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Thread: fluorescent fixtures

  1. #1
    Join Date
    Oct 2003
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    117

    fluorescent fixtures

    Recently replaced a 2-foot rapid-start fluorescent fixture that is paralleled with a ceiling fan/fixture combo off of one switch. When the circuit is closed the fluorescent fixture will not start every time on it's own. However, (when it doesn't start) if I shuffle my feet to build up static electricity and simply wave my hand in front of the fluorescent lamp it will start right up. What is happening here? I noticed that there appears to be some type of flexible cable run from the fan to the fluorescent fixture that is powering the equipment with 18 AWG size wires. Couldn't tell what type because of the numerous coats of paint that it has gotten over the years (the cable was exposed on the inside of the cabinet leading to the fixture). All the splices are good. Is it simply a matter of the small wire size feeding the fixture not allowing enough initial power to start the lamp? I already recommended replacing the flexible cable with proper cabling, but the homeowner wants to wait until the kitchen is remodeled in a couple of months.

  2. #2
    Join Date
    Sep 2003
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    969

    Re: fluorescent fixtures

    Bad/no ground perhaps? Is this flex (like FMC flex) or some 2-wire lead cord someone just hacked in?

  3. #3
    Join Date
    Sep 2003
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    26

    Re: fluorescent fixtures

    Aside from the issue of whether or not the cable is up to code. Does the cable contain a ground wire? I have heard of problems with florescent fixtures not operating properly when they are not grounded.

    Opps, Tonyi types a little faster than I do.

    [ December 21, 2003, 08:46 PM: Message edited by: crash ]

  4. #4
    Join Date
    Nov 2003
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    Hudson Valley, NY
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    Re: fluorescent fixtures

    I'll add my vote for bad ground.

  5. #5
    Join Date
    Oct 2003
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    117

    Re: fluorescent fixtures

    Yes...There is no grounding wire in the cable that was rigged to the fixture. So with that being the case, why does the fixture act like it does?

  6. #6
    Join Date
    Feb 2003
    Location
    Portage, Indiana NEC: 2008
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    9,770

    Re: fluorescent fixtures

    Florescent lights require the bulbs to be mounted within 1/2" of a grounded reflector. the capacitance between the bulb and grounded reflector causes the electrons to flow down the tub more efficiently. The are some two wire fixtures that use a higher voltage or a cap in the ballast to get the same results but many older fixtures needed the ground. try connection a ground wire to it. it will probably light on its own.
    But I have seen where someone would cheat and use the neutral but this is not safe as if the neutral ever opens to the fixture it will leave lethal voltages on the fixture case.

    [ December 21, 2003, 10:30 PM: Message edited by: hurk27 ]
    Wayne A. From: N.W.Indiana
    Be Fair, Be Safe
    Just don't be fairly safe

  7. #7
    Join Date
    Feb 2003
    Location
    Massachusetts
    Posts
    32

    Re: fluorescent fixtures

    yup, no ground, run your finger along the length of the tube and it will probably light.

  8. #8
    Join Date
    Jun 2003
    Location
    Hawthorne, NY
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    2,410

    Re: fluorescent fixtures

    Have you got the correct lamps installed as listed on the ballast? Grounded or not I've seen fixtures do this and I refuse to believe a fixture has to have a ground to operate.

  9. #9
    Join Date
    Feb 2003
    Location
    Massachusetts
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    48,482

    Re: fluorescent fixtures

    Originally posted by hbiss:
    I refuse to believe a fixture has to have a ground to operate.
    Why is a Grounded Fixture Needed for Reliable Starting?

    Many fluorescent fixtures will not start reliably unless they are connected to a solid earth (safety) ground. This is most likely the case with rapid or trigger start magnetic ballasts. These will usually state on the label: "Mount tube within 1/2 inch of grounded metal reflector". If this is not done or if the entire fixture is not grounded, starting will be erratic - possibly taking a long or random amount of time to start or waiting until you brush your hand along the tube.
    The reason is straightforward:

    The metal reflector or your hand provides a capacitive path to ground through the wall of the fluorescent tube. This helps to ionize the gases inside the tube and initiate conduction in the tube. However, once current is flowing from end-to-end, the impedance in the ballast circuit is much much lower than this capacitive path. Thus, the added capacitance is irrelevant once the tube has started.

    The reason that this is required is probably partly one of cost: it is cheaper to manufacture a ballast with slightly lower starting voltage but require the fixture to be grounded - as it should be for safety anyhow.
    Go to this site "Sam's F-Lamp FAQ
    Fluorescent Lamps, Ballasts, and Fixtures
    Principles of Operation, Circuits, Troubleshooting, Repair"

  10. #10
    Join Date
    Jun 2003
    Location
    Hawthorne, NY
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    Re: fluorescent fixtures

    Iwire, your quote is only somebodies opinion.

    I think that the confusion is caused by the ballast manufacturers literature that indeed says that the fixture must be grounded for the lamps to light or light reliably. What you need to understand is that by "grounded" they are saying that the sheet metal fixture must be electrically connected or "grounded" to the metal ballast case. Further, this "grounded" sheetmetal must be within 1/2 inch of the lamps. THAT makes sense, especially if you consider that the circuitry within the ballast could have a connection to the case for this purpose.

    I have not seen a plausable explanation yet for why connecting the fixture to an equipment grounding conductor that just goes back to the neutral would have anything to do with the lamps not lighting.

    Anytime I've seen a problem like this it has been caused by a bad ballast, improper wiring within the fixture, bad lamps or improper lamps. The latter is very easy to do these days and is probably the most common reason for a new fixture not working.

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