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[250.56]. No. This is a commonly misunderstood
point. The NEC doesn't require the earth resistance to be any
particular number. The actual number is an engineering decision. The
NEC simply says that if you drive one ground rod and the earth
resistance is 25 ohms or less then you need to supplement it with an
additional grounding electrode. It's common for electricians to pee
on the rod and then measure--the salty urine is a strong electrolyte
that brings the reading way down from just dry soil. If ever called
upon to repeat the measurement, they pee first and then get people
gathered around to witness the testing.
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[250.58]. No. Doing so would create a dangerous
difference of potential. In fact, they must be grounded to the same
electrode(s).
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[250.60]. No. Air terminals are of entirely
different construction and dimensions. You can use grounding
electrodes in lieu of air terminals, but you can't go the other way
around.
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[250.62]. No. Such pipe can serve as an auxiliary
electrode and it should be bonded to your grounding system to
eliminate dangerous differences of potential. But you cannot use it
as part of the grounding path. It may seem wasteful to run a length
of copper or aluminum wire along a water pipe instead of just
hooking onto it on each end, but it's not wasteful at all. The pipe
is for carrying water, not for carrying lighting or other
undesirable electrical current.
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[250.62]. The answer depends on what materials
your cathodic protection system is constructed of. Use the
appropriate conductor based on that, remembering that it must be
"resistant to any corrosive condition existing at the installation."
On any give site, the cathodic protection systems and grounding
system must be engineered as a unified system, not as separate
systems.
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