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#46545 12/26/04 01:22 PM
Joined: Mar 2004
Posts: 391
BigJohn Offline OP
I have two flourescent fixtures that keep turning on and off at random intervals.

One fixture would occassionally trip the CB so that was replaced with a used fixture that was known to function properly. The CB stopped tripping, however when installed in this new spot the previously-operable fixture also started turning on and off at random times.

I managed to catch the light while it was inoperable to put a volt meter on the branch wiring and it all reads like it should, so for some reason, the problem is somewhere in the fixture. I replaced the ballast on this newly installed fixture and even then the problem continues.

I have not begun to trouble-shoot the second light aside from re-making the fixture-to-branch connections and correcting the missing EGC. However, out of 50+ lights like this, these are the only two that exhibit this problem, and all the fixtures are identical.

All I can remember about them at the moment is that they have 40W universal ballasts, which, I think are for parallel tubes...? Not sure. All these fixtures are underlamped at 32W. I seem to recall hearing that is hard on ballasts-will shorten operational life-but I can't remember positively. Even with all the fixtures underlamped, however, those two are the only ones that have ever had a problem.

I'm thinking from the way these things turn on and off (no flickering or blinking) that it's the thermal disconnect in the ballast kicking out. But what are the odds of three ballasts all having bad thermal cutouts?

I've done the usual tightening- connections/jiggling-conductors test to see if something is loose and that hasn't yeilded any change.

Anyone have any clues?



#46546 12/26/04 04:23 PM
Joined: Oct 2004
Posts: 806

Two thoughts immediately came to mind here:

First, since you had previously had a breaker tripping before replacing the one bad fixture, I'd look very closely for burned feed wiring. The old ballast may have cooked a connection or wire someplace...

Second, since now you have two fixtures acting up, possibly harmonics on the neutral. How many fixtures total on this one circuit? Are these electronic ballasts?

Do any of the ballasts feel excessively hot? Compare temperatures with the fixtures not having problems.

Hope some of this helps!

Stupid should be painful.
#46547 12/26/04 07:13 PM
Joined: Mar 2004
Posts: 391
BigJohn Offline OP

I'm pretty sure the fault was in the fixture itself because I was in the room at one point when I heard a *pop* come from the light and the CB tripped. I inspected all the branch wiring I could see and saw no evidence of arcing or damaged insulation. And though I couldn't find obvious damage in the fixture either, since I replaced it I haven't had any problems with the breaker tripping.

The total number of fixtures on this circuit is only five or six, it's a night-light circuit that's on 24 hours a day. Incidentially, both the problematic lights are on this circuit. However, none of the other lights have been affected.

To be honest I'm not even sure what I'd do to check for or resolve neutral harmonics. Would I check with a averaging current meter and then a true RMS meter and compare the measurements to determine if there's a difference?

And as far as temperature, all these ballasts are toasty. I know they're too hot to handle, but because they require 90C wire, I'd expect them to be at least 75C+. Even if the problem is thermal, I don't understand why it'd only be these two locations. There would have to be something in the enviroment that was making these two fixtures excessively hot and I haven't been able to find anything like that.



#46548 12/26/04 09:04 PM
Joined: Oct 2000
Posts: 2,723
Likes: 1
Broom Pusher and


And as far as temperature, all these ballasts are toasty.

Ahh, that sounds like the problem!

Type P Ballasts have thermal cutouts, which will turn off the incoming AC Power if temperature is excessive.


I know they're too hot to handle, but because they require 90C wire, I'd expect them to be at least 75C+. Even if the problem is thermal, I don't understand why it'd only be these two locations.

Check for wiring faults in the Lamp Circuitry. Things like Lamp Leads faulted to the metallic fixture, pinched Leads, shorted Lamp Holders, etc., and also use of incorrect Lamps / Ballast.

Lastly, check for methods for heat to be entrapped within the fixture - or around the fixture, and also check for methods for heat to become concentrated in the fixtures' area - like an HVAC Supply Diffuser, Infrared Heater(s) aimed at or near the fixture, Incandescent or Quartz-Halogen Lamps operated near the fixture, and etc.


Scott " 35 " Thompson
Just Say NO To Green Eggs And Ham!
#46549 03/14/05 02:34 AM
Joined: Mar 2004
Posts: 391
BigJohn Offline OP
Here's a new twist on this:

Because I can't identify any unusual sources of heat that would affect only these two fixtures, I decided to run one fixture with the sheet-metal reflector over the ballast removed. I figured this would allow more heat to dissipate, so if the problem persisted, I was on the wrong track. I removed the reflector, and here's the kicker: Not only did that fixture stop blinking off as often, but so did the other fixture that's all the way across the room and that I hadn't touched at all!

I am racking my brains and I cannot understand what is going on with these two lights. I'm on the verge of just ripping them out and replacing everything. But I really don't like doing that without knowing what problem it may or may not even correct.

Any ideas at all?


#46550 03/14/05 03:53 PM
Joined: Dec 2000
Posts: 4,294
Magnetic 40 watt ballasts don't have 32 watt T8 lamps listed on their application charts.
They are made to be used with 40 watt or 34 watt T12 lamps.

I've replaced a few of them that have had 32 watt lamps installed.

Could this be the problem? Could the 2 fixtures be just the weakest of the group?
If they run as Night Lights, then they don't get a cooling off period like the others.

There are some major differences in the way magnetic 40w and electronic 32w ballasts operate.

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