Previously I wrote about streetlights, with a special emphasis on the freeways since these were about to be converted to LED. Now Xcel Energy has announced 100,000 local streetlights in its service area will be converted, so it is time to look at some of these.
The NEMA Lights
In the early days, every manufacturer had it’s own design. Here’s a beautiful example from Lexington, MA near Battle Green.
The pole is decorative, but the luminaire itself, although beautiful, is simply designed to be functional. The white porcelain protects the metal, and the waves add strength to the reflector and help throw light to the sides. This was known as the “radial wave” design and these are highly desired by lighting collectors.
In 1941 all the manufactures got together and developed the NEMA (The National Electrical Manufacturers Association) standard. NEMA luminaires standardized the placement of two latches to mount the optics into the rest of the luminaire. This form had two advantages.
- An optical assembly for any manufacturer would work on the luminaire base for any manufacturer. If a globe got smashed, a city could just take a Westinghouse globe out and mount it to a GE luminaire, without needing a different stock for every different manufacturer on the streets.
- Optics could be changed, i.e., if a new more efficient reflector was developed, or more light was desired and thus a bigger globe was needed, they could just go out and pop in a new, bigger lamp (since incandescent lamps didn’t need specialized control gear for each different wattage) and new optics on the existing luminaire.
Originally there were a wide variety of NEMA optics– “Radial Wave”, “Half Moon”, “Admiral Hat”, “Gumdrop”, “Teardrop”, “Bucket” to name some. Here’s a NEMA “Gumdrop” I own. It has a 327 watt, 4000 hour streetlight lamp in it.
The “Teardrops” were the same idea, but the refractor extended down farther to accommodate larger lamps for major streets. This style is now very popular with those gaudy, atrocious “fake history” lights I’ve seen in Chicago, on Lake Street, the Minneapolis parkways, and numerous other places. I get that as much as I like the old mid-century modern lights the installation is now a maintenance nightmare and need to be replaced, but when the teardrops first went up it was the style of the time; they weren’t putting in fake chimneys to imitate Victorian or Federalist style lighting. There’s plenty of designs that while modern, are still decorative.
And an “Admiral Hat”, using an economical open reflector.
There were some differences in the lamps used in early streetlights. They cared more about life than efficiency, so they were under-driven to the point of having a decidedly yellow cast. And they cared about lumen output rather than watts, so they have even lumen ratings, like 2000 and 4000, and weird watt ratings, like 327.
Rather than line voltage, many incandescent streetlights, in a blocks-long version of Christmas lights, were run in series, the supply voltage was around 2000 volts with each lamp running at 50 volts. This controlled voltage drop for the enormously power hungry incandescent lamps. Early models had porcelain insulators but improved wire insulation later enabled them to be wired through the luminaire neck like paralleled lights. Obviously you didn’t want the street to go dark if a lamp burned out or was removed, so these had “Jones Sockets”, so 1) If the bulb was removed, a spring in the mogul socket shorted the contacts together, 2) the mogul socket itself fit into another special socket. The receptacle in the base would short if the socket were pulled out, and in between the prongs was a clay disc to short out if the lamp burned out.
To relamp, you would pull the socket out, replace the shunt, screw in a new lamp, and push the socket back in, all without affecting the rest of the street. My light was originally a series string so I pulled out the shunt and replaced it with a piece of credit card wrapped several times with electrical tape.
With the coming of High Intensity Discharge (HID) lamps (mercury vapor and it’s relative metal halide, and high-pressure sodium vapor) these were much more efficient, but required other components in or near the luminaire to operate. The problem is that they appear as a dead short when run directly off the mains voltage, so a device to limit that current and reduce the voltage is needed. Originally this was just a ballast, but with newer HID lamps also requiring ignitors, the term “control gear” is sometimes used.
The simplest ballast is a two-wire choke ballast, a coil of fine wire wrapped around an iron core many times. This both limits current and reduces the voltage by a substantial, fixed ratio. More complicated ballasts are 4 or more wire autotransformer types. They are more flexible in input vs output voltage and often have multiple taps for different input voltages. When designing the lamp plus ballast combination, there is a trade-off vs the optimal arc voltage for the lamp vs what voltage is obtainable from the mains with a simple choke ballast. Mercury vapor lamps were designed in the UK with 240 volt mains. With an arc voltage around 100 volts, they require an autotransformer in the US on 120 volts, although many older streetlights used chokes with a 240 volt supply. Here’s a 240 volt choke mercury ballast from my 1960s streetlight
High pressure sodium lamps were designed in the US and the smaller ones (150 watts and less) can use choke ballasts with an arc voltage of 55. That would be inefficient in Europe, where 120 volts is unavailable, so they designed their own sodium lamps for 100 volts (and for good measure called them “SON” lamps and used smaller, higher strength glass envelopes). Larger sodium lamps are designed to use autotransformer ballasts only to have the most efficient arc voltage for each size lamp regardless of mains voltage and the additional cost of an autotransformer ballast is less of an issue.
Even though there’s little resistance when a lamp is hot, there is significant resistance when it is cold. Mercury and older metal halide lamps use a starting electrode; when the lamps starts, the arc forms between two electrodes very near each other on one end, and eventually the end heats up enough to form the arc from one end to the other. Newer lamps use a several thousand volt zap to initialize the arc, generated by the ignitor.
Here’s the gear of an American Electric model 315 cobrahead. The ballast is at lower left, the ignitor is the circuit board at the lower right. At top is a capacitor for power factor correction. This is important to utilities and businesses (who the utilities charge if it’s not corrected) but not strictly necessary for lamp operation and is omitted in inexpensive fixtures for home use.
The most modern ballasts use switching power supplies rather than heavy magnetics to produce the desired current and voltage. These are now standard for fluorescent lamps, but were never adopted for HID use in the US except for interior ceramic metal halide fixtures.
As a side note, Europe developed “White SON” for indoor use, which drives high pressure sodium hard enough that it produces a pleasant incandescent like white color common in shops, but it was never common in the US and even in Europe was less common than ceramic metal halide. White SON is more expensive, less efficient, and shorter lived then regular high pressure sodium so it was never used for street lighting.
NEMA Lights Today
Back to our discussion on NEMA lights. Some of the previous optics attempted to direct the light in specific directions, but the “open bucket” refractor, which blasts light equally in all directions, is the only one still in common use. However, Xcel has recently introduced a dark sky friendly full cutoff reflector for new installations.
Here’s a NEMA light in St. Paul. Uniquely, Xcel supplies the power and the mast, while St. Paul owns and maintains the luminaire. Since they blast light in all directions, they tend to be on very long poles to have them towards the center of the street. With the newer, more efficient lamps, St. Paul and many small towns still use NEMA heads for streetlighting the much smaller lamp size of smaller and medium sized HID lamps enabled them to still fit even with the required control gear.
A pair of old and new Xcel NEMA area lights, these are a design by GE called the “Power Bracket”. The much shorter length of HID lamps opposed to the large incandescent lamps for which NEMA luminaires were designed enabled the ballast to be stacked on top in the conventional design, but these moved it into short, integrated arms, which eliminated heat rising and building up around it. More or less the same model has been in production for many decades.
Also notable are what are pejoratively called “yardblasters” by lighting enthusiasts; cheap fixtures from big box stores that look somewhat like true NEMA heads equipped with a bucket, but the refractor is bolted on, they’re made of stamped metal, and the ballast is usually sketchy; it’s the buzzing mercury light on your neighbor’s garage that’s shining in your window.
Fluorescent and Induction Street Lights
The ubiquitous fluorescent tube was also once used for streetlighting. Municipalities were eager to discard the old fashioned looking and inefficient acorn lanterns and “teardrop” overhead lights in their downtown districts in favor of these bright, streamlined luminaires. However their bulk, lack of control over the broad diffuse light sources, and poor cold weather performance eventually led to their extinction for streetlighting. All the major streets in Minneapolis were once lit by fluorescent luminaires but these were all gone by 1990 and no lighting enthusiast knows of any in service or in private collections. The last ones I saw in the field were in Waconia in the late 1990s.
A modern development has been induction lights. These basically use a high frequency magnetic field to put energy into the lamp. They’re very efficient, more compact than standard fluorescents, and last a very, very long time because they lack a starting electrode, which is the usual failure point on conventional fluorescents. Like fluorescents, they give a very diffuse, omnidirectional light. They’re good for lanterns, good cold weather performance, less good for overhead lighting where you want it focused down. There’s a test installation in the lanterns on 46th street west of Hiawatha, and a few cobraheads near the river in Hudson. Had LED technology not progressed we would be seeing a lot more of these.
Although NEMA luminaires were able to accommodate HID lamps, soon came luminaires designed from the start to take advantage of them: the “clamshell” which evolved into the cobrahead. More about those and a spotter’s guide to cobraheads in the next article.
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