Societies of the past handed down traditions, stories and knowledge orally from generation to generation. One hopes the original basis for the inherited ideas remains true as time progresses. The Old Testament is probably one of the best examples, as it took centuries before the spoken word was transcribed to the written word, which went through numerous languages and authors until reaching the Information Age.
In the realm of power quality, ages pass much faster than they did for those ancient societies, and some statements made by those in the field have lost their basis due to rapid technological evolution. One of these is that 70, 80 or 90 percent of power quality problems originate within a facility, with the percentage varying among the authors but always in the clear majority of facility-originating problems. Fact or fiction? Finding the original survey that led to this tidbit, in addition to finding any current benchmark survey to determine if it is still valid, has been a fruitless effort, but you will find it in many papers and articles.
There is one written item that really affects designing, installing and monitoring for the optimal quality of supply (which also means minimal power quality problems). It is Article 250.56, which reads, “Resistance of Rod, Pipe, and Plate Electrodes. A single electrode consisting of a rod, pipe, or plate that does not have a resistance to ground of 25 ohms or less shall be augmented by one additional electrode of any of the types specified by 250.52(A)(2) through (A)(7).” The 25-ohm number has been taken further by some organizations to state much lower numbers, including, “the installation manual stated the ground must be less than 1 ohm in the reference section.” This is missing the reason for the original requirement, which was not intended to improve the quality of the supply but rather to provide an adequate low-impedance path for fault current to flow, which would allow for the overcurrent circuit protection (breakers or fuses) to trip and clear the fault quickly. MIL-HDBK-419A and others indicate that 10 ohms is the maximum for adequate lightning protection, while the Institute of Electrical and Electronics Engineers (IEEE) Std. 141 (The Red Book) points to a 5-ohm recommendation.
Having a low impedance is good. From the power quality perspective, however, the grounding system for the interconnected electronic and other susceptible equipment should be at the same reference potential, i.e., an equipotential plane. Proof of this concept can be found in vehicles, ships and aircraft, which have a lot of electronic equipment but no low-impedance earth connection (though one might argue about the salt water and a ship). There is clearly no grounding conductor attached to any of those. If there are relatively low impedences (much less than 5 ohms) in the supply, grounding and grounded conductors back to the service entrance or separately derived source, any root-mean-square (rms) voltage variations (sags and swells) will be much lower in amplitude for the same current changes; hence, there’s less impact.
If the impedance at the harmonic frequencies is also low, the current harmonics will result in lower voltage harmonics. If the equipment-grounding conductor impedances are low back to the bonding-jumper connection, all of the equipment will move up in potential together, and few, if any, problems should result when a disturbance occurs—such as the coupled energy from lightning, which elevates the ground potential relative to earth ground. However, if communication lines run between equipment on one grounding system and another grounding system, this may not be the case, and destructive current can flow between those two points.
Another tradition that has had the misfortune of missing the original point of its creation in the National Electrical Code (NEC) is the “isolated ground,” those orange outlets common in data centers and other places where information technology (IT) equipment is plugged in. The intent was to have an insulated equipment--grounding conductor that is not connected to other grounding conductors that may be carrying significant current levels of any frequency that would affect the “sensitive” equipment. Ohm’s and Kirchoff’s Laws apply to ground circuits as well. Current through the grounding conductor will result in a voltage. If the current has harmonic components, so will the ground voltage at the equipment-grounding connection. An extreme example of misconception is the case of an installation manual that called for an isolated ground. The installers placed a ground rod at the installation location and did not bond it to the electrical--grounding system. The intent is not to develop a series of isolated grounding systems within a facility—a practice that can be downright dangerous.
Hopefully, over time—through the spoken word at training courses and seminars and the written word through this magazine and the Internet—we can steer some of the misguided myths back to the truth in power quality and make the world a safer place for susceptible equipment that occurs when constructing new or when modifying an existing facility. Of course, that leads to the obvious question, “Why not just make the equipment less susceptible?” That’s another story for another time.
BINGHAM, a contributing editor for power quality, can be reached at 732.287.3680.