How can I tell if my building plumbing provides an adequate ground?
http://homeguides.sfgate.com/plumbing-system-electrical-grounding-94939.html

This author above suggests plumbing can supplement and earth ground rod, but not be a primary connection.

Building codes in your area determine the ultimate answer.

Ask relevant city authorities for reference to building codes for commercial ground methods or consult an electrician.

Certainly deep water lines are better than no ground for equipment that needs to protect you from stray leakage from dust and humidity on metal shell electrical tools.

I might measure the AC voltage from the plumbing to Neutral and see how much voltage drop there is.  ( must be &lt; 5% line) ) ... this is grounded outside at the transformer but current can cause a voltage drop.