Yes, the primary function of grounding is to protect against shocks and
electrocution. Of course, it can also protect against outside forces like
lightning or a live wire contacting something it shouldn't (powerline
fall, shorted equipment, etc.). Assuming a broken neutral, a good ground
should give a current path that would trip the breaker/fuse/etc.
According to the 2002 NEC:
250.52A1 - Use metal water pipe as ground if 10 feet or more is in
contact with the earth.
250.53D2 - Water pipe must be supplemented with other grounds (not the
Metal water piping actually makes a good electrical ground, since it
often runs a long distance through the earth. The main issue with water
pipe as the sole ground is that old plumbing is often replaced with
plastic pipe which would leave the electrical system ungrounded.
Pipe doesn't need to be grounded. Until it comes in contact with
electricity, it's just a metal tube (barring static or galvanic
Plastic pipe doesn't conduct electricity anyway, and if it's metal and
running through the earth it is grounded already.
250.50 - Bond all grounding electrodes (ground rods, water piping, etc.)
The point is to ensure there is always a ground (in case the pipe is
replaced, or the ground rods are damaged) for the electrical system.
Also, bonding the various grounds together prevents a voltage difference
between the grounding systems (for instance, in case a power line and
waterline are in a trench and hit by someone digging. Breaks the
waterline as the ground, but energizes the pipe. Thus the need to
supplement with additional grounds).
To my knowledge, there is no mention of grounding in the plumbing codes.
But the electrical codes are obsessed with grounding.
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