Which of the following is specifically not permitted to be used as a grounding electrode?
- AA metal underground water pipe in direct contact with earth for 10 feet or more
Underground metal water pipe with at least 10 feet of earth contact is the first electrode listed, subject to the requirement that it be supplemented by another electrode.
- A metal underground gas piping system
- CA concrete-encased 20-foot length of 1/2 inch steel reinforcing bar
Twenty feet of half-inch reinforcing bar encased in concrete in contact with earth is the concrete-encased electrode, one of the most effective electrodes available.
- DA ground ring of 20 feet of bare 2 AWG copper buried below the frost line
A ground ring of at least 20 feet of bare copper not smaller than 2 AWG is a recognized electrode when installed at the required depth.
Why B is correct
The list of recognized electrodes is short, and the list of things specifically excluded is shorter still: underground metal gas piping and aluminum electrodes. Gas piping is excluded because deliberately routing fault or lightning current through a fuel line is unacceptable, and because gas systems commonly include insulating fittings that break the path anyway.
What this question is testing
The item tests whether the candidate knows the short list of excluded electrodes and can separate bonding a metal piping system from using it as an electrode.
On the job
Gas piping gets proposed as an electrode surprisingly often on rural and older jobs where nothing else is convenient, and the clamp is easy to install. It is a flat violation and a safety problem that a plumber replacing a section with a dielectric union can make worse without knowing. The correct move is a driven rod or a concrete-encased electrode, plus bonding the gas piping through the equipment grounding conductor of the circuit likely to energize it.
Memory technique
Bond the gas line. Never ground to it.
Exam tip
Two things are excluded outright as electrodes: underground gas piping and aluminum. Everything else is a question of meeting the conditions.
Where to look it up
Look in the grounding article at the section listing grounding electrodes, and read the subsection covering electrodes not permitted for grounding.