Traffic Signal and Street Light Information Sheets
Willis Lamm
  Street Light Information Sheet -
Understanding Shunts


Part Two

Note: This document is a continuation from Part One, in which autotransformers and magnetic cutouts were discussed.

  Film Cutouts

By the end of the 19th century, incandescent lamps were starting to make inroads and replace maintenance intensive carbon arc lamps for street light service. The development of the much more brilliant Mazda lamp rapidly accelerated this change.

Most of these lamps operated on series circuits so each lamp had to have some form of shunt. Two issues confronted engineers and inventors as incandescent lighting gained a foothold.

(For a complete discussion on series circuits, please visit Understanding Series Circuits.)

Autotransformers were effective shunts however they consumed some power in addition to the lamp itself. This power consumption not only increased operating costs but it reduced the total number of lamps that could be operated from a given constant current regulator.

Magnetic cutouts did not consume any measurable power when their associated lamps were operating, however they were mechanical in nature and there could be issues related to the time it took them to actuate.

If a magnetic cutout did not close quickly enough, the voltage in a series luminaire (usually less than 50 volts) could spike to well over 2,000 volts. If not shunted quickly, the components inside the lamp bulb, not designed to be insulated from such high voltage, could arc, creating a small lightning storm of sorts. If not stopped, continued high voltage arcing could generate enough heat to melt the lamp, damage the lamp socket and sometimes the luminaire itself.

Engineers understood that a cutout could be designed by separating two pieces of metal with destructible insulating material, such as very thin paper or film. When voltage reached a critical level, such as when a lamp burned out, an arc would pass through the insulating material and the pieces of metal together thus shunting the burned out lamp. The beauty in this concept lay in the fact that film cutouts could be designed to create a small internal arc and shunt failed lamps before voltage spiked and arced across the components inside the lamp.

Some of the earlier film cutouts were included as parts of the lamps themselves. The record is not clear as to whether these original designs were intended as replacements for autotransformers and magnetic cutouts, or as backups to prevent radio interference or damage caused by arcing inside a lamp.

Two examples are illustrated below. Elihu Thomson developed a film cutout lamp in 1886, patented in 1891. Thomas Edison designed a similar lamp that appeared in The Electrical World in 1888. Thomson's concept was also replicated in socket inserts that provided film cutout protection for ordinary series lamps.

Film cutout socket insert based on Thomson's design in a Holophane luminaire.

The greatest drawback with respect to lamps that contained their own film cutouts was that linemen could not remove burned out lamps at night while circuits were energized without the string going dark as soon as the lamp was removed, and without risking dangerous high voltage peaks inside the luminaire and potential arcing as they screwed in a fresh lamp.

As a result, these internal cutout designs required either an autotransformer to supply and isolate the lamp or some kind of mechanical cutout to bypass the entire luminaire until a fresh bulb was properly installed.

An old technology that was held over from arc lamps, and that was particularly useful where it was difficult to change lamps with a lamp changing pole, involved using a cutout switch that would be automatically operated when the lamp was lowered for changing. Once the lamp was lowered, it was manually bypassed by the cutout until it was again raised to its operating position.

If you look carefully above the radial wave luminaire you can see a lowering pulley and cable.

  Magazine and Disc Cutouts

In 1900 Walter Jones invented what was to become the industry standard for series street lights that is still used today. Jones' invention was the basis for the present day film-disc cutout or "rupture disc."

Jones actually invented a socket system. The luminaire was equipped with a receptacle for a removable lamp socket. The receptacle employed two spring loaded contacts that stayed closed against each other until a socket with two blades was inserted. The contacts and blades were designed to keep the socket from falling out of the luminaire.

Lamps screwed into the the removable socket. A film cutout was clamped between the blades of the socket that kept current from conducting between the two blades while the lamp was functioning, but would melt when the lamp failed and voltage spiked.

Thus the circuit remained intact if a lamp burned out as well as when the socket was removed by allowing current to flow directly across the touching receptacle contacts. With the socket installed a working lamp would light since the fresh film cutout would prevent current from bypassing the lamp. When a lamp failed the film cutout would melt and complete the circuit.

Jones patent was not awarded until 1906 due to challenges that his design infringed upon the Thomson design. Eventually Jones' patent description convinced the court that his design was unique and his design became widely used.

With widespread adoption of Jones' socket, inventors set to work designing improved film cutouts to use with this system. Early cutouts were little more than strips of metal and film. There was a degree of unreliability with respect to the cutouts' breakdown voltage, with some cutouts breaking down before lamps actually burned out.

Inventors experimented with a number of shapes, types of insulating materials and thicknesses in order to produce cutouts that had more consistent and reliable breakdown characteristics.

Two concepts were ultimately advanced, the magazine type cutout that had a rather thick body and the disc cutout that earned the nickname, "dime" for its shape and size.

Ultimately the disc design was deemed to be the most reliable for the money and became the industry standard.

Removable Jones socket in a Line Material Co. luminaire.

Film disc cutouts are still manufactured and additional discussion about them can be found at

Jones sockets can still be special ordered from GE until existing inventory is depleted. Here's a link to the GE catalog.

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