One fixture sometimes won't light

Old 09-02-20, 04:41 AM
pugsl's Avatar
Forum Topic Moderator
Thread Starter
Join Date: Mar 2008
Location: United States
Posts: 8,161
Received 77 Upvotes on 70 Posts
One fixture sometimes won't light

One fixture sometimes won't light in a string of fixtures.Last fixture on string (I think) 2 bulbs (i think) per fixture. Have not gone up there to see Looks like a commercial fixture with 2 lights or florescent bulbs in each fixture. The last one in string sometimes need the switch to be clicked a couple of times before it lights. Have 2 other of the same sets on lights that work fine. Each set on a separate switch. Why would I need to flick switch a couple of times to work?
Old 09-02-20, 05:40 AM
Geochurchi's Avatar
Join Date: Nov 2012
Posts: 5,082
Received 161 Upvotes on 147 Posts
Hi, have you changed the lamps? are they black or grey on the the ends? Maybe time to switch to LED replacement lamps.

Old 09-02-20, 05:50 AM
Join Date: Sep 2019
Location: Canada
Posts: 1,955
Received 295 Upvotes on 265 Posts
Try replaci9ng the bulbs.
If that does nothing then odds are that the ballast is going.
So you might want to switch it over to LEDs.
Old 09-02-20, 05:54 PM
Join Date: Jul 2014
Posts: 58
Upvotes: 0
Received 5 Upvotes on 5 Posts
Oh, this fun question again. It's old rapid-start fluorescent won't light unless I touch the lamps or flip the switch repeatedly. Most of the time these days people will tell you to swap them out with LED or at least install a modern electronic fluorescent ballast. However, you can troubleshoot it if you wish. There are several possible causes.

1. The fixture is not grounded. Rapid-start requires a ground in order to offer a capacitive path aiding to ionize the gas and enable conduction through the lamps. If you've ever seen a plasma ball where you touch it and the filament of light jumps to your finger, it's a similar concept. Check that the ground wire or conduit is secure, and if possible, use a multimeter to verify the reflector is in fact grounded. The reflector should be present and secure.

2. One of two pins at an end of the lamp is not making good contact. Rapid-start fluorescents may unreliably start if only one of the two pins at one side or the other is connected. Unlike the grounding issue, this will result in rapid blackening on that end of the lamp and after a while, it will quit working altogether. This might be caused by a broken/bent socket or bad wire connection to the socket.

2. The lamps (tubes) cathodes are simply worn out. Once they wear out completely, they won't light at all (beyond a dim glow, perhaps). Although borderline lamps do not always start intermittently and sporadic starting does not always suggest failing tubes, sometimes it simply is the bulbs.

3. The lamps cathodes are not technically worn (little blackening), but the tube is either low on mercury or contains contaminates that make it tougher to start. Regardless, the solution in this case is to replace the tube. Note some brand new low-mercury lamps may have trouble starting or be dim out of the package, but they should improve after a few hours of run-time.

4. The ambient humidity is excessively high or the temperature is very low. High humidity can interfere with the capacitive path to ground and cold temperatures can increase the required starting voltage beyond what a non-cold temp rated ballast can provide.

5. The wrong type of lamps/tubes are installed. For instance, F32T8 lamps have a higher starting voltage requirement than the older F40T12 and might not light dependably in a ballast not rated for them. Check the other fixtures, or even better, the ballast label to verify the lamp type.

6. The ballast itself isn't delivering sufficient OCV or cathode heating voltage due to a defect. The ballast isn't as often to blame as some might suspect, but it can be the point of failure.

Even if the ballast is technically good, replacing it might fix the issue due the higher voltage output of modern electronic ballasts. Although it's best for a fixture to be grounded and for all lamp pins to be making snug contact regardless, often an electronic ballast will still operate with one of these defects present (there might be some increased cathode wear resulting in if only one of the two pins is in the circuit, however, so best to correct it to avoid premature lamp burn-out).

If everything looks good but this happens anyway, or it's due to a factor such as temperature or humidity that you can't change, there are some quick workarounds. One is to adhere a strip of foil tape lengthwise along the tube, on the side facing the metal reflector. This increases conductivity. I also heard not long ago about somebody trying silicone spray and apparently that helped, but I've never done that. I used the foil trick in a cold garage before and it actually made a difference. I just had to be careful the foil didn't come into contact with any electrical components. If the tubes are dirty, merely cleaning them might be enough.

Also, if these are single-pin slimline lamps, you can disregard any faults that involve one of two pins making contact, as that would not be possible.

So there you go, I think that's about everything. Not unsurprisingly many people jump right to swapping out the fixture, converting to LED, or changing the ballast rather than deal with all the possible causes. But if you have the time, you can track down where the fault lies.

Last edited by pjcpc1; 09-02-20 at 06:08 PM. Reason: extra suggestion

Thread Tools
Search this Thread
Ask a Question
Question Title:
Your question will be posted in: