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When you connect the
electrical tester to the hot and ground wires, you draw a
small amount of power. The Ground Fault Circuit Interrupter
senses that a small amount of current is going out the hot
wire and not coming back down the neutral (white) wire,
so it trips. It can't tell if the power is going through a
circuit tester or your left arm, but it knows some electrons
are going in the wrong place.
Some electrical testers (such
as the little plug-in-the-wall testers) have indicators for "open
hot". This means that there is no voltage on the hot
wire. It could be caused by something as simple as a switch
being off or a GFI being tripped. If the photocell is
receiving light, then it could be holding a relay open
somewhere, a situation I've seen in commercial wiring. But if
the photocell is built into your fixture (common with
residential fixtures) then there are probably some internal
electronics that control the power to the bulb socket. You may
not be able to repair such a fixture.
My first approach to
something like this is make sure there is power available to
the devices that require it. If your photocell is separate
from the fixture, there may be a relay somewhere (I believe
they are called "contactors") that needs to have
power applied to it. Once that is verified, the photocell
needs to be tricked (by covering it with electrical tape) to
think it's night. But... some photocells won't activate
immediately, rather, they have a time delay. You may need to
wait a few minutes.
When I worked in hotel
maintenance, I once spent half a day resolving a tangle of
outdoor flood lights and a photocell. There were problems with
corroded wiring at some light fixtures, and there was also a
problem with a corroded connection in the photocell wiring.
Sometimes the only way to resolve such problems is to isolate
each and every wire and verify continuity with a voltage
tester. Even then, I've seen wires show continuity with a
digital meter but not show continuity with a battery powered
automotive-type continuity light, which applies a higher
voltage. That's a sure sign of corrosion... it almost appears
that Ohm's Law is being violated, but it's really a matter of
what amounts to a "variable resistance" at some
point of corrosion... higher current causes the corroded metal
to change somehow, and raises the resistance. It sounds
bizarre, but I've seen it on at least three occasions.
One thing I would do is dis-assemble
all of the outdoor connections, and examine them for
corrosion, and re-assemble everything correctly. Of course, do
one connection at a time.
Most photocells I've seen
produce a very small voltage when exposed to light, perhaps
around 1 volt. But your unit may employ some internal
electronics that make the device act like a switch. It would
help to have specifications for the photocell, you may be able
to call some electrical supply stores and find out what the
photocell is supposed to do, does it have an internal relay or
something?
If that doesn't help, maybe
an electrician is needed. Ouch, that hurts the wallet! I
imagine they'll charge by the hour until they find the
problem, and then they'll charge you some more.
Bruce W. Maki, Editor.
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