...that is the question. Whether 'tis nobler in the minds of men to earth test with four leads, or three. Megger^® Ground Resistance Testers are available in both three- and four-terminal models. Deciding on which type suits your needs may be your first decision on the way to selecting a Ground Resistance Tester. Of the six available models, two are three-terminal types (DET62D & 250260), and are comparatively economical. The remaining four (DET3/2, DET5/4R, DET5/4D, & DET2/2) have a full complement of four terminals, and cost more. What is involved in making this determination?

There are just two prime considerations in deciding upon the number of terminals (hence, number of test leads) required: resistivity and resolution. This note will deal with resistivity.

The term resistivity is easily confused with resistance, and sometimes the two terms are used interchangeably (and incorrectly). Resistance refers to the primary measurement performed by an earth tester, the resistance relationship between a buried electrode and the surrounding soil. It is given in Ohms. Resistivity, on the other hand, has nothing to due with any installed electrical structure, but is a pure measurement of the electrical conductivity of the soil itself, and is commonly rendered in Ohm-centimeters.

All can perform a resistance test, in full conformance with the Fall of Potential Method accepted by IEEE. However, to perform resistivity measurements, in accordance with the established Wenner Method, you must have a four-terminal model.

The full four terminals are used to improve the accuracy and resolution of the measurement by eliminating lead and contact resistance. All resistance measurements can be made optionally between either a three- or four-terminal configuration. Three-terminal is the quicker and simpler, with one less lead to string. (Four-terminal models can all be set up in this manner: DET3/2 has an internal connection that can be engaged between the C₁ and P₁, the operator selects the "3 POLE" test button on DET5/4R & D, and the terminals can be physically connected with a jumper on DET2/2. Be sure to connect the lead to the current terminal of an electrically shunted pair, not the potential terminal, as they have a resistive connection internally and are not dead shorted.)

However, use of all four terminals establishes a true four-wire Kelvin bridge measurement that is free of any lead or contact resistance. If the operator is only trying to meet the standard NEC requirement of 25W or less, the few tenths that are normally contributed by the common lead are not a significant part of the measurement, and a quicker three-terminal test is sufficient. But to check, for instance, a computer room ground, that may be mandated as low as 1W , the extra work of running a fourth lead may be worth the effort, by resulting in a clear, sharp measurement of only the ground resistance.

Once the number of terminals has been considered, go on to examine ranges, noise protection, power source, and other features, to make the final determination of the model that best suits the operator's requirements and preferences. Just remember, if some other feature catches your attention, you can use a four-terminal model in a three-terminal application, but not the other way around.