Streetlight Poles, Lamps, and Control Gear

October 7, 2016 at 1:52 am | Posted in Streetlights | Leave a comment

Here is a guide to street lighting pole types and the lamps that they use.

Types of Poles:

Davit Poles: These are the Mn/DOT standard, and although not used by most other states, a few do, like Rhode Island and North Dakota. They are actually two pieces, the top piece slips over the bottom and curves between the pole and luminaire, giving a modern and seamless appearance.

Some poles are mounted on the concrete in the median, and some are on the sides of the road.  Lights in the median are more economical to install, make use of wasted light emitted towards the back of the fixture, are less vulnerable to knockdowns. However they must be maintained on the left side of the road, and lane closures are required if there is not a full shoulder. Current practice is to mount them on a concrete median if one exists and there is a full shoulder.


Relamping a luminaire on a davit pole, Bloomington.

Tenon Poles (“Turnpike” Style): These use poles farther back (less likely to be hit by texting drivers), with floodlights aimed forward at about a 45 degree angle. For a while Mn/DOT liked these due to reduced maintenance, but they caused a lot of light spill into neighboring residential neighborhoods, and equipping them with shields reduced their effectiveness. They’ve also been problematic to convert to LED, but now they figured out that a standard LED fixture pointed down will work for roads up to four lanes, and have found special fixtures for wider applications.

“Turnpike” luminaire on I-494, Plymouth

“Turnpike” luminaire on I-494, Plymouth

Bent Straw Poles: In some areas a more decorative look is desired. Mn/DOT uses bronze “bent straw” pole poles with square “shoebox” luminaires to provide something somewhat better looking than standard davit poles while still being efficient and easy to maintain. Notable locations are the I-35E “Parkway”, The Wakota Bridge, and MN 36 and Stillwater Blvd.

Bent Straw Installation on MN 36, Stillwater

“Bent Straw Installation” on MN 36, Stillwater

Truss Arm Poles are the oldest poles that can still be seen occasionally in Minnesota.  Mn/DOT has not installed new ones since the 1970s and almost all have been replaced, but there’s a few stragglers around. These came in both the straight and curved varieties. Also, the Hudson Bridge, built by Wisconsin, has straight truss lights on both sides of the river (While the St. Croix Crossing, built by Mn/DOT, will have davit poles). Generally truss arm poles are used in the eastern half of the country while a different style, called the “upsweep” style is used in the west.


Curved Truss Arm, Bloomington.

Straight Truss Arm Pole, Shakopee

Straight Truss Arm Pole, Shakopee

High Mast Luminaires are the ones on those super tall 100-120 foot poles. Mn/DOT has traditionally liked them due to ease of maintenance (the fixtures are mechanically lowered on a pulley for relamping) and they give a smooth, even light enabling drivers to easily discern complicated interchange. After complaints about light pollution, Mn/DOT now avoids installing them in residential areas, and in fact has replaced one (at MN 62, US 169, and US 212) with davit poles.


High Mast Lighting

High Mast light in front of Best Buy.

High Mast light in front of Best Buy.

Types of Lamps

Incandescent. Although we all know about incandescent lamps, 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.


Line Materials NEMA “Gumdrop”

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.


Jones Socket Patent Drawing

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

Since the 1950s, arc discharge lighting, and then High Intensity Discharge (HID) lighting has been predominate for lighting streets, due to their compact size, efficiency, long life, and lack of concern about color rendering. Light is produced when electrons are bumped outwards away from the nucleus due to the input of energy, when they fall back into place they emit the extra energy as light. In fire and incandescent lamps the energy comes from heat; HID lamps are much more efficient using an electrical arc directly.

Low Pressure Sodium Vapor: Commercialized in the 1930s. these arc discharge lights use sodium metal (combined with neon and argon to get it started). They are bulky and produce a pure yellow light. Astronomers like them because if they see something pure yellow in their telescope they know what it is. These were used years ago around Met Stadium, and on major north-south streets in Richfield. Mn/DOT never liked them because it was difficult to control the light. A surviving installation is at the Thunderbird Hotel parking lot in Bloomington. Another notable use is bridges over the Mississippi, for example the “Big Blue” bridge in La Crosse, where the yellow light doesn’t attract mayflies.

Original Low Pressure Sodium Light from the Golden Gate Bridge, at the Visitor’s Center

Original Low Pressure Sodium Light from the Golden Gate Bridge, at the Visitor’s Center


Low Pressure Sodium Light, San Jose, CA

High Pressure Sodium Vapor: In the 1960s High Pressure Sodium was  (HPS) was developed. These have a golden hue and are now the de-facto standard for street lighting. The gasses are at higher pressure and mercury is added to decrease the size needed and add other colors than pure yellow to the spectrum. Usually 100 and 150 watts are used for residential streets, 200, 250, and 400 for wider streets and freeways, and 1000 watts for the 100 foot towers on the freeways. The 1000 watt lamps are roughly 8 times as efficient as household incandescent. The initial HPS installations in the Twin Cities that I recall, from at least the 1970s, were I-35W in south Minneapolis. I-35E north of dowtown St. Paul, and Lyndale Ave in Bloomington.  New installations from around 1980 on were HPS, with mass conversion in the early 1990s.

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.


White “SON” lamp, note European style lamp with small, high strength glass instead of the huge bulging shapes of US lamps.

18 Watt Low Pressure and 35 watt High Pressure Sodium Vapor lamps. These are smaller than are used for street lights, more typical for small wall mounted outdoor lights. They each produce about as much light as a 100 watt incandescent.

18 Watt Low Pressure and 35 watt High Pressure Sodium Vapor lamps. These are smaller than are used for street lights, more typical for small wall mounted outdoor lights. They each produce about as much light as a 100 watt incandescent.

Closeup of a new Low Pressure Sodium lamp. The silver drops are sodium metal. Since sodium is violently reactive and will attack stadard glass, water, air, and just anything else the inner tube is made of the same material as laboratory beakers.

Closeup of a new Low Pressure Sodium lamp. The silver drops are sodium metal. Since sodium is violently reactive and will attack stadard glass, water, air, and just anything else the inner tube is made of the same material as laboratory beakers.

Mercury Vapor.  Dating from the early part of the last century and refined in the 1930s, mercury vapor gives a bluish-green light, unless a red phosphor has been added on the color enhanced version, which gives them a reddish tinge.  Mn/DOT has long since eliminated the last of these, but older installations still exist on some city maintained lighting and on older Minneapolis parkway fixtures. Mercury Vapor, while extremely long lived, is relatively inefficient compared to other HID bulbs. New ballasts have been banned for several years, and new bulbs will be banned starting in 2016. Although not describe as such, standard fluorescent lighting can be though of as “low pressure mercury vapor”.


Mercury Vapor Light

Metal Halide is a refinement of mercury vapor.  Developed in the 1960s, Metal Halide adds additional substances to improve color rendering and efficiency at the cost of bulb life. They last 10,000 hours, or half that of other HID lamps.  Metal Halide was always the choice for car dealerships and indoor applications where color was important; in the 1990s as the designs improved and before the LED invasion it became much more common for general outdoor use. Newer Minneapolis parkway fixtures are metal halide. A further refinement is ceramic metal halide that produces a warm, pleasing light. They’re not used here on highways here but Chicago had been adopting them.

Metal Halide decorative lights on the Blatnik Bridge, now replaced with LED lights that illuminate the bridge internally.

Metal Halide decorative lights on the Blatnik Bridge, now replaced with LED lights that illuminate the bridge internally.

Fluorescent and Induction Street Lights

The ubiquitous fluorescent tube (which can also be thought of as “low pressure mercury vapor) 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.


Fluorescent Streetlights in Spooner

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.

Jersey Series Induction Light

Jersey Series Induction Light

Control Gear for HID Lighting

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.


240 Volt only choke type ballast

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.


American Electric HPS Control Gear

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 fixture costs have gone down the energy efficiency and long life of LED fixtures are becoming attractive. The I-35W bridge was notable as the first use of LED lighting on an interstate highway in the country. At the time fixtures that were direct replacements for conventional luminaires weren’t available so they had to space the poles closer together.


Early Mn/DOT LED light,

With Xcel Energy and Mn/DOT converting most of their fixtures to LEDs, the last remaining ones will be  high mast fixtures,  where an LED fixture that can put out enough light that has been fully evaluated and approved by Mn/DOT does not exist yet. (A LED high mast setup is being tested at US 61 and I-494, but it does not equal the brightness of the conventional fixtures).  Maine has had a retrofit made that does equal the light output, but some agencies are leery about modifying existing fixtures due to UL approval issues.


High Mast LED test setup, US 61 and I-494.


High Mast LEDs, Houlton, Maine

Also excluded are the tunnels, which are relatively new installations using fiber optic light pipes to provide a very even light.

There have been some issues. At one point I noticed about a fifth of the lights on the I-35W Minnesota  River Bridge were out. Mn/DOT told me that they had been hit with a power surge and the power supplies were blown out. Being a test installation they had to have more shipped in from the manufacturer. HPS fixtures with their magnetic ballasts are essentially immune to power spikes. With the old sodium fixtures they would not go around chasing individual burned out bulbs, but their intent is to address individual LED fixture failures due to warranty issues.

The Health Effects of LEDs.

Recently an article came out claiming LEDs were detrimental to human health. My friends kept emailing me about the article, and my official response is “I don’t know”. First of all I’m a bulb geek, not a doctor. Secondly, it’s not clear if they’re objecting to the light blue “ice white” tint of LEDs, or that pure blue makes up a large part of the spectrum. (LEDs are essentially blue emitters with yellow phosphors, the mix of those appears white to us. If it’s the former, we have a lot of other bluish lights around, in our offices and computer screens and whatnot, to say nothing about the fact that mercury vapor streetlights were once extremely common, so I’m not sure why they’re suddenly sounding the alarm. If it’s the portion of blue in the spectrum, they seem to be advocating the use of “warm white” LEDs, which also contain pure blue, just more yellow to mask it.


LED Spectrum. Older mercury based light sources had more greens and purples instead of blues and yellows.

I’m old enough to have remembered the transition from the moonlight glow of mercury vapor to the golden hue of sodium lights, and now back to the blue of LEDs.

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