1972 GE 2500L (6.6 A 147.2 W)

The lamp shown here has become quite a rarity nowadays as municipalities across North America have retired their obsolete incandescent streetlight luminaires with LED counterparts (or are in the process of doing so). The present 2500L is a special incandescent lamp which was designed specifically for an operation on 6.6 A series circuits that once were popular in the USA in street-lighting installations. The low lamp voltage resulting from the elevated drive current results in a higher lumen efficacy thanks to a shorter and fatter filament, which limits heat losses compared to the situation in mains-voltage GLS lamps of comparable life rating. Moreover, the smaller low-voltage filament increases the source’s luminance, which is beneficial to the optical control of the emitted light. Finally, because those lamps are driven at constant current, they have a better lumen maintenance that standard GLS lamps due to an effective wattage rise over time as the filament evaporates and its ohmic resistance rises.

An interesting feature of series-circuit incandescent lamps made after the mid-1950s is their integrated voltage fuse which ensures continuity in the circuit after the lamp failure. This feature was most often implemented in the form of lead wires brought close together so as to facilitate the ignition of an arc in the event of filament rupture (the OCV of street-series circuits is several kV). This arc melts the lead wires which are quickly fused together, thereby quenching the arc and forming a robust short circuit. Such design greatly simplified the maintenance procedure since it eliminated the cut-out device that was usually inserted across the terminals of the lamp socket.

The GE 2500L shown here is the second smallest model of street-series lamps (after the 1000L) that were sold in North America. The lamp has an output of 2500 lumens, which was adequate for the illumination of secondary roads and of (small) town centers. This particular model is rated for a 3,000 h average service life, best suited for the group replacement of lamps during maintenance. Although the number of incandescent streetlight installations began to decline strongly during the 1980s, these lamps remained in production for several decades more. GE eventually discontinued them around 2010, which is quite remarkable given how inefficient the technology is compared to its HID counterparts.

- Not only do incandescent lamps operate more efficiently at low voltage and high current, interestingly their flux output is also more constant through life. Over time, the filament impedance increases as a result of evaporation, causing a slowly increasing wattage and output at source level (at constant current that is) which compensates for the optical absorption resulting from bulb blackening.

What is even more peculiar is the fact that those current-driven lamps are less prone to failure via filament hot spot formation than standard voltage-driven incandescent lamps. That's because when their thicker and stiffer filament breaks following hot spot melting, the resulting arc can weld the filament extremities back together. As a result it will take several of such transient phenomena before the filament fail so catastrophically that an arc will initiate between the lead wires and short the lamp permanently. On that subject, it's interesting to note that most of those lamps were operated in the vertical cap up position, and when a long-enough arc is struck between two filament sections after local melting, then a gliding arc effect (Jacob's ladder) occurs, i.e., convection and self induction force the arc to move upward and to transfer itself to the section where lead wires are close together, thus making the process of end-of-life shorting more reliable than at any other positions.

However, each re-welding of the filament lowers its impedance, causing a decrease in output flux. Lamps have been found with up to ten welds, resulting in a very low light output. Those lamps were referred to as “slumpers” and prompted manufacturers to push for group replacement, typically three times a year, in order to maintain a nominal output flux in installations. Nevertheless, despite the drawback of filament re-welding, this process also curbs early failures, which was a bonus from a safety point of view of a reduced light output is a much better outcome than no light at all in the first occurrence of filament rupture.