This standard seeks to bring order to the apparent chaos of power quality assurance by describing recommended practices for powering and grounding sensitive electronic equipment.
As electronic loads proliferate and take on more important roles in companies and our lives, so do power quality concerns. A recent addition to the Institute of Electrical and Electronic Engineers (IEEE) color book series, IEEE Standard 1100 (Emerald Book), Recommended Practice for Powering and Grounding Sensitive Electronic Equipment, seeks to bring order to the apparent chaos of power quality assurance by doing exactly what its title says, particularly in industrial and commercial power systems. The book is the result of efforts of a number of IEEE experts in the relatively new power quality industry. The origin of the book was May 1985, when the industry's help in developing this new IEEE standard was solicited. The culmination of the working group's efforts was the first edition of the Emerald Book, which was approved by the IEEE Standards Board in June 1992. It's a regular reference in EC&M's PQ Corner and is a virtual tutorial on the subject of power quality.
The Emerald Book is divided into nine chapters. Chapter 1 is the introduction; it describes the scope and background of the standard. The highlights of the other eight chapters are as follows.
Chapter 2, Definitions
The power quality industry, being relatively new, suffers from a lack of consistent terms, with definitions having widespread use. This lack of common terminology inhibits clear communication and promotes confusion. A sampling of the definitions is included to promote the cause of understanding and with the hope of banishing the imprecise street language still encountered.
"Power quality. The concept of powering and grounding sensitive electronic equipment in a manner that is suitable to the operation of that equipment."
This definition lays the foundation for understanding power quality problems that result when the powering and grounding are not suitable for the equipment. Stated another way, a power quality problem results when there is a difference between the quality of power supplied and the quality of power required for reliable load equipment operation. When viewed in this context, power quality problems can be solved in one of three ways:
* Reducing the magnitude or frequency of occurrence of power supply variations, including wiring and grounding problems.
* Improving the tolerance of the load equipment to the power supply variations.
* Adding power conditioning equipment between the power supply and sensitive load equipment to mitigate the power supply variations to a level acceptable to the load equipment.
"Ground. A conducting connection, whether intentional or accidental, by which an electric circuit or equipment is connected to the earth, or to some conducting body of relatively large extent that serves in place of the earth."
It's important you note that ground connections can be "intentional or accidental." Many accidental ground connections not shown on any drawings occur in normal building electrical systems, such as capacitive ground connections and the multitude of ground connections that occur every time a metal conduit or enclosure is attached to the building. Further, it's important you note that "ground" is not necessarily the "earth." For example, the airframe becomes the ground for an airplane. Likewise, the surroundings (building steel) on the 40th floor of a high-rise becomes "ground" rather than the earth 40 floors away.
"Ground loop. A potentially detrimental loop formed when two or more points in an electrical system that are nominally at ground potential are connected by a conducting path such that either or both points are not at the same ground potential." You should note that the term "ground loop" is not a dirty term; the above definition states that a ground loop is only potentially detrimental. In subsequent chapters of the Emerald Book, you'll find that ground loops may be detrimental or helpful, depending upon the frequency range of interest. For interconnected analog systems (audio and video systems and some process controls where the signal frequencies of interest include the power system frequencies), ground loops are detrimental and need to be avoided. However, for digital circuits, where higher frequencies are of interest, multiple ground connections (with ground loops) become necessary to equalize potentials at high frequencies.
"Isolated equipment ground. An insulated equipment grounding conductor run in the same conduit or raceway as the supply conductors...originates at an isolated ground type receptacle or equipment input terminal block and terminates at the point where the neutral and ground are bonded at the power source."
Isolated ground (IG), contrary to its name, is not isolated from the power system ground. Rather, it is insulated at the receptacle and intermediate panels so as to control where the connection to the power system ground is made. [ILLUSTRATION FOR FIG. 1 OMITTED] Per the NEC, IG is allowed by exception and should be used only where required to reduce electrical noise. In fact, in certain interconnected systems (particularly audio and video systems), IG has been found to actually increase the noise on the interconnecting signal cabling and should be avoided in most instances.
"Surge reference equalizer. A surge-protective device used for connecting equipment to external systems whereby all conductors connected to the protected loads are routed, physically and electrically, through a single enclosure with a shared reference point between the input and output ports of each system." [ILLUSTRATION FOR FIG. 2 OMITTED].
It has been found that, with multiport loads (such as computers with AC power input and data communication ports, televisions with AC power and CATV ports, or fax machines with AC power and telephone ports), a transient voltage surge event on one port, even if protected by transient voltage surge suppressor (TVSS), causes a transient voltage surge to be impressed across the other ports, often causing damage to the load equipment. One potential solution is the use of a surge reference equalizer to prevent differences in the ports' ground references under transient voltage surge conditions.
"Power disturbance. Any deviation from the nominal value (or from some selected thresholds based on load tolerance) of the input AC power characteristics."
With today's sensitive powerline monitoring equipment, it's possible for you to measure and record very slight changes in any of a number of characteristics of the AC power supply. A common faulty line of thinking is that the larger the roll of the power monitor printout, the worse the power quality problem. Just because a deviation from the ideal nominal value can be detected does not mean that it's a power disturbance; the load equipment may or may not be affected by the power deviation. It's proposed that the term "power disturbance" be used to indicate only those variations that actually disturb the load. Unfortunately, this definition leads to a variable threshold for power disturbances because the tolerance of load equipment varies significantly.
The terms used to describe the variations in the power supply have been defined by the Emerald Book, in conjunction with another recently published standard, IEEE Standard 1159 on monitoring power quality. Table 1 below shows the recommended terminology for power disturbances.
The Emerald Book also lists words that have a history of varied usage, some without specific meaning. These terms are recommended to be avoided and include "blackout," "clean ground, "computer grade ground," "dedicated ground," "dirty ground," "dirty power," "glitch," "raw power," and "spike."
[TABULAR DATA FOR TABLE 1 OMITTED]
Chapter 3, General Need Guidelines
This chapter discusses the nature and origin of power quality problems. It attempts to describe the origin of power supply variations and the typical sensitivities of load equipment. Fig. 3, on page 49, is a generalized view of the origins of power supply variations. Lightning is an obvious source of transient voltage surges (both directly coupled onto the power lines and indirectly coupled by induction and electromagnetic fields), but it's also a major source of voltage sags as the overvoltages cause flashovers and faults in the power system.
Load switching is identified as a major cause of disturbances. Switching currents cause transient voltage surges; large inrush currents cause voltage sags; and turning off large loads causes voltage swells.
The other part of the power quality equation is the sensitivity of the load equipment to power supply variations. Information on the load equipment sensitivity is generally not available. The de facto standard curve for the tolerance of computer equipment to power supply variations, first published in IEEE Standard 446 (The Orange Book), is the Computer Business Equipment Manufacturers Association (CBEMA) curve, which is reproduced in the Emerald Book. [ILLUSTRATION FOR FIG. 4 OMITTED]. The concept of the CBEMA curve, based on energy considerations of the power supply, is that for shorter durations, the load can tolerate wider variations from nominal. Recent reconsideration of the CBEMA curve, based on testing of various computers, indicates that many of today's computers actually perform better than the curve indicates. Nevertheless, the curve still has been shown to be valid for the worst-case performers. However, it has been shown to be inadequate in representing the tolerance of computer systems to high frequency noise (very short duration disturbances), where the energy content is not sufficient to be damaging, but the noise can disrupt the logic and communication.
Chapter 4, Fundamentals
This chapter reviews the fundamental concepts related to power quality to help you understand the "why's" of the recommended practices presented in Chapter 9. Chapter topics include impedance considerations, frequency effects, load and power source interactions, harmonic current effects, sources and characteristics of voltage surges, coupling mechanisms, and grounding, bonding, and shielding fundamentals.
The grounding requirements for sensitive electronic equipment are divided into three distinct but interconnected subsystems:
* The safety (fault and personnel protection) subsystem following the NEC requirements.
* The signal reference subsystem for equipment performance by maintaining equal ground potentials over a broad range of frequencies.
* The lightning protection subsystem.
Chapter 5, Instrumentation
Chapter 5 describes 18 types of instruments that may be useful to you in performing power quality site surveys. Some of the important lessons of this chapter are as follows:
* Using the right instrument for the right task.
* Knowing the characteristics of the instrument.
* Always using true-rms instruments when measuring nonsinusoidal waveforms associated with electronic equipment.
Chapter 6, Site Surveys and Site Power Analyses
Conducting a site survey is often the only effective approach to solving a power quality problem. This chapter is a primer on conducting site surveys, including forms and procedures, what to look out for, and typical problems encountered. A thorough investigation of the site wiring and grounding is recommended when troubleshooting power quality problems before resorting to the more expensive and time-consuming task of power line monitoring. Careful testing, troubleshooting, and documentation techniques are necessary to collect meaningful power quality data.
Chapter 7, Case Histories
The case study chapter briefly describes 25 problems and solutions organized into nine typical problem categories, including utility problems, premises-generated surges, electronic load problems, premises wiring problems, TVSS problems, radiated electromagnetic interference (EMI) problems, electrical inspection problems, life-safety system problems, and equipment application problems.
Chapter 8, Specification and Selection of Equipment and Materials
This chapter describes commonly used power conditioning equipment. Table 2 is a summary of the performance of the various types of power conditioning equipment. Rather than the typical black-and-white solution charts, the Emerald Book introduces gray solutions that indicate there are significant variations in the capabilities of power conditioning technologies, and a power problem may not be fully correctable by a particular technology.
The chapter also includes a discussion on writing and reading product specifications, with particular emphasis on uninterruptible power supply (UPS) product specifications.
Chapter 9, Recommended Design/Installation Practices
This chapter represents the meat of the book, with the recommended powering and grounding practices organized by power system components from the service entrance to the outlet box. The recommended practices include the following:
* Strictly following the requirements of the NEC.
* Using solidly grounded AC power systems.
* Using dedicated circuits for sensitive loads.
* Using an insulated grounding conductor to supplement the Code-minimum raceway grounding path.
* Using a separately derived source close to the sensitive loads.
A major limitation of the Emerald Book is the lack of attention to distributed, interconnected computers that are in widespread use today. For computer rooms, signal reference structures, such as raised floor signal reference grounding grids, are recommended. Surge protection is recommended for both the power and signal (data, communication) ports. For multiport devices, surge reference equalizers as shown in Fig. 2 are recommended. For interconnected sensitive loads, particularly those located remote from each other, fiberoptic interconnections or opto-coupler isolation is recommended. For safety and performance, all grounding subsystems for power, lightning/surge protection, and communication systems must be connected together. Isolated grounding subsystems are unsafe and are not recommended.
Presently, IEEE has authorized a working group to revise the Emerald Book. The working group's goal is to complete the revision in time for the five-year review of the standard in 1997. Proposed revisions include revising and updating all chapters, adding more figures for clarifications, and adding a new chapter specifically addressing recommended practices for distributed computer and telecommunications equipment.
Any IEEE members or other power quality industry experts who are interested in contributing to the revision or commenting on the current standard should contact the author (FAX 1-614-841-8126).
Thomas M. Gruzs is Manager, Power Products, Liebert Corp., Senior Member of IIEE, and Chairman of IEEE's P1100 Working Group.