Thank you for your question regarding Ufer grounding, it is our pleasure to help.
Ufer grounding (steel rebar and/or copper encased in concrete) can be an excellent safety grid, but it must be analyzed as there are a number of factors involved. Human safety in high-voltage environments is based on Step & Touch Voltage Hazards, and those can certainly form even on concrete foundations. Especially if there are no copper paths in the Ufer system. Some of the determining factors include: the soil resistivity model for the site (does the resistivity go up or down with depth), impedance and magnetism of the steel rebar, fault currents, X/R ratio, clearing time, leakage currents, physical makeup of the structure (second story and higher systems are particularly bad locations), and many more.
Of course, if your steel rebar is plastic coated or if you have a vapor barrier preventing direct contact of the concrete to the earth, you cannot use it as a Ufer ground. Also, the latest addition of the NEC 2011 requires at least one dedicated grounding electrode under 25-ohms. A second ground rod is required if you cannot test the electrode or if it is not under 25 ohms. You may no longer simply rely on building steel and water pipe. You must drive ground rods.
We hope this helps. If you should have any further questions please do not hesitate to contact us at 310-718-7151 California time.
The Engineering Team at E&S Grounding Solutions
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