Why did they install a metalarc bulb?


Old 05-05-06, 04:11 PM
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Why did they install a metalarc bulb?

In this city facility, they put 100 watt metalarc bulbs under the entry way canopy. Easily accessible off a 6 foot step ladder.

The bulb burned out after being continously on for 5 years. I noted where it says on the box that these bulbs require being shut off for at least 15 minutes a week, but people I talked to , even at the electrical supply house, did not realy know why. (Common sense would say that it be either to cool the bulb or the balast, but i don't know which, or why, or ...wouldn't you think that after running for 6 days and 23 hours and 45 minutes that the bulb/fixture could get no hotter?, and any damage would already have been done by that length of tiime?)

What strikes me odd is why the electrical contractor that wired the building, opted to put these obviously expensive set-ups in, that require balast fixtures to match the wattage of the bulb (that is what the guy behind the desk at the suply house told me who has worked there for years)...when all they would have had to do is put in a conventional socket and a $2.69 halogen bulb that is guaranteed to last 2 years. The metalarc bulb would have cost me probably about $25 if our home center had that watt bulb, but they only had 70 watt in that size, for $20. So I had go to the supply house and pay $32. $32...for a bulb! And I noticed the bulb near it is getting dark and probably ready to go next.

So why did they put in those bulbs..with balasts?

The building also has 1000 watt metalarc bulbs (giant size) up by the 30 foot ceiling inside. If longevity is the key (although those are burning out right and left now, even though these are kept off whenever possible, and are actually off for about half the business hours time ...and we have about 60 of them up there), I can see why they chose that bulb when up 30 feet.

But why did they put in the 100 watt metalarc ones where I can easily change them off a step ladder? Do these bulbs also produce more lumens, per watt?,or no?
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Old 05-05-06, 08:23 PM
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Dave, if you check the package to see how the halogen bulb lifespan is calculated you will probably find it says 4 hr usage per day over 2 years. If you were to burn this bulb for 12 hour periods during the winter months you should olny expect 1/3 the life. Other factors like how often thebulb is cycled on and off will effect the lifespan.

You said that you have gotten 5 years of life out of the metalarc lamps. The city has saved quite a bit of labor
by not having the maintenance staff changing the bulbs.

You should also compare the electricity consumed vs the lumen output when comparing bulbs.
Old 05-05-06, 11:32 PM
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A 100 watt Phillips Halogena lamp lamp puts out 1750 lumens and lasts about 3000 hours This website listes them at $4.99.


A 100 watt metal halide lamp puts out 6800 (mean) lumens, and yours lasted 5 years (43,800 hours.) (They're not rated for that many hours, but a lot of them do last that long)

You'd have to install 4.5 100 watt halogens to equal the light output of the 100 watt metal halide you have now. If you had done that, over the past 43,800 hours you would have had to replace those lamps 14.6 times at a cost of around $325 (@ $4.99/lamp), exclusive of labor.

During that time, you would also have burned up 4.5 times the amount of electricity.

Makes that $32 lamp look pretty good, no?
Old 05-06-06, 07:02 AM
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As for the bulb cycling once a week has to do with the arc tube inside the bulb. These bulbs operate under high pressure and over time the quartz arc tube will sucumb to the pressure and rupture. Cycling the bulb will cause the bulbs that have cracks in the arc tube to not start since and reduce the chance of a rupture.

There are new codes now that say that HID bulbs with arc tubes must have a shielding of some type in case of a rupture the hot glass don't fly everywhere. The new tech even has ballasts that cycle the lamps for you every so many hours.

As for why they used them is simple. They outlast halogens by a long shot and hold there color longer, plus their color is more like day light and you'de never get that from a halogen. They are probably operated on 277V allowing for smaller wires and more fixtures in a circuit. Also you must think of labor cost of replacement, over 5 years you'll have to pay the labor of a guy once to replace a metal halide and you'de have to pay him 4-5 times for a halogen. The best practice for these bulbs or any large facility whether they are flourescent tubes or metal halides is group relamping when they get toward the end of their life.
Old 05-08-06, 06:09 PM
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I appreciate all the answers. I read all posts. Very informative and all makes great sense. Thanks.
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