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How to tell if a fuse is a fast- or slow-blow?

Updated April 17, 2017

A fuse is a sacrificial device that is meant to suffer the damage caused by a surge of electrical current running through a device. When the fuse becomes blown, you can just replace it instead of needing to buy a new device entirely. Fuses differ in the way they are blown by a current resulting in one type of fuse that blows immediately upon taking in a peak of electricity while the other is able to sustain a spike and will gradually blow out after absorbing the shock.

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  1. Look around the fuse for a large label or sticker. This label should be a manufacturer’s label and will indicate the speed at which the fuse will blow.

  2. Read the manufacturer’s label, if it says fast or slow, the fuse type is as it is labelled. If it says F it means the fuse is a fast-blow, and if it says T or S it means it is a slow-blow. Other letters will indicate unique fuses for your specific device.

  3. Feel for raised lettering on the case of the fuse itself if you cannot find a sticker or label. This lettering will be imprinted on the fuse housing and identify the fuse in the same way the label would.

  4. Check the fuse itself to make a clear identification. You may need to use a magnifying glass to see the fuse details if it is small or in an inaccessible location.

  5. Look through the tube of the fuse glass and check the wire filament within. If there is a thin wire, you have a fast-blow fuse. If you see a thick wire that has a very small spring at one end, you will know it is a slow-blow fuse.

  6. Tip

    Read the instruction manual of any new product you receive to find out its specifications in advance.


    Do not plug in a device into an inhospitable outlet as occasionally the fuse will be passed by the current and will destroy your device.

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Things You'll Need

  • Magnifying glass

About the Author

Based in Santa Rosa, Calif., Cindy Paterson has been writing articles on travel and lifestyle since 1991. Her work has appeared on ForbesTraveler.com and MSNBC.com. Paterson holds a Bachelor of Arts degree in creative writing from Columbia University in New York.

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