Engineering Question #1133

Sophie, a 23 year old female from Winnipeg asks on December 23, 2002,

Why do street lights shut down, often, when we drive underneath them? Oftentimes, my boyfriend and I notice that lights go off when we drive by - about half of these occurrences are from one specific light post on our street. This happens at least once a day, if not a few times a day.. every single day.

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The answer

Gord Engstrom (Vancouver City Engineer) answered on January 8, 2003

The occurrences are not related to the person. The most likely explanation would be that the lights are reaching the end of their life span. Most cities, including Vancouver and Winnipeg, use High Pressure Sodium lamps. High Pressure Sodium Lamps have long rated average lives. Depending on lamp type, most are rated at 24,000+ hours. Normal end-of-life occurs when the lamp begins to cycle on and off. This on-off pattern will begin when the lamp voltage has increased to the point that the ballast voltage isn't sufficient to keep the lamp lighted. The City of Vancouver attempts to use a four year re-lamping schedule for high pressure sodium street lamps based on 70% of the 24000 hour rating. One must keep in mind that a rated life for lamps is the point when 50% of the lamps will still be lit. Old lamps are less tolerant of voltage dips than new lamps. In Winnipeg report failing street lamps to Manitoba Hydro.

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