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. 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. Occasionally, as in the image above, a by-pass wire across the spring is used to adjust the characteristics of the fuse. There are many variations on this design and it gives manufacturers a lot of parameters for adjusting the fuse characteristics. 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. 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 main wire, that heats more slowly than the wire. 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 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 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. I have not seen an explanation of the fuse with the wavy wire. Pictures of this type and the next are in McMahon's answer. 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. Three main techniques are used to accomplish this. 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. For a short circuit it must blow fast in the normal way. I think it is relevant to the discussion here.Ī surge protection fuse must accommodate three overload regions. 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 was an electrical engineer in the 1950s, part of my work was concerned with testing and selecting fuses.
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