Letters: Reader Feedback – 2006-05-01

Kudos on defining switchgear vs. switchboard

I am not in the habit of writing to magazines, but I felt compelled to compliment CSE and especially author Brian Rener for writing “Switchboard or Switchgear?” ( Spring ’06 Pure Power, pg. 10 ).

The difference between these two pieces of electrical distribution equipment has been well defined by IEEE over the years, but has been ignored by most engineers—a pet peeve of mine, especially since I have been in the business for more than 30 years.

This article very well delineates the differences, but does so in a manner that is simple to understand, yet comprehensive. I think that engineers not involved in power quality would profit from this piece as much as those that do, and maybe even more so. This is even more the case for new electrical engineers who must be very confused about this issue.

As I mentioned, this article found its way to me via the Pure Power supplement, but I hope it, or something like it, finds its way into the main magazine as an article in the near future.

Jean-Jacques Majon, RTM & Assocs., Inc., South Barrington, Ill.

Don’t discount high-pressure sodium bulbs

I’m in disagreement with the article “Safe Parking” (CSE 2/06, pg. 48) . The author deceives readers into thinking that pulse-start metal-halide lighting provides the most economic means of lighting parking lots. The article begins by touting the color rendering index advantage pulse-start metal-halide lighting has over high-pressure sodium, yet the author spends the rest of the article touting the economic advantages that pulse-start metal-halide bulbs have in comparison to traditional probe-start metal-halide bulbs. No mention is made of the economic difference between pulse-start metal-halide and high-pressure sodium. Why? In my opinion, it’s because high-pressure sodium bulbs are still more economical, despite the advances in bulb life and lumen maintenance for pulse-start metal-halide lamps.

Metal-halide lamp proponents have been touting improvements in lamp life and CRI ratings as having narrowed the cost gap and widened the performance gap between metal-halide and high-pressure sodium. Unfortunately, these improvements have been represented generically, when in fact, they are often mutually exclusive—the higher CRI pulse-start lamps have an even lower life span than standard metal-halide lamps—and wattage specific; the 20,000-hour lamp life often cited is only seen in pulse-start bulbs of 320 watts and higher.

Pulse-start metal-halide proponents don’t mention that the rated life of these bulbs is based on different criteria from high-pressure sodium lamps. Pulse-start metal-halide lamps have a rated life of 15,000 to 20,000 hours, while high-pressure sodium lamps have a minimum life of 24,000 hours. For lamps with a rated life of less than 24,000 hours, the criteria used is the median life expectancy—that is, the operating time at which 50% of the lamps would still be functioning. For lamps rated 24,000-plus hours, the criteria is more stringent; rated life is the time at which 67% of the lamps would still be operating. Thus, the high-pressure sodium lamps would have only a one-third failure rate at 24,000 hours, while the pulse-start metal-halide lamps would experience a one-half failure rate at 15,000 to 20,000 hours.

It should also be considered that the high-pressure sodium lamps have a significantly higher lumen maintenance ratio. Lumen maintenance is the mean light output maintained as the lamp ages. For high-pressure sodium lamps, this is taken at the midpoint of their rated life, while it is taken at 40% of the rated life for metal-halide lamps. For 150-watt lamps of each type, this would mean the high-pressure sodium lamps retain 85% to 90% of their initial intensity at 12,000 hours, while pulse-start metal-halide would maintain 69% to 78% (the author says 75%) at 6,000 hours.

The life-cycle cost for high-pressure sodium lamps is about one-third that of standard metal-halide lamps, and just under half that of the pulse-start metal-halide lamps. The numbers are less dramatic if we were talking about higher wattage lights—e.g., 400 watts and above—but economically, would still favor high-pressure sodium. As stated above, that your author begins his piece mentioning the difference in CRI ratings between metal-halide and high-pressure sodium lamps, then spends the rest of his article building a case for pulse-start metal-halide lamps by comparing them only to standard metal-halide bulbs without ever mentioning the continuing economic advantage of high-pressure sodium bulbs, is misleading and a disservice to your readers.

Kelly A. Giblin, P.E., NJ TRANSIT Corp., Newark, N.J.

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