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

Written by cindy paterson
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How to tell if a fuse is a fast- or slow-blow?
(Jupiterimages/liquidlibrary/Getty Images)

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.

Skill level:

Things you need

  • Magnifying glass

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  1. 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. 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. 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. 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. 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.

Tips and warnings

  • 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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