A switch controls a bank of 2x4 drop in, 277, 3 tube bi-pin fluorescent lights with electronic ballasts. One fixture, in the middle of the run is not working. The ballast was replaced, 2 new tubes are put in and it works, put the third tube in an the light goes out. The fixture is replaced with another from a different location. The third tube is put in and the light goes out. The switch is flipped a couple of times and all of the light come on. The switch is flipped again, the fixture and the one next to it go out. The original fixture is put in another location and works fine. What could it be?
First, I'd bypass the switch to see if that's the problem.
Then I'd do a voltage reading under load at the middle fixture to see if there is a bad connection somewhere causing a VD or intermittent power.
Then, I'd go through the lamp sockets and internal wiring of the fixtures to see if there is anything causing the short-circuit protection of the ballasts to activate. Get the fixture lit and then jiggle the lamp a few times to see if causes the fault/shutdown.
IS IT POSSIBLE THAT THE BANK OF LIGHTS THAT YOU ARE HAVING PROBLEMS WITH HAS VOLTAGE PROBLEMS? THERE MAY BE VOLTAGE DROP ISSUES DUE TO DISTANCE OR WIRE SIZE. DON'T FORGET ELECTRONIC BALLASTS GENERATE ALOT OF HARMONICS THAT MIGHT AFFECT THE CIRCUIT.
Why is it the fixture in the middle and the one next to it, in a bank of about 10 lights. All of the lights are controlled by one switch. The other lights work fine. There is also another problem in another office in the same building, but on another floor.
Are you lamping these while energized? I have read on certain fluorescent ballasts before "do not lamp while energized. Lamp will not properly light"
Is this switch rarely operated and left on overnight? Just a stab, but.... excersing the switch reset the ballast that was lamped live and the light next to it was just waiting for a reason not to come back on, it was close to its end of life.