I have two adjacent ceiling fixtures holding two 40W 48" fluorescent
tubes, both controlled by a wall switch. Sometimes, they go on
properly; other times they'll go on if their glass tubes are touched;
still other times they need to be rotated slightly in their sockets.
And sometimes they won't go on at all.
What should I check?
The fixtures are at least 18 years old, but do not have the little
starter cans that older fixtures used to.
Even if the fixtures are properly gounded, if there is a thin film of
dirt on the tubes, it can be so thin it is invisible, that film may
keep the bulbs from lighting. Especially if the humidity is high.
Flakey fixtures, such as you describe, are often traced to a problematic
ground connection. The ground for a florescent fixture, unlike its use for
most other electrical devices, is a necessary component of the device in
creating a flux this close to the galactic core - or something like that.
It starts the arc from one end of the 4' tube toward the other end by
initially arcing from the "hot" side of the ballast output toward the
"ground" of the metal fixture, and the arc progresses down the tube
until it stikes fully. That is why a greasy film on the outside of
the tube will sometimes inhibit the initial arc from stiking, in
effect it insulates the tube from the "ground".
This is a generalization/simplification, but is how it really works
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