I was an electrical engineer in the 1950s, part of my work was concerned with testing and selecting fuses. I recently gave a talk to my local amateur radio club on the subject, and what follows is from the script I wrote for that talk. I think it is relevant to the discussion here.
A surge protection fuse must accommodate three overload regions. For a short circuit it must blow fast in the normal way. It must also blow for steady overload currents just like an F fuse, but it must tolerate continual brief over-currents -- say ten times its rating -- without blowing or deteriorating.
Three main techniques are used to accomplish this. The simplest is to increase the thermal mass of the element, using a thicker, and therefore longer wire (to get sufficient resistance to heat up), wound round an insulating core, with careful control of the spacing for consistent operation. Pictures of this type and the next are in @Russell McMahon's answer. I have not seen an explanation of the fuse with the wavy wire.
The second technique employs a three part fusible element.The first part is a wire with a high melting point so that it will absorb surges, while still blowing fast on extreme overload. This is similar to an F fuse working at well below its rating, so it will not protect against overloads close to the rated current. The second part gets round this, providing the protection for currents that are closer to the rated value but not high enough to blow the thin wire itself, and consists of a lump of lower melting point material in series with
the main wire, that heats more slowly than the wire. The third part of the element is a stout spring of relatively high resistance material, helping to heat up the lump, and pulling it rapidly apart when it melts. The combination of lump and spring, with its relatively high thermal mass, also allows the surge to pass, but provides the protection for longer term but lesser overloads. There are many variations on this design and it gives manufacturers a lot of parameters for adjusting the fuse characteristics. Occasionally, as in the image above, a by-pass wire across the spring is used to adjust the characteristics of the fuse.
The third method employs the 'M' effect. In the 1930s Prof. A.W.Metcalf (hence the 'M') researched a phenomenon where the tin alloy used to solder the ends of the fuse seemed to affect the time to blow, reducing it in a strange way. He found that a spot (the 'M' spot) of solder on a silver wire element did not affect the short circuit performance, but it did reduce the time to blow on a sustained lower current. In this case, at the lower temperature of the wire, the solder diffused into and alloyed with the silver to create a region of high resistance in the spot, which would glow red hot, with the wire rupturing next to it. This, with suitably chosen alloys, nicely gives the characteristic needed for a surge resistant fuse. A problem with this type of fuse is that occasional currents just above the rated value may cause some unwanted diffusion to occur, altering the fuse characteristics without visible change.
Here is a picture of three M spot fuses, and yes there is a tiny spot on the top one.
You need to bear in mind that there are many different modes of damage possible,
- Large current passed though tissue may burn the tissue
- Modest current passed through a vital muscle (like that of the heart) may cause that muscle to operate in a manner that it's not used to and strain it so that it is damaged from the strain.
- Very tiny current may interfere with the normal nerve signals that drive the heart's operation, throwing off the heart rhythm leading to ineffective pumping for long enough to be fatal.
So it's not a matter of stating a particular voltage, current, charge, energy or power. It's a matter of stating a range of very particular and quite different circumstances that pertain to different body systems.
Best Answer
That is a nice observation - I am only speculating here about the how, and I hope a fuse manufacturer will jump in with a better answer.
The fast fuses I looked at contain a relatively thin wire, slack, while the slow blow fuses contain a thicker wire, held under tension by a spring.
As such, they operate in different ways : you need to melt the wire in the fast fuse, to break the circuit. But the slow blow fuse only needs to soften the wire until it yields under the spring tension, at a somewhat lower temperature (for the same wire material). That leads to the use of a thicker wire, as can be confirmed by observation.
Thus the fast fuse, with its thinner wire, has better cooling - greater radiating surface per unit mass of material - than the slow fuse, as well as requiring a higher ultimate temperature before failing.
So a modest overload will leave it at a high enough temperature to radiate away enough heat energy to limit its temperature, or at least slow its temperature rise, and delay its melting.