I was searching today for a new 24V transformer, and I found that some of them have a "fuse" symbol on their label, and I was wondering what this fuse symbol means?

Does the fuse symbol on a transformer (toroidal or not) mean that the fuse is integrated in the transformer's winding?

Or does the "fuse" symbol try to suggest what kind of fuse has to be used with that transformer?

Here you can find an image with the transformer label:

Photo of toroidal transformer showing label, from RS Components

(Image source: RS Components - 2 Output Toroidal Transformer, 120VA, 2 x 25V ac)

  • 2
    \$\begingroup\$ There is no room to integrate the fuse. I have worked with such toroids and found that to install even a thermo-disc cutoff I had to epoxy it to the inside of the core, where there is the most amount of heat. \$\endgroup\$
    – user105652
    Mar 24, 2019 at 16:07

1 Answer 1


In this case the fuse symbol indicates the fuse rating you should use. 2.40AT means you should use a 2.40A slow acting fuse (type T, T stands for Trage). Trage can be translated from German to sluggish, slow-moving. You can verify that the 2.40A matches the rated current for each secondary of the device you are considering. device specs The datasheet mentions no fuse, so it should not be integrated.

  • \$\begingroup\$ So what is the 'T' in 2.40AT? \$\endgroup\$
    – Transistor
    Mar 24, 2019 at 18:07
  • \$\begingroup\$ @Transistor it's the speed of the fuse, I will change my answer. \$\endgroup\$
    – xuva
    Mar 24, 2019 at 18:24
  • 1
    \$\begingroup\$ I found the same info in the RS catalog. See 2.25A Glass Cartridge Fuse, 5 x 20mm, Speed T. The datasheet is a bit vague although it does give the \$ I^2t \$ ratings. \$\endgroup\$
    – Transistor
    Mar 24, 2019 at 18:28
  • \$\begingroup\$ 'T' stands for Trage which stands for ...? (I'm making you work for your +1!) \$\endgroup\$
    – Transistor
    Mar 24, 2019 at 18:30
  • \$\begingroup\$ Yes, you are, haha. Learned more than I expected from it though. \$\endgroup\$
    – xuva
    Mar 24, 2019 at 18:33

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