What reading should I be looking for when performing a ground test?
When testing the resistance of a grounding system (rod, grid, etc), the objective is low resistance. The grounding electrode is expected to be capable of carrying large fault currents to ground, while safely diverting around the electrical system, equipment and people. Therefore, the requirement is quite simple: the lower, the better.
National Electrical Code®
The only prevailing standard, however, is that of the National Electrical Code® (NEC®), which is 25Ω. This code defines ‘effectively grounded’ as intentionally connected to earth through a ground connection or connections of sufficiently low impedance and having sufficient current-carrying capacity to prevent the buildup of voltages that may result in undue hazards to connected equipment or persons.
This is a rather forgiving standard, based on practicality, as well as performance. A residential home can’t be expected to be built on top of a quarter acre grid, like a substation. If a single rod doesn’t meet 25Ω, the Code then only requires a second rod be paralleled. The two together aren’t even required to meet the 25Ω; something is better than nothing, and the facility thereby has basic protection against fire and electrocution.
There are many different types of ground testers to choose from depending on the application or other factors. A four-terminal model is a must for testing conductivity of the soil itself, whereas a three-terminal model is used for installation or maintenance testing. It can be useful to have extra digits on display for improved accuracy and resolution for both types of measurements. A fourth terminal can come in handy if the small amount of lead resistance must be eliminated in order to make especially low resistance measurements.
The code isn’t about performance
Commercial facilities also need to be grounded for noise mitigation and stray currents that, while not hazardous in terms of fire and electrocution, can wreak havoc on the operation of sensitive high-tech equipment. By this yardstick, the basic rule of thumb for commercial and industrial grounding is 5Ω. Even 10Ω may be marginally allowable if the performance requirements aren’t considered too demanding. But in the right direction, even 5Ω can be a bit high for optimal performance in the most sensitive situations.
Therefore, computer room grounds, telephone central offices, utility substations and the like are often required to have a 2Ω ground, or even less than 1Ω. Certain industries have established their own standard, while individual companies may have done likewise. The ultimate authority, though, is the electrical design engineer who drew the plans.