||Upon failure of a high voltage linear halogen lamp an arc will often form between the broken pieces of filament. Owing to the low electrical resistance, very high currents of several hundred amperes can flow. This leads to severe heating of the extremely thin molybdenum foils within the pinch-seals at either of end the lamp, which can result in cracking of the surrounding quartz and a lamp explosion.
Manufacturers have investigated several methods of building fast-acting fuses into these lamps, but the small dimensions do not leave much space for a traditional fuse. One of the earliest successful designs is featured here, invented by Staaf Siaens and Eduard Janssen when Philips transferred its linear halogen production to Belgium in the early 1980s.
In this design a small cavity is formed in the pinch seals, surrounding the area where the filament tails are welded to the moly foils. If arcing occurs the exposed part of the moly foil heats up very rapidly, since it is no longer embedded in the heat-sinking quartz body. The foil is instantly vaporised thus breaking the circuit, and any subsequent arcing in the chambers is rapidly quenched due the high gas pressures that are rapidly built up within this small volume. The design was implemented in Philips' lamps of 500W and below until the early 1990s when a simpler method was adopted.
Incidentally this lamp is filled with a mixture of both bromine and iodine. It is suspected that the exposed filament tails, somewhat longer and therefore colder than in a normal lamp, would be prone to excessive corrosion leading to short life in case the usual bromine-only gas filling was employed. The presence of iodine restricts operation to horizontal ± 4°.