Which plug fuse has a screw in base similar to a light bulb?
A plug fuse with a screw-in base that resembles a light bulb is not a hypothetical or a regional quirk. It was a real product, made by Dorman & Smith Ltd. of Manchester, and it was one of the earliest attempts to put a fuse inside a domestic plug for ring-main protection. The fuse was not merely housed in the plug. The fuse was the live pin. One end of the fuse carried a brass screw thread that mated with a threaded socket inside the plug body, and the exposed end of the fuse formed the current-carrying pin that entered the wall socket. You replaced a blown fuse by unscrewing it with your thumb and fingers, no tools at all, and screwing in a new one the way you would replace a lamp.
The dimensions tell you why the comparison to a lamp base is structurally literal rather than poetic. The D&S fuse measured roughly 31 mm in length and 9 mm in diameter, with a copper body, a coloured band in the centre, and that threaded brass nose. The plug and socket system was rated at 13 A, competing directly with the Wylex flat-pin design and ultimately losing to the completely new BS 1363 square-pin standard that arrived in 1947. D&S sockets were never popular in private houses, but they were deployed in prefabricated houses, council housing, and BBC installations, and some local authorities kept specifying them for new work until the late 1950s.
I have to be honest about why this design did not survive. It was more expensive to make than the eventual BS 1363 solution, because the fuse had to be "heavily built with a brass screw thread on one end," while the BS 1363 fuse was a cheap cartridge that sat in spring clips inside the plug. But cost alone would not have killed it. The D&S plug had a mechanical failure mode that is difficult to read about without wincing. The live pin was the fuse, and the fuse was held in place by a screw thread, and that thread could work loose in service. A fuse that backed out of the plug body would remain lodged in the socket when the plug was withdrawn, leaving a live threaded conductor protruding from the wall. The IET discussion on pre-BS 1363 plugs describes the same failure in the same words: the fuse "tended to come unscrewed on its own in use," and a loose fuse "could end up protruding from the socket, electrically live and posing a shock hazard". One contributor to that discussion recalled the live pin having "a distressing habit to snap off in the live hole of the socket". That is not a nuisance failure. That is a retained live part in a domestic socket outlet.
The contrast with the Wylex alternative is instructive, because Wylex was also an early ring-main competitor, and Wylex put its fuse inside the plug body rather than making the fuse part of the pin. The Wylex plug kept the fuse captive, and the BS 1363 standard that displaced both designs kept the fuse captive as well. The cartridge fuse in a BS 1363 plug is a 25.4 mm by 6.3 mm ceramic tube, rated up to 13 A at 250 V AC, with a rated breaking capacity of 6,000 A at 240 V. It is a completely different physical concept from the D&S screw-in fuse, even though both were 13 A plug fuses serving the same protective function. The BS 1362 fuse is inserted into spring contacts and removed with a screwdriver or a fingernail, not rotated into a threaded socket.
I need to separate this device from two nearby concepts that often get conflated with it. The first is the Edison-base screw-in fuse used in North American fuse panels, the Type T plug fuse, which does screw into a socket "the way a lightbulb screws into a light fixture socket". That fuse shares the screw base geometry, but it is not a plug fuse. It sits in a distribution panel, not in a cord-connected plug. The second is the BS 646 cartridge fuse, a 19 mm by 5.3 mm miniature tube used in clock connectors, shaver adaptors, and dimmers, rated up to 5 A. The BS 646 fuse is smaller than a BS 1362 plug fuse and has no screw base at all. The Dorman & Smith fuse is the only device in this family that combined three properties in one component: a screw-in base, a plug-pin function, and a domestic plug-fuse rating.
If someone today encounters a plug fuse with a threaded base and assumes it is a variant of the modern plug fuse, the assumption is understandable but wrong in a way that matters for safety. The D&S fuse was withdrawn not because the fuse element was unreliable, but because the mechanical interface between the fuse and the plug could degrade in a way that left a live part accessible. The BS 1362 cartridge fuse eliminated that interface. It is held in place by spring pressure and cannot unscrew itself into the socket. The screw-in plug fuse is a genuine artifact of the 1940s British ring-main transition, and it is a useful reminder that the shape of a fuse is never separate from the failure modes that shape permits.