I purchased a few fuses to get familiar with their workings, and I was surprised to see that the 100mA fast-blowing fuses I bought happily conducted up to 215mA (6V power supply, 10\$\Omega\$ resistor), where the filament just started glowing. I could reproduce this with a second fuse.
Am I gravely misunderstanding something here, or is this an issue with the fuses? They are Bel Fuse Inc. 5SF 100-R parts.
Answer
Even fast fuses don't respond immediately after the rated current is reached. Most fuses require a significant over current to fire almost instantly. This is an obvious requirement since its all about heat, and minor heating (a raise ambient temperature) shouldn't affect the fuse too much (or at least not cause it to "blow").
Judging from the data-sheet, your 100mA fuse should blow after about 80 seconds @215mA @25°C.
How long did you wait?
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