Find out why the National Electrical Code "25 ohms or less" rule may have less to do with power quality than you think.
Nearly all electricians and electrical inspectors are familiar with the National Electrical Code requirement in Sec. 250-54, which requires the resistance to ground of a single-made electrode (e.g., ground rod) to be 25 ohms or less. Unfortunately, it seems many electrical professionals don't actually test the grounding electrode system (GES) to ensure they're meeting this requirement. Even fewer of you feel testing the earth ground system is worthwhile. From a power quality perspective, you may be right.
A GES provides:
• A zero-volt reference for the supplied or derived power systems.
• A path to dissipate lightning or fault current (for higher voltage systems).
• A path for the dissipation of electrostatic currents.
A GES consists of two components: the grounding electrode conductor (GEC) and the grounding electrode.
You can choose a bare or insulated GEC (sized per Table 250-66) in copper or aluminum. The GEC connects the grounding electrode to the grounded circuit conductor, the equipment-grounding conductor, or both, at the main service equipment or the source of a separately derived system.
The most common types of grounding electrodes (identified by Sec. 250-50 and 250-52) are:
• Structural steel
• Metal underground water pipe
• Ground ring
• Ground rods
How to test. You should measure the resistance of an electrode with respect to the surrounding soil in the area. You can only do this by using the fall-of-potential method with a three-terminal, earth ground resistance tester. To properly test the resistance of a GES, you must follow some simple rules:
1. Disconnect the electrode under test from the rest of the electrical system. Considering this, it is not possible to test the grounding electrode system in nearly all circumstances.
2. Don't use a meter that injects DC current into the ground rod. Do not use standard VOMs.
3. Don't perform test measurements if the current on a GES is greater than 5A.
Contrary to popular belief, clamp-on earth ground resistance testers can be inaccurate in field applications. These testers require a low-resistance feedback loop with adequate spacing between electrode systems to provide meaningful readings. Many people often add a high resistance (caused by loose connections in the feedback loop) to the displayed value of the meter. Also, inadequate spacing between electrodes results in the meter only making a comparative bonding test, which almost always results in a low-resistance value.
Why do I need to reach 25 ohms? The most credible answer to this question is: 25 ohms is a reasonable value to strive for, given the average soil resistivity for most regions of the United States. Keep in mind, however, that 25 ohms is not a requirement when you install multiple electrodes. This is only a requirement for single-made electrodes, per Sec. 250-56. If you drive the first rod and get a resistance reading greater than 25 ohms, the NEC allows you to drive an additional rod 6 ft away from the first rod.
Let's say, for example, you drive a ground rod into the soil, but instead of testing that rod to see if it meets the 25-ohm criteria you drive the second. Once the two rods bond together, consider the GES complete. But if you don't take a measurement, how do you know your installation meets Code?
Reality check. In most commercial and industrial low-voltage power systems, technicians do not perform earth ground resistance testing. But this shouldn't surprise you. An informal poll of 50 electricians found only four performed earth ground testing in the past. The reasons cited for not testing were:
• The testers were too expensive.
• The test was too confusing and took too much time.
• Two rods are good enough (most common response).
Impact of power quality. Believe it or not, nearly all electronic equipment will operate properly without the benefit of a low-resistance GES. Power quality site surveys have shown that in situations where the grounding electrode resistance is between 5 ohms and 105 ohms, it doesn't affect equipment. However, you can trace most problems to poor quality connections on the equipment-grounding system. Therefore, you should pay less attention to the GES measurement and more to the impedance of the equipment-grounding system and the verification of low-resistance bonding connections between grounding planes.
What's the lesson here? Spend less time on the testing and qualification of the GES resistance and more time checking the bonding between locations and the equipment-grounding conductor impedance.