For electrical workers, installing a standard grounding system is almost an afterthought — drive two ground rods and you're done. But this practice is inadequate for today's demanding applications because it leaves them open to unreliable operation and exposes their owners to huge financial losses.
The requirements for ground resistance levels are becoming 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 under 5 ohms, which is less than half the resistance that was 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. Believe it or not, we're now 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. Let's look at how that process works.
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 content, 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. You can 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 design process. Supply your soil resistivity data, a site sketch, and the performance objective to the grounding system designer.
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 is not feasible, bonding and other requirements, such as TVSS, are more critical.
If you are using an outside design firm, make sure it provides a written recommendation along with a drawing of the site footprint that illustrates the recommended grounding configuration, quantity and location of electrodes, electrode spacing, and models.
Several electrode types are available. Initial low cost, familiarity, and availability make the copper-clad driven rod a common choice. The disadvantages are that driven rods often have short lives and provide unstable performance. Though the NEC still allows you to use infrastructure components like buried water pipes and building steel as ground electrodes, you can't them and you cannot depend on them in a high-performance ground system (see Sidebar on page xx). 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 2 1/8-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 on the market provides high conductivity (60 ohm-cm resistivity), neutral pH, and moisture retention capabilities. Electrolytic ground rods are maintenance-free and have proven to be stable over long periods — they typically have a 30-yr warranty.
Some people purposely install isolated ground points in the earth without bonding them to the rest of the facility grounding system. That 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 unbonded electrode creates a difference of potential that can damage equipment, or worse, electrocute somebody. The answer is simple: Without violating the NEC, engineer the grounding system for the ohmic value you need.
Bonding allows charges to equalize between objects or surfaces, eliminating the voltage potential between them. Electricity always tries to get back to the source, so when the bond is absent, the potential increases until the electricity can flow through whatever is between it and the source. Even one bonding error can result in circulating loop conditions, shared noise, and a ground system that is unpredictable, unstable and untestable. If you're still unsure about bonding, remember this: Lethal, undesirable current doesn't know equipment from people.
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 obvious use, periodic testing has a serious weakness. 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 overcomes this weakness by warning you early of almost any grounding problems that can possibly arise — and you don't have to disconnect the grounding conductor to find out what your ground system is doing. Look for a monitor with high-input noise filtration. Also look for remote communications abilities, which will allow you to incorporate the instrument into your larger predictive monitoring scheme. And don't forget the interface — you want something you can use intuitively, not something that requires extensive training.
Your grounding system will be only as good as the effort you put into it. Make sure you follow a four-step process: test the soil, design the grounding system, install the system per industry standards, and do the testing. Monitoring helps you keep the system working properly and maintain the high performance you paid for — and the protection it provides.
Barsu is an applications engineer with Lyncole Industries in Torrance, Calif.
Article 90.1 clearly says the purpose of the NEC is 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 does not violate NEC requirements; it exceeds them.