Flourescent Puzzle

I have two adjacent flourescent fixtures of two 40W bulbs each. Both fixtures (all four lamps) went out at once. The fixtures are both getting 120v and there is a slight humming noise from one (or both?) of the ballasts.
Well, I can understand one of the ballasts going out but not both at once.
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Blue wrote:

I'd put my money on a bad ground. Also consider that it may be getting colder and it may be below their operating temperature.
--
Joseph E. Meehan

26 + 6 = 1 It's Irish Math
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The black and white leads are connected to the fixtures and the voltmeter shows 110vac on them. The ground wire is attached to the fixture also and a voltmeter shows 0v between the fixture and a good remote ground (plumbing) when the power is applied.
The ground is a safety feature but don't see how it could prevent the lamps from lighting even if the ground was defective. What is your thinking on how a bad ground could cause the problem?

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They use the ground plane of the fixture to start the bulbs. I don't know all the ionization details but I know it is true. If the ground is bad they don't start well.
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Blue wrote:

The voltage measurement between the fixture and a good ground does not indicate the fixture is grounded. You need to measure resistance. Also don't every assume plumbing is a good ground. In most newer homes it is not and in many older homes it is not. It may not be grounded at all.

Trust me on this. Most units need a ground. Not all, many of the new electronic ballast don't need it, but the magnetic ones do. It is need to set up the magnetic field.

--
Joseph E. Meehan

26 + 6 = 1 It's Irish Math
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Sympathy failure, perhaps? ;-)

getting
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Sounds like you lost the earth ground connection to the lites. Or, it could be the cold, but not real likely unless something changed in your environment. Usually low temps make them get dimmer, then you'll see spirals inside the bulbs and finally they wont' come on when it gets cold enough depending on the temp, age of the bulbs, xfmr, and ground connections. KILL POWER BEFORE THOUCHING ANYTHING! You don't sound real experienced.
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|I have two adjacent flourescent fixtures of two 40W bulbs each. Both | fixtures (all four lamps) went out at once. The fixtures are both getting | 120v and there is a slight humming noise from one (or both?) of the | ballasts. | | Well, I can understand one of the ballasts going out but not both at once. | |
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I investigated the missing ground theory in howthings work" and was fascinated by what it said. Seems that the flourescent metal box should be grounded to give reliable ionization of the tubes. that's a new one on me and still puzzles me. E.g., I have a two tube flourescent fixture in my garage with a two prong plug feeding it with no problem.
Well, I lashed an extra ground to the fixtures with no effect. Then I bought one new fixture as that was as cheap as a ballast, my second suspect. Bought two new tubes too, just in case. Just for grins I tried replacing the tubes and you guessed it - lights came on. Problem is that I only replaced TWO tubes and lost track of whether they both went into the same fixture or one into each fixtures. Also, all four of the old tubes had continuity through the filaments on each end!
I don't want any more problems like this!

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Magnetic florescent fixtures have always been a nuisance. Especially 2 light four footers, with all the connections on the ballast, four contacts on each lamp and eight contacts between the four sockets, there's a lot of week links. I too have had incidents where I'll change bulbs and ballasts and the thing still doesn't work. What is nice is the current electronic ballasted florescent fixtures. They use a 32 watt T8 lamp instead of the 34 -40 watt T 12. The ballasts are in like their fourth generation, and really work well. Each lamp is in parallel as well so one lamp can burn out and the other work fine. There's also no blink blink blink before they come on

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Blue wrote:

<snipped>
Grounding of the metal "reflector" above the bulbs is not always essential, but can be helpful if the ballast (or maybe the bulbs) are a little wimpy. The symptoms of an ungrounded fixture can be bulbs glowing just at their ends, but not "firing up" all along their length.
The grounding works by providing an electrode to create an electrostatic field extending away from the electrodes at the ends of the bulbs. That field helps get the ionization of the gas (mercury vapor)inside the tube extend the full length of the bulb. The ionized gas is electrically conductive and current keeps flowing through it from one end of the bulb to the other, keeping it ionized. (At that point you could probably remove the fixture ground wire and it'd stay lit fine 'till you switched it off.)
The ionized gas emits ultraviolet light which excites the phosphor on the inside of the glass and makes it flouresce white, much like an ultraviolet "black light" will make certain inks and stuff glow brightly. That's why they call them "flourescent" lights BTW.
Doubtlessly Don Klipstein can do a more elegant job of presenting this
HTH,
Jeff
--
Jeffry Wisnia

(W1BSV + Brass Rat '57 EE)
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TRUE!
If the bulbs glow throughout their length, grounding is not the problem.
When grounding is the problem, the ill effects of lack of grounding may be intermittent, and may vary with temperature and/or humidity.

Close enough and I would award a cigar!
The way I see it, grounding makes unionized gas in most of the length of the bulb more alike in potential, and that increases the voltage gradient (electric field) in the gas approaching the electrode at either end of the bulb - which is where ionization of the gas normally starts. Once ionization happens where it can be initiated, then the boundary between ionized gas and unionized gas is where the voltage gradient is high - resulting in ionization there, and thus the boundary between ionized and unionized gas propagates (usually in quite a small fraction of a second if at all).

True!
True! But anyone trying to make the coating glow from a blacklight will usually not get that to work - the phsophor coating in most fluorescent bulbs works well from shortwave UV and not from the longwave UV from a blacklight. And most "fluorescent style" blacklights are true fluorescent lamps - they have a phosphor converting shortwave UV (UVC) to longwave UV (UVA, and at wavelengths too long for suntanning). Any UVC passing through the phosphor is well-blocked by the glass.
Actually in a fluorescent lamp, maybe about 20% of the light is visible light (bluish) from the glowing low pressure mercury vapor and about 80% is from the phosphor. In a compact fluorescent, it's more like 25-75 or so. Very roughly!!!

Maybe not quite if I work 2 jobs and then get online after working 13 hours.... :) :)
- Don Klipstein ( snipped-for-privacy@misty.com)
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Grounding may not have been your problem, but it is a common one. Having grounded sheet metal close to the bulbs affects the electric field distribution within a bulb that is trying to start. Sometimes cleaning the bulbs also affects this, if the bulbs get coated with dust that is hygroscopic and the humidity is high (more likely in coastal areas).

Anyone who does this ought to replace the ballast with a good "commercial grade" one suitable for the bulbs in question once things start working! Cheap fixtures, especially for "T12" (1.5 inch diameter) bulbs as opposed to "T8" (1 inch diameter ones), tend to have "residential grade" balasts. "Residential grade" ballasts for T12 or F40 "lamps" (bulbs) are all too often something that I like to refer to as "turds". More politely, paperweights that have wires coming out.

2 tubes go out in a 2-tube fixture - chances are only one is dead. 2 tubes go out in a 4-tube fixture - chances are only 1 is dead. Same story if 2 tubes are dim or dim-and-flickering (but not outright blinking and doing so out of sync with each other), or if one is dim with one completely out. In this usual failure mode, the dead tube usually has some major darkening at one end - about 3 inches long in a 1.5 inch diameter tube, about 2 inches long in a 1 inch diameter one. Less likely you see such discoloration at both ends. That bulbs's viable mate usually has much more minor but easily visible discoloration at the ends, usually at least somewhat similar from one end to the other. Much newer bulbs that got mixed in should have much less end discoloration, and possibly different markings. If you can identify the mate to the dead bulb, its life is probably mostly expended and the bulb can be disposed of like the actually dead one.

- Don Klipstein ( snipped-for-privacy@misty.com, http://www.misty.com/~don/ltrouble.html )
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