Exploring Ground Testing Methods: Enhancing Earth Resistance Checks

In a previous article, I mentioned that Fluke was asking readers to share their experiences with ground testing. So I thought it would be interesting to talk about this topic in more detail this month.

Grounding and equipotential bonding conductors

What does grounding mean? What does a link mean?

Sections 100 and 250 of the 2008 National Electrical Code (NEC) contain several references to “connecting conductors and ground wires”. Unfortunately, this imprecise wording was confusing. This was fixed in the 2011 code. It now refers to “bonding and grounding conductors”.

When installing equipment in a facility, do you ground the metal enclosure with a ground rod? Nb Must be connected to a connecting conductor,sometimes called an equipment ground wire. The goal is to carry the fault current back to the source, where the conductor is connected to the neutral. The current will then trip the circuit breaker. This is a safety measure for personnel in case of overheating of the metal casing of the equipment following a short circuit.

What is grounding used for in the electrical panel of a plant?

Will your ground rod (or “ground electrode” according to the 2011 code) be effective for its intended use, which is to direct surges caused by lightning strikes and other causes of grounding conductors?

When you drive an 8-foot (2.4-meter) earth rod into the ground near an electrical panel and plug it in, how do you know if it will be effective for your intended use?

The usual practice is to ensure that the earth electrode resistance value is less than 25 ohms, and if not, add parallel electrodes spaced at least 1.8 meters (6 feet) apart (more on that later), until the condition is not satisfied .

25 ohms to what?

When the earth electrode to be tested has not yet been connected to the electrical system, or has been temporarily disconnected, it is possible to carry out the potential drop test.

In a typical test on a 2.4 meter (8 ft) by 15 mm (5/8 in) copper grounding rod, the measurement circuit is similar to Figure 1.

During this connection, one of the Fluke 162x Series ground testers generates a known current between the outer rod and the ground electrode, which creates a voltage gradient in the ground. The tester then measures the voltage drop between the inner rod and ground. Ohm’s Law calculations give the resistance between the ground electrode and the inner rod.

Ground test without stakes

In an active system, where the earth electrode is already connected to the neutral, there is a safer solution than the potential drop test. It is not necessary to disconnect the earth from the installation.

This method requires two current clamps (see Figure 2). The first current clamp is used as the primary of a transformer whose secondary is the earth conductor. Using this transformer, the tester applies a voltage source in the circuit formed by the system of grounded equipment outside the plant and switchboard. The second clamp senses current flow in the loop, then Ohm’s Law is again used to calculate the resistance of the loop, which is primarily that of the ground between the plant ground connection and the equipment ground .

I had indicated that parallel earth rods may be required in some installations. It seems common to overlook the need for a test: instead of testing the quality of a single earth rod, a second earth rod parallel to and close to it (but at least 1.8 meters (6 ft) away) is considered a suitable solution. The fact is that due to highly variable ground conditions, there is no guarantee that the “less than 25 ohm” requirement will be met. Nothing can really replace testing. Also, even if the grounding system is tested during installation, there is no guarantee that it will remain functional over time. Corrosion and changes in ground conditions can render the system ineffective. The only solution is to test the system regularly.

Range of earth testers

The good news is that Fluke have a range of earth testers, from the compact 1630 which uses the stakeless method, to the 1625 which can test using four methods, including the stakeless method and the above potential drop method.

The stakeless method is the fastest and most convenient as it does not require disconnection or reconnection of the system earth. It is not suitable for new installations where the power service has not yet been connected. In this case the potential drop test using the three connections is more suitable.

I look forward to hearing what Fluke customer inquiries have yielded on this matter. Maybe new features will be added to make earth resistance checks easier.