Just Wires: Power Quality and the NEC

At a small telecommunications facility, equipment mysteriously started self-destructing for no apparent reason. Laptops, routers, printers and cell phones were apparently just becoming nonfunctional. The facility operator’s first call was to the electric utility, because the operator figured “dirty” power must have been the culprit. When someone recalled that there had been a thunderstorm the week before the problems started, the operator doubled down on those suspicions. The utility monitored the power and determined something within the facility was causing excessive current levels. Meanwhile, more equipment was getting damaged.

This is a typical opening sequence in a power quality investigation. There is a lot of data but not many facts. There is usually a scramble to try different solutions without an action plan or working knowledge of the potential effects. Equipment and cable grounds are removed, swapped or added. Equipment is moved from one circuit to another. The problem may get worse, stay the same, change, get a little better or even go away for a while. But is it solved, and will it stay fixed? That’s unknown and usually unlikely.

The National Electrical Code (NEC) has its origins and purpose in safety. However, wiring designed to protect the occupants and structures obeys the same laws of physics that wiring designed to provide a quality of supply compatible with the equipment to be powered from it. Ohm’s and Kirchhoff’s Laws work for both, and they can work together, provided there is an understanding of them.

Sometimes, outside resources are brought in to help find the source. Statements such as, “The wiring is the problem, but it can’t be fixed and still maintain compliance with the Code,” are usually a good clue that you need to hire someone else.

In most facility electrical systems, the grounding system that is designed so that an electrical fault in the system will generate enough current to cause the circuit protection to trip, interrupting the electrical fault and any resulting hazard. Over time, it was found that some faults could still injure or kill people who came in contact with such energized equipment. So, in locations where human conductivity was greater, such as bathrooms, kitchens, outside receptacles, and other damp areas, the NEC required ground fault circuit interrupters. These devices can trip much faster than a 20-ampere breaker, and they are designed to trip before harm to a human occurs; however, they don’t always trip before a 500-peak-volts, 2-microsecond transient destroys a capacitor or semiconductor device. So, transient voltage surge suppressors and other voltage-clamping devices were added.

The NEC requires a single bonding point between the grounding conductors and the grounded conductors (except with separately derived systems). This helps prevent ground loop currents from occurring and causing the voltage potential at equipment chassis ground connections from elevating above earth ground. Electronic and other susceptible electrical equipment requires an equipotential ground plane; that means all of the equipment that is connected together, either through electrical or communication means, is to be at the same ground potential. It doesn’t have to be 0.0 volts. It doesn’t necessarily have to be connected to ground, for if it did, all of the electronic equipment in airplanes would cease to work properly. 

Over the past few decades, many rectified input, switch mode power supplies have had filters to protect the equipment from radiated and conducted emissions as well as to prevent such emissions coming from within the equipment getting out and polluting the electrical worlds of other equipment by use of the equipment grounding conductor as a return path for the electrical noise.

So, just as a ground loop from an illegal neutral-to-ground bond can cause the safety ground potential to elevate, so can this situation, though generally at much lower level. But “sensitive” equipment from both an electrical and human interface perspective, such as is found in hospitals and data centers, needed a lower limit on this current level and to keep others from contaminating it. Hence, the use of an isolated ground, those orange receptacles, allowed by NEC and compatible with PQ.

The same type of power supplies also resulted in harmonic currents, as they draw current in a nonlinear manner. In three-phase wye circuits, this can result in the current in the grounded conductor, the neutral, no longer being neutral or near zero but as high as the phase currents. Triplen harmonics (3, 6, 9, 12, 15 …) add the neutral rather than canceling out. The PQ community fed knowledge of this phenomenon back to the NEC Code-Making panels, resulting in a change from undersized neutral conductor to similar to the ungrounded conductors using the demand and unbalanced loading calculations.

The NEC and PQ can coexist, and the related standards groups have discussions to foster the flow of information in both directions. Passing the electrical inspection and the PQ commission tests are good for the end-users for safety and performance.

About the Author

Richard P. Bingham

Power Quality Columnist

Richard P. Bingham, a contributing editor for power quality, can be reached at 732.287.3680.

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