Found this fuse at work. Normally these fuses have a single wire inside but this one has a little black "resistor"-looking thing on a spring. What is up?

First picture is of a "normal" fuse. The second one is the fuse i found at work. Hard to take pictures sorry.

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It's a "slow-blow" fuse designed to take a short high current impulse without opening, but to open for a long term overload or a brief very high current (such as a short circuit). Quite common.

As always, replace with the same type and rating for continued protection (and no nuisance blowing).


The spring puts the element under tension which allows them more degrees of freedom to adjust the characteristics. I believe the black thing is a resistive element. See more info in US2386094.

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  • \$\begingroup\$ You forgot to explain the purpose of the spring! \$\endgroup\$ – Transistor Jul 29 '18 at 22:38
  • \$\begingroup\$ Okay it's slow-blow. But how does it work? What is the black thing and the spring doing? Wouldnt the thin wire between the terminal and the black thing just burn out like the normal looking one. \$\endgroup\$ – Swagministeren Jul 29 '18 at 23:03
  • 2
    \$\begingroup\$ @Swagministeren : the 'black thing' has higher heat capacity than a length of thin wire, it is both the resistive element (heater) and the time delay element (temperature ramps up slowly because of its heat capacity). \$\endgroup\$ – Whit3rd Jul 30 '18 at 0:56

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