Grounding a Plumbing Drain Pipe

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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 sole electrode)
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 potentials).
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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