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Sir Sparks
Sir Sparks, President/Owner/ Master Electrician
Category: Electrical
Satisfied Customers: 2532
Experience:  US State Certified & UK Electrical Contractor for all electrical fields/disciplines
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On a 4oow metal halide low bay fixture when the light goes

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On a 4oow metal halide low bay fixture when the light goes completely out what is the problem? The schematic shows no thermal protector in the circut. These fixtures have a capacitor ballast and bulb. Also when the capacitor goes bad why can't a run capacitor with the same mfd be used to replace the orignal one?
Hi James, obviously there is no "always true" answer for when a metal halide fixture is out, but from experience, the light will usually flicker when the ballast is going out. If this light was working fine and then suddenly went out, I'd say it's either the bulb, the capacitor, or a problem with the wiring itself. Assuming you have correct voltage at the wiring in the fixture junction box and all connections are secure and the fixture still isn't working, I'd typically replace the capacitor first as this is most likely the problem.



A run capacitor and a start capacitor serve two completely different purposes James. But in short, a run capacitor is designed to run indefinitely while a start capacitor is designed to run for a finite amount of time.



Regards,

Mike




Customer: replied 7 years ago.
I new that about the capacitors but why could't a run capacitor be use to replace the capacitor when one goes bad in a metal halide fixture? I know for a fact it will work but I don't know for how long it will work. Is the capacitor in a MH fixture for starting the light or for continous runing of the fixture?
As for how long it will work, I'm not sure how anyone can answer that. I, nor anyone else that I know, goes around installing run capacitors in a fixture that is UL Listed to be used with a start capacitor. I'll opt out though so that other experts will have the opportunity to "guess", so hopefully you can get the answer you're looking for.
Customer: replied 7 years ago.
You are right on your way of thinking. My thoughts were since the capacitor is wired in series with the light and ballast it would be a run cap. A start cap is only on line (as you said) for a brief period. I guess what I am wanting is the science of how this fixture works.
Hi James, I would be pleased to explain the "science" of this to you but first I need to find out what depth I can go to.So I am going to mention a few words and I would like you tell me what briefly you understand about them then I shall gauge my answer accordingly; Power Factor, Lagging and Leading Loads, Vector Diagrams. Capacitive and Inductive Reactance.

Customer: replied 7 years ago.
I am a Journeyman Meterman for a municpaly electric co. I have been doing this for 33 years and also a 33 tear member of the IBEW Local 702. I am trying to help a friend out with a lighting problem. With out a thermal protector in the ballast circut it appears to me that the ballast doesn't work off and on. The cap to me would be the same way and can be checked with an ohm meter. Do the bulbs intermit?
The purpose of the capacitor is to correct a very large LAG in the current/voltage supply characteristics. A large inductance alone ( the ballast) would have the current lagging the voltage by a great deal and make the lamp life shorter and the efficiency less. A capacitor being LEAD and in effect the exact opposite of an inductance brings the overal current/voltage back in line (close to unity) and therefore increases lamp life, Lumen output and efficiency.
Sir Sparks, President/Owner/ Master Electrician
Category: Electrical
Satisfied Customers: 2532
Experience: US State Certified & UK Electrical Contractor for all electrical fields/disciplines
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