We recommend that single-electrode grounds be tested when installed, and periodically afterward. We italicized that last phrase because of its importance. Resistance to earth can vary with changes in climate and temperature. Such changes can be considerable. An earth electrode that was good (low-resistance) when installed may not stay that way; to be sure, you must check it periodically.

We cannot tell you what your maximum earth resistance should be. For specific systems in definite locations, specification are often set. Some call for 5 ohms maximum; others accept no more than 1 ohm. In certain cases, resistances as low as a small fraction of an ohm are required.