If the metal is not connected to earth then that is not a good "ground"
| What is meant by "connect the copper wire to the grounding conductor of the
| supply circuit." I though any screw and metal made a good ground.
It means to connect the "copper wire", whatever wire that refers to, to the
bare wire of the supply circuit using a wire nut or equivalent (assuming of
course that your supply circuit has the normal ground wire--if this is an
old house with pre-1970s or so wiring, there may be circuits using
ungrounded cable, and in this case you won't be able to produce a ground).
If you mean just attaching the "copper wire" to the box will provide a
ground, this is not generally true, and even if the box is grounded it is
not an acceptable way to ground the fixture. The circuit ground conductor
needs to be directly connected to the fixture ground wire; the box also
needs to be connected to the circuit ground unless it is a plastic box.
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