To Tell if a Glass Fuse Is Blown? (Without Multimeter)

1. Check The Fuse Casing
To tell if a glass fuse is blown, you have to look if there is a metallic or dark smear inside the glass and if there is a gap in the wire; these are the indications of a blown glass fuse.

2. Disconnect The Power Source
You need to disconnect the circuit from the power source to protect yourself from any electrical shocks or injuries caused by electricity.

3. Disconnect The Fuse
Then, use a screwdriver to disconnect the glass fuse from its holder cap and look for any gaps in the wire of the fuse, a metallic or a dark smear in the fuse body. If you found any of the mentioned indications, your fuse has blown, and you need to replace it as soon as possible.

How To Tell if a Glass Fuse Is Blown? (With Multimeter)

If you didn’t see the indications of a blown glass fuse, like any gaps in the wire of the fuse, a metallic or a dark smear in the fuse body, you would need to use a multimeter to know if the fuse is blown, you can follow these steps:

  1. Adjust Your Multimeter 
    Grab a multimeter; any multimeter will work, whether analog or digital and then adjust it to the resistance or the ohms mode. 
  2. Test The Fuse
    Use the multimeter’s probes to test the fuse by connecting one of the multimeter’s probes to the fuse’s first terminal and connecting the other end to the other multimeter’s probe. Suppose the reading of the multimeter is between 0-5 ohms. In that case, you have a good fuse, if the reading of the multimeter reading shows a higher resistance, you have a degraded or bad fuse, but if the multimeter reading shows over the limit “OL,” your fuse has blown.
  3. Replace The Fuse
    If your fuse has blown, you need to replace it as soon as possible because the fuse is the safety barrier in any circuit. It alarms you when there is an overcurrent or heavy load on the circuit. Before buying another one, you need to check the blown fuse ratings, ampere ratings, and voltage ratings. The new fuse must have the same ratings as the blown one; they need to have the same size and type; slow-blow or fast-blow fuse.
  4. Check The Operating Manual
    If you can find the fuse ratings for the old fuse on its body or casing, you can check the panel label around it. Sometimes the manufacturer imprints the ratings of components on the panel around them, so you should check it also. If you didn’t find the ratings anywhere near the fuse, you would have to check the operating manual to know the ratings of the old fuse.

Important Warnings For Glass Fuse Replacement

  • Always disconnect the electrical circuit the glass fuse is connected to before checking whether it’s blown or not because it’s very dangerous to work or test equipment; when there’s power in the circuit, it can result in serious injuries and electrical shocks.
  • When replacing a glass fuse, you need to replace it with a fuse with the same amperes rating and voltage ratings. The new fuse must have the same ratings as the blown one; they need to have the same size and type; slow-blow or fast-blow fuse.
  • Store enough fuses for emergency cases, store each type in a different container and label the container with the fuse type, whether it is fast-blow or slow-blow, in addition to the fuse amperes rating and voltage rating. The size of the fuse is also a piece of important information to label when storing fuses.
  • Check the containers of fuses frequently and see if any of them needs to be refilled or restocked; this way, you can save a lot of time and reduce the damage if any fuse has blown. You can also keep a fuse near your equipment to use it quickly and save more time.

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Can You Test A Glass Fuse Without Removing It?

Yes, you can test a glass fuse without removing it from its holder cap, and you can do this throw continuity or voltage checks. You can either use the voltage or the continuity checks; however, the voltage checks allow testing the fuses without de-energizing the circuit.

1). Voltage Checks

    1. Adjust Your Multimeter
      The electrical circuit can be either AC or DC, so you must adjust your multimeter to the correct settings depending on your electrical circuit current type. Some multimeters have more advanced features that can detect the current type without adjusting it manually.
    2. Test Your Fuse
      Multimeters have two probes; you can use any one of them and connect it to the fuse line side and connect the other probe to the fuse neutral/common or ground point.
    3. Verify The Correct Voltage
      Any fuse would have an expected line voltage, so you should know and use the meter to verify it, for example, 120 volts. 
    4. Test Your Fuse Again
      Connect the multimeter probes; you can use any one of them and connect it to the fuse line side and connect the other probe to the fuse load side.
    5. Check The Readings
      After connecting the probes to the load and line sides, check the multimeter reading; if the reading is 0 volts, the fuse is good and has not blown. If the multimeter shows the same reading for the line side, for example, if the line reading is 120 volts and the multimeter shows 120 volts, your fuse is blown, and you need to replace it.

2). Continuity Checks

  1. De-energize The Circuit
    You need to disconnect the circuit from the power source to protect yourself from any electrical shocks or injuries caused by electricity. In addition, if there are any capacitors in the circuit, you need to discharge them first before testing the fuse.
  2. Adjust Your Multimeter
    You can’t use a multimeter to check for continuity or resistance. At the same time, the circuit is connected to a voltage source; adjust your multimeter to voltage settings. Then you can use any one of the multimeter’s probes, connect it to the fuse line side, and connect the other probe to the fuse neutral/common or ground point.
  3. Verify That There is No Voltage
    Check the multimeter readings; the reading should be zero volts; if the multimeter shows voltage, you need to de-energize your circuit properly and verify again.
  4. Adjust Your Multimeter Again
    Adjust your multimeter to continuity settings, and connect the multimeter probes; you can use any one of them and connect it to the fuse line side and connect the other probe to the fuse load side.
  5. Check The Readings
    If the multimeter shows over the limit or “OL” reading, your fuse has blown, and you need to replace it. 

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Conclusion

To sum up, you can tell if a glass fuse is blown by looking for a metallic or dark smear inside the glass and also if there is a gap in the wire; these are the indications of a blown glass fuse before you start looking for these indications you need to disconnect the circuit from the power source to protect yourself from any electrical shocks or injuries that can be caused by electricity. 

Then, use a screwdriver to disconnect the glass fuse from its holder cap and look for any gaps in the wire of the fuse, a metallic or a dark smear in the fuse body. If you found any of the mentioned indications, your fuse has blown, and you need to replace it as soon as possible. If you can see the indications, you need to use a multimeter.

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