This is a standard sized glass fuse like what cars used before the 80s. I can faintly make out 20A, but I’m not sure what to call it in order to find a replacement. The fuse in the picture has not blown, but it is the only one I have. I don’t think it matters, but this happens to protect a transformer for a pool salt generator.

I’m also interested in what purpose the spring serves. I’m sure I can find another 20A fuse, but I’m not sure I can find one exactly like this, so I’d like to know why it is special.

glass fuse


2 Answers 2


20 amp glass cartridge type slow blow. It has a thermal mass that generates a delay before the solder melts at the left hand end. The spring pulls the mass away from the wire, making sure the two conductors separate and stay separated.

  • \$\begingroup\$ Thanks, so if I happened to use a non-slow-blow, the risk would be greater chance of a nuisance trip? Doesn’t sound like there would be a chance of harming the protected device... \$\endgroup\$
    – JPhi1618
    Commented Dec 8, 2019 at 15:18
  • 10
    \$\begingroup\$ Probably not, but depending on where it's used a 'fast' blow may not be the best choice. Slow blows are often used in devices that have short current spikes, an example would be inrush current on power supplies, or filament lamp circuits. \$\endgroup\$
    – BobT
    Commented Dec 8, 2019 at 16:52
  • 1
    \$\begingroup\$ Thanks for that description, Bob. This is for a family member that happened to have a standard fuse and they were worried about using it. I'm going to get the proper one installed as soon as I can, but wanted to make sure the device wasn't in danger in the mean time. \$\endgroup\$
    – JPhi1618
    Commented Dec 9, 2019 at 4:53

BobT has it right - my answer serves only to add information.

This class of fuse is commonly known as a "3AG" or "3AB" fuse. The Buss number would be AGC-20 (fast blow) or ABC-20 (Slow Blow).

Although they are still available with a clear glass housing, most modern versions now use an opaque ceramic housing for safety reasons (ceramic doesn't shatter or explode under extreme fault conditions).

I did a quick search at Digikey: 20A Slow-Blow 3AG fuses - 8 versions available in stock ready for immediate shipment as of this date. However, the link that I provided should be active for as long as Digikey still uses that search engine.


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