Test a Fuse With a Multimeter

Cars and older homes that don’t use modern circuit breakers use fuses to prevent damage from electrical surges. Sometimes these fuses require testing to check that they are still in good working order. Testing fuses can be done using a multimeter, and doing so is both fast and easy to learn.

Steps

Learning about Fuses and Multimeters

  1. Understand fuses. Fuses are really just wires that are designed not to last, but their purpose is to prevent damage to more valuable electrical equipment or prevent fires (especially in homes) caused by power surges. If too much power runs through the fuse, it will "burn out," quite literally, and break the circuit. There are number of varieties of fuse, but their differences are primarily in appearance. Here is a description of the two you are most likely to see:
    • The cartridge fuse is a cylindrical fuse that has been common in a wide variety of devices for many years, from homes to small electronic devices. They have metal contact or terminal points on either end and consist primarily of a tube that contains the wire.[1]
    • The blade fuse is a common type of automotive fuse that has come into use in the last 20-30 years. They vaguely resemble the plug of a power cord, with two metal prongs emanating from a plastic housing that contains the wire. Previously, most vehicles also contained small glass cartridge fuses. Blade fuses plug conveniently into banks, and relatively little space is required to house a large number of them together.[2]
  2. Learn how a multimeter works. Multimeters measure AC and DC voltage, electrical resistance, and the flow of current. For testing a fuse, you can either use it as an ohmmeter (that is, a meter that tests resistance) or an ammeter (a meter that tests the flow of current). [3]
    • A multimeter has a positive and negative lead. When testing resistance or current flow in a circuit, the meter will transmit a small quantity of electricity from its own battery and then measure the amount that passes through the circuit or object.
  3. Understand why you must test fuses. Testing fuses is the simplest way to examine what is going on in the electrical systems of your car or home, and for that reason it is a vital skill to possess.
    • It is easier to test fuses than it is to test other electrical equipment. Other car or home components involve complex wiring systems that run on for some length. In addition, most car parts can only be tested at repair shops, and to do so will usually cost quite a lot of money. Testing fuses with a multimeter is comparatively simple to do, and the equipment involved is cheap and easy to operate.
    • Many types of fuses allow for visual confirmation that the fuse is still functional. They are made clear so that you can see if the wire remains intact. If the translucent area is blackened, it is usually because the fuse has burned out. However, some fuses will create that blackened stain after only slightly overheating, and that may have even been the result of an unnoticed incident weeks or months earlier. If a device is not working, you should test the fuses. If the fuses are all still working, there is likely a more serious problem and it may be time to call on an expert.

Testing the Fuse

  1. Turn the device off first and Remove power.
  2. Remove the fuse from the vehicle or device. Make sure it is turned off before the fuse is removed.
  3. Turn the meter on and set it.
    • You can rotate the selector to Ω, or OHMS. This will measure resistance. Before you test the fuse, put the positive and negative leads together and look at the reading. The number it provides should be close to the one you see when you test the fuse.
    • Alternatively, you can test the flow of current by setting the meter to the symbol that looks like an arrow traveling along a line.
  4. Put one lead on each end of the fuse and look at the display.
    • Because the fuse is little more than a single wire--and no complex parts to worry about--it does not matter which side receives the positive or negative lead.
  5. Test the fuse.
    • If using the multimeter to measure ohms, the reading should match (or nearly match) the one provided when you touched the two leads. If the fuse is blown, it will not read anything at all, or the meter will display O.L. depending on the brand and type of meter you are using.
    • If using a digital multimeter set to measure continuity, the meter should beep continuously as you hold the leads to the ends of the fuse. That means the circuit is complete. If it does not do so, the fuse is blown.

Tips

  • Regarding car fuses, most cars use a colored 'blade' type fuse, and looking at the top of the fuse in situ in the fuse box will show that the metal strip that runs along the visible top of the fuse will either be intact (fuse good) or broken (fuse blown).
  • Household installations these days shouldn't really be protected by just fuses. Modern circuit breakers and protective devices are fuse-less and much safer. Consider having an old fuse installation upgraded to modern standards.

Warnings

  • Never test a fuse on a machine that is still on.
  • Never replace a blown or suspect fuse with one of a higher rating. They are rated for safety reasons and you should always replace a fuse with exactly (or sometimes lower) rating as the old one.

Sources and Citations

  1. ↑ http://electrical.about.com/od/panelsdistribution/a/cartridgefuses.htm
  2. ↑ http://www.edmunds.com/how-to/how-to-inspect-and-replace-automotive-fuses.html
  3. ↑ http://www.wisegeek.org/what-is-a-multimeter.htm
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