| I couldn't find any other user group, so if this is not the right one, I
This is a good enough place for this issue.
| I have some knob and tube wiring accessible from my basement going to
| several old (2 wire) receptacles. Can I simply connect a ground(ing)
| wire from a near-by gas pipe to new 3 wire receptacles, or must the
| ground(ing) wire be connected to the ground bus of the main circuit
| panel box?
It MUST be connected to the ground bus of the main circuit panel. Since
you mentioned "main" I presume either the circuits involved truly originate
only at the main panel, as opposed to a sub panel, or you only have one
Although the grounding wire does not normally carry electrical current, it
would carry such current when there is a short circuit between a line wire
inside an appliance or outlet box, and the containing grounded box. The
intent is for this short circuit to briefly carry a high enough current to
quickly burn out a fuse or trip a circuit breaker. What you do not want
is for such a short circuit to ignite a gas explosion.
Furthermore, electricity travels in a loop (called a circuit). That's why
there are two wires (or more in certain cases) carrying current in any
circuit. There is also a magnetic field established between these wires.
By keeping the wires close together, that magnetic field is limited in size.
If the wires were spread apart too far, the magenetic field would be wider
and would induce electrical currents on other metallic objects. The
grounding wire needs to follow the other two wires in the circuit for this
reason. That short circuit that could happen could result in a very large
current and strong magnetic field for a brief instant (until the fuse or
breaker opens the circuit), and that field could cause other problems in
addition to impeding the fault current flow intended to blow the fuse or
trip the breaker.
Also, the gas pipe itself may not have a good solid return path to the
electrical source to allow the full fault current flow.
Ask yourself: Do I think it is cool to have 1000 amps of electricity flow
over my gas pipe? Do I even want to live nearby anyone who does that?
With all that said, I would not add grounding to any knob and tube circuit.
Either I would leave the K&T circuit untouched, or replace it entirely with
an appropriate wiring job with NM, UF, or MC cables, or THHN or THWN in
conduit, depending on the circumstances and local requirements.
Do note that a GFCI outlet _will_
work (it will both provide the intended
protection as well as having the test button function correctly) on an
ungrounded circuit. You would just need to mark the outlet as having no
grounding (I might fill in the grounding pin hole with a hard resin, but
I don't know if that is legal to modify it as such).
| Phil Howard KA9WGN (ka9wgn.ham.org) / Do not send to the address below |
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