A high-performance grounding system enhances reliability, reduces maintenance costs, and improves uptime.
For electrical workers, installing a standard grounding system is almost an afterthought — drive two ground rods and you're done. But this is inadequate for today's applications because it creates the potential for unreliable operation and exposes owners to large financial losses.
The requirements for ground resistance levels are more stringent because of the lower operating voltages and higher operating speeds in today's electronics. For example, telecommunications specifiers want an earth resistance of less than 5 ohms, which is less than half the resistance considered high-performance a few years ago. Specifications even vary among industries — some manufacturing processes and medical applications demand less than 3 ohms resistance to earth. We're even seeing “less than 1 ohm” in some specifications.
However, high-performance grounding is more than achieving a low earth resistance. A high-performance ground gives predictable, long-lasting, seasonably stable performance — and it's testable. Testability is important because grounding systems degrade, and damage to them may not be apparent until a catastrophic failure occurs.
The only way to achieve a high-performance grounding system is to engineer it. This allows you to achieve your desired goals without expensive overkill. In other words, an engineered system gives you what you need at the right price.
Begin with the soil.
Soil characteristics play a significant role in total system performance and cost. Accurate soil resistivity data enable precise designs and predictable results. The type of soil, moisture, electrolytic content, and temperature affect soil resistance. Frost line depth, water table level, bedrock, and available space dictate some specifics of the grounding system design. Determine soil resistivity using the Wenner 4 Point test method, or by analyzing soil samples. Soares Book on Grounding contains tables and other resources to help you understand this part of the process.
The design process.
A design firm specializing in grounding system design can provide the best performance ground system available, within the constraints of available area, cost, and equipment being protected. In addition to specialized knowledge, such firms use tools like design software to model a grounding system based on the soil data, design goals, and other factors. After modeling the area, the firm will engineer a grounding system that meets the performance objective — or is the best reasonable effort. When the soil resistivity of an area is so high that a low-resistance ground isn't feasible, bonding and other requirements, such as TVSS, are more critical.
If you're using an outside design firm, make sure it provides a written recommendation along with a drawing of the site footprint illustrating the recommended grounding configuration, quantity and location of electrodes, electrode spacing, and models.
Electrode selection and configuration.
Several electrode types are available. Initial low cost, familiarity, and availability make the copper-clad driven rod a common choice. However, driven rods can have short lives and provide unstable performance. Although the NEC still allows you to use infrastructure components like buried water pipes and building steel as ground electrodes, you can't test them and you can't depend on them in a high-performance ground system (see Sidebar above). Further, the ground path they provide is often a shared lane for noise and transients. Other choices include plates and UFER grounds, but plates are unstable and UFERS are typically not testable.
It's beneficial to consider self-activating, electrolytic grounding systems as well. A 2⅛-in. diameter copper cylinder has better energy-handling capability than conventional driven rods and the electrolytic action dispenses a highly conductive solution that improves performance and provides seasonable stability. At least one such system provides high conductivity (60 ohm-cm resistivity), neutral pH, and moisture retention capabilities.
Bonding vs. isolation.
Some people install isolated ground points in the earth without bonding them to the rest of the facility grounding system, which is dangerous because it creates a potential between electrodes. The myth of the isolated ground as a requirement for sensitive circuits violates the NEC, Ohm's Law, and Kirchoff's Law. An unbounded electrode creates a difference of potential that can damage equipment or electrocute someone.
Bonding allows charges to equalize between objects or surfaces, eliminating the voltage potential between them. Electricity wants to return to the source, so without a bond, the potential increases until the electricity can flow through whatever is between it and the source. Even one bonding error can cause circulating loop conditions, shared noise, and an unpredictable, unstable, and untestable ground system. Remember, lethal, undesirable current doesn't distinguish equipment from people.
Testing and monitoring.
Regardless of grounding system layout, resistance testing is a standard procedure for informed owners of sensitive equipment. The verification of the first resistance results should be part of your maintenance program. This validates design efforts, provides a written statement necessary to obtain the manufacturer's warranty, and establishes a baseline value for trending.
Despite its importance, periodic testing has its weaknesses. If a fault burns a grounding conductor, someone makes a neutral ground bond on the load side, or some other problem arises with your grounding system, you won't know until your next grounding check. And most likely, you'll conduct that check while fixing an urgent, but mysterious, power quality problem causing you downtime.
Continuous monitoring will help you overcome this weakness by warning you early of almost any grounding problems that can arise — and you don't have to disconnect the grounding conductor to find out what your system is doing. Look for a monitor with high-input noise filtration and remote communications abilities, which will allow you to incorporate the instrument into your predictive monitoring scheme. And don't forget the interface — you want something intuitive.
Your grounding system will be only as good as the effort you put into it, and these four-steps are key: test the soil, design the grounding system, install the system per industry standards, and test it frequently. Monitoring helps you keep your system working properly and maintains the high performance you want.
Barsu is an applications engineer with Lyncole Industries, Torrance, Calif.
Sidebar: High-Performance Grounding and the NEC
The purpose of the NEC is to codify the requirements for safety — not performance. The NEC Art. 250 requirement of “25 ohms or less” is a minimum safety standard, long surpassed by the stringent demands of sensitive equipment. Article 250.2 requires every grounding system to be permanent and electrically continuous, and capable of safely carrying the maximum fault current while providing low impedance. A properly designed high performance grounding system doesn't meet NEC requirements — it exceeds them.