Good Vibes Only: How Vibration Dampeners Extend Light Pole Life
Light poles are typically very tall and heavy and installed in public places, so it is very dangerous if they collapse. According to the Illuminating Engineering Society and many pole manufacturers, the incident of pole failure has increased. For example, a recent study in Chicago found that 33,000 (or 10%) of the city’s light poles were graded as having some sort of serious issue. It makes sense that light poles are failing in the Windy City. Light poles fall when wind creates vibration in the pole. The vibration causes the metal pole to deteriorate. Deterioration where the base of the pole meets the base plate causes the pole to eventually break and fall over.
TWO TYPES OF LIGHT POLE VIBRATION
Harmonic Vibration
Harmonic vibration causes the pole to move from side to side. The vibration and movement are greatest at the top of the pole. Harmonic vibration is relatively small and takes longer to deteriorate the metal pole.
Hubbell Lighting - Pole Vibration Application Guide (PDF)
Resonance Vibration
Resonance vibration causes the pole to rotate around the midpoint of the pole. This causes the pole to vibrate at a higher frequency. The pole can start to crack or deteriorate more quickly under these conditions.
EFFECTIVE PROJECTED AREA RATING
Every pole has an Effective Projected Area (EPA) rating . EPA is used to determine how much force a fixture applies to the mounting brackets or pole at a given wind velocity. To calculate the EPA, multiply the frontal projected area by the drag coefficient to find the square footage. The EPA rating varies based on the pole's shape, composition and how it is anchored.
Underloading a lighting pole is nearly as bad as overloading it because both create damaging harmonic vibrations. Many new LED fixture heads weigh less than 70% of the HID fixture heads they replace. Some people only look at the max weight allowance and load the new LED heads. They do not realize the weight of the old legacy fixture head was helping to dampen vibration and stabilize the pole. They do not think it is a problem, but the lighter LED fixtures can cause increased vibrations. For instance, poles that have been in place for five or 10 years may already have stress cracks at the base of the pole. When retrofitting these poles, removing an old 40-pound HID fixture and replacing it with a 10-pound LED fixture can accelerate the pole’s metal fatigue even faster because now the pole is experiencing enhanced vibration.
HOW TO PROLONG THE LIFE OF THE POLE
The solution is to use a vibration dampener, which comes in several forms. The most common vibration dampener is a length of chain that mounts at the top of the pole and runs down the inside of the pole. The chain counters or disrupts the vibrations and keeps the pole steady in high winds or wind gusts. It is a simple solution for electricians and electrical distributors to keep poles secure and minimize damage.
For example, consider a contractor who wants to replace a double-head HID fixture on a 25-foot pole with LED lights. They have chosen an LED fixture type that has an EPA rating of 2.02 in a back-to-back configuration. The max weight is 54 pounds combined. The previous HID fixture heads had a higher EPA rating of 3.4 and a total combined weight of 95 pounds. The 25-foot pole has an EPA rating of 7.7 and a max weight of 188 pounds for an area that is rated for 90 mph of wind. The replacement heads seem to be a perfect replacement to the HID heads, but the overall reduction in weight and EPA by more than 50% could be a concern, especially if the pole is a square, steel pole. This example also assumes the pole is a heavy steel construction, but it could be a thinner gauge of metal. That would reduce the weight and EPA allowance by nearly half. Retrofitting existing light poles with LED fixtures is not an exact science unless the contractor is measuring the steel thickness of the pole. Either way, vibration dampeners can prolong the life of lighting poles and help prevent a dangerous situation where a pole would break and fall.
Pole vibration dampeners are a simple solution to a potentially serious problem. Before retrofitting light poles, consider the EPA rating of the pole and fixture, and use a vibration dampener to extend the life of the pole.
