We have a glass fuse for a 12v lighting circuit that has "Siran No. F 13 4A" written on paper inside it. The circuit takes 5.5A at the moment with all the lights switched on without the fuse blowing. Can someone tell me what this is rated at? It seems to be a fast blow (F) so can it really be a 4A?

I figure my options are

13A (so what does the 4 mean?)


4A (why is it still working at 5.5A?)

Thanks for reading, Steve

Edit: Images added as requested. Fuse shown with left side in holder Fuse with writing fuse wire

  • \$\begingroup\$ Can you make a picture? Normally the F13 stays for dimensioning and type and 4A the current. It still can be a slow blow fuse . \$\endgroup\$ Dec 31, 2015 at 9:59

1 Answer 1


It might be a 4A fuse allowing 5.5A at the moment. To understand why/how this can happen, you need to know the working of a fuse. The fuse will have some finite resistance R and a current I through it will start adding heat to the fuse as per the equation - IxIxRxt where t is the time. This heat energy will raise the temperature of the fuse and at one point, it will melt. However this isn't the only thing going on in there. The fuse is also losing some of the heat to its surroundings and hence the surroundings also play a role in deciding the current at which the fuse will blow and the time it will take to blow. Here is an excerpt from a fuse sizing guide:

Fuse selection guide

Fast fuse

In your case, one of the following might be the case:

1) Lack of quality control - Maybe the fuse company didn't control the tolerance very well.

2) Fuse is losing too much heat to its surroundings.

3) Fuse is not a fast one and it might blow given enough time.

4) It's not a 4A fuse.

  • \$\begingroup\$ Thanks for that. I'm guessing that the next step would be to figure out the gauge of the wiring and get a suitable fuse? This is the only time I've ever switched on all 5 lights - regular use is 2 or 3 at the most. \$\endgroup\$
    – Steve
    Dec 31, 2015 at 12:15

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