Consistent adherence to IEEE Standard 142 would save billions of dollars annually over today's common practices. What makes this standard so powerful?
To many of us, properly designing and installing a commercial or industrial grounding system is often a confusing process. Fortunately, IEEE Standard 142 provides all the information you need to clear up confusion and develop a good grounding design. Its references and bibliographies are impressive evidence of the authority and technical accuracy by which this standard speaks. However, the key to this standard lies in the IEEE logo watermarked on each page.
The standard applies to industrial and commercial power systems. The first chapter is basically an introduction to grounding theory. You can divide the types of grounding systems into two main categories: solid grounding and impedance grounding. Solid grounding is straightforward, and the standard describes how to do it. Impedance grounding is more complicated, but the standard provides clear explanations of the key concepts. To understand this subject, you can divide impedance grounding into the subcategories of reactance grounding, resistance grounding, and ground-fault-neutralizer grounding. The standard describes each of these in detail. It also gives you guidelines for a third category: ungrounded systems.
How do you know which type of grounding to use for your particular situation? The standard will tell you. It will explain what works best with a single power source, multiple power sources, parallel generators, unparalleled generators, and uninterruptible power supply (UPS) systems. And if you want to know how to reduce common-mode noise through proper grounding-system design, Chapter 5 will tell you how to do that, too!
The first chapter explains terms you should know. You can find similar definitions in IEEE Standard 100 (a 1300-page glossary) and the National Electrical Code (NEC) Art. 100. You should be using the NEC, so we won't repeat the material here. Misunderstanding of Arts. 90 and 100 is widespread, as proven by case histories and the responses given in quizzes at grounding and bonding seminars. If you haven't reviewed these articles lately, you'll benefit from doing so now.
Equipment grounding. This is an area of neglect, and - in some cases - contention in the industry. Chapter 2 of this standard, written many years before last year's attack on NFPA 780, is clear about the need to bond all nonelectrical metallic elements of a system so there is no potential between them - and thus no hazard of flashover. The idea that any device could eliminate the need for bonding violates fundamental principles of electrical engineering. This chapter explains the theory and application of equipment grounding, which has three main objectives:
- To reduce electric-shock hazard to personnel;
- To provide adequate current-carrying capability - in magnitude and duration - to accept the ground-fault current permitted by the overcurrent protection system without creating a fire or explosive hazard to building or contents; and
- To provide a low-impedance return path for ground-fault current.
The problems in designing an effective equipment-grounding system vary immensely with the class of use. The objectives are the same - it's the getting there that changes, depending on what you are trying to ground. The standard provides a road map for each classification:
1. Outdoor, open-frame substations,
2. Outdoor unit stations,
3. Outdoor, portable heavy equipment,
4. Interior wiring systems, and
5. Interior substations and wiring centers.
Static- and lightning-protection grounding. You can think of Chapter 3 as a complement to NFPA 780. The first part of this chapter focuses on detecting and controlling static, and forms the bulk of the chapter. It gives details on how to ground metal rollers and shafts, how to use active means of static reduction, and how to reduce static ondrive belts. It also contains specific recommendations for specific industries. The second part gives you a concise overview of lightning protection theory and application.
Earth isn't necessarily ground. A common mistake in this industry is confusing earth and ground. As explained in this month's cover story, they aren't always the same. When you consider aircraft and spacecraft, this point is obvious - but the information in Chapter 4 tells you why this is so. This chapter also explains connections to earth, and why you must bond grounding electrodes.
From the tables and charts in Chapter 4, you can determine electrode resistance as a function of distance from the electrode surface; the resistivity of the soil you are working with; the effect of a given moisture content on a given soil type; the effect of temperature on soil resistivity; and the current-carrying ability of a given size of rebar. And there's more. For example, one table has formulas for calculating resistance to ground for any of 14 different grounding configurations.
According to the standard, copper-clad steel isn't the best choice of ground rod material. The copper can cause severe corrosion of nearby structures if used in the wrong application. One misperception people have is that the copper coating results in a lower resistance to earth. In fact, there is no measurable difference in earth resistance if you use galvanized or stainless rods instead of copper-clad. This fact is apparent when you look at the formulas for calculating earth resistance - the rod material isn't even a consideration.
A new chapter on grounding. The 1980s saw a rapid expansion of personal computers, programmable logic controllers (PLCs), and distributed control systems (DCSs). Causing that expansion were developments such as the Motorola 68000 microprocessor (used in Bailey Controls' DCSs and Apple computers) and the Intel 80286 processor (whose 80386 descendant ushered in the Wintel era). These devices crammed millions of transistors into small physical packages - vastly increasing vulnerability to power anomalies. To the chagrin of many owners of these devices, the very systems that powered them often destroyed them.
While you can buy a Pentium III processor for about $100 today, those early processors - and their support chips - were costly. And downtime is always costly. Thus, in 1991, the IEEE addressed electronic equipment - all of which is sensitive - with a new chapter in this standard.
This chapter points out the fallacy of the popular notion that the NEC favors ground rods. What gives you protection is bonding. To maximize equipment protection, form your grounding electrode system by bonding together certain items first (see sidebar, above) and then consider supplementing it with ground rods. Doing this right means a lower cost of installation and a better grounding system.
One impetus for developing this chapter is the misunderstood "isolated ground." The standard states, "there is no real isolation between electrodes." It goes on to say the earth has a resistance, and you must consider the earth an electrical element (e.g., impedance) in an electrical circuit.
The standard states that "there is always a current flowing in the earth, which produces voltage differences in electrodes - even a few feet apart. Not only are there differences, but these differences vary from second to second, particularly in an electrical storm."
The isolated ground often recommended or required by manufacturers is effectively a capacitor waiting to discharge. Sure, such systems often eliminate continuous low-level noise. But case histories show such systems inject high voltages into the equipment they are allegedly protecting. You can use Ohm's Law to predict this result. The standard explains exactly how it happens and how to prevent it without introducing a "noisy ground."
The standards-making bodies codify and explain the collective wisdom of our industry. These bodies include NETA (electrical testing), the NFPA (fire prevention and safety), and the IEEE (electrical engineering). You'll find crossover and cross-reference between these standards, because all of these groups base their standards on proven science and acceptable practice.
Without standards, you are unlikely to achieve a good grounding scheme. Fortunately, IEEE-142 is a solid foundation on which to build your grounding system.
Sidebar 1: A More Powerful Standard
IEEE-142-1991 is nearly twice the size of its predecessor. It includes more definitions and explanations. It incorporates and references the National Electrical Code (NEC) and other standards. These changes endowed the standard with great practical value. The revision also introduced Chapter 5, "Sensitive Electronic Equipment Grounding."
Sidebar 2: Building a Grounding System
When you design a grounding system, use these items first and bond them together:
1. Metal underground water pipe,
2. Metal frame of the building (where effectively grounded),
3. Concrete-encased electrode, and
4. Ground ring.
If these items aren't available, Standard 142 says, "then and only then can you use any of the following:"
1. Other local metal underground systems or structures,
2. Rod and pipe electrodes, and
3. Plate electrodes.