The National Electrical Code (NEC) permits, in certain very limited situations, installation of an ungrounded electrical system. But even here, as in all electrical work, an equipment grounding conductor is required to establish ground continuity for any non-current carrying metal parts including enclosures, fittings, conduit, and so on. It follows that there must be one or more grounding electrodes. It is not, as a rule, permitted to pick up the equipment ground from a grounded neutral that is downstream from the entrance panel.
The grounding electrode is a conducting object that establishes a direct connection to the earth. This grounding system may take the form of one or more ground rods, a ground plate, buried metal waterline, ground ring, reinforcing rod embedded in concrete that is in contact with the earth, or structural building steel that is solidly grounded. These types of grounding electrodes can be used to good effect in combination. Multiple units can be connected in series, the more the merrier.
The fact is that if you simply drive a ground rod below grade, it may have a ground impedance of many ohms. To measure ground impedance with an ohmmeter would require an adjacent known perfect ground, in which case further grounding would not be needed. Ground impedance can be measured using specialized test equipment and procedures, but this is not generally done.
The NEC states that a single rod, pipe or plate electrode is to be supplemented by an additional grounding electrode. An exception provides that if the single grounding electrode has a resistance to earth of 25 Ω or less, the supplemental grounding electrode is not required. Rather than make this difficult measurement, most electricians simply install the second grounding electrode. The usual installation consists of two ground rods.
Two ground rods should be more than six feet apart. This is because overlapping electrical fields interfere with one another, reducing their efficiency. The recommended procedure is to drive below grade two or more ground rods including ground clamps and attached copper grounding electrode wire of adequate size. It can be bare, covered or insulated, solid or stranded.
In a future article, we’ll discuss bonding. This is separate though related to grounding. You could say that bonding is more important than grounding, but that would be misleading because both are essential. Design and installation of both should be impeccable to ensure safety from the standpoint of electrical fire and shock.
Filed Under: Test & Measurement Tips