While researching grounding and power quality, the information was divided between how to design the grounding systems of an electrical infrastructure that was optimized to maximize the quality of the electrical power, and the National Electrical Code’s requirements for grounding electrical systems in Article 250, “Grounding and Bonding.” While both topics were an interesting refresher, I wondered “on what grounds” is the information relevant to electrical contractors dealing with power quality problems.
A commonly quoted statistic is that “nearly 80% of the power quality problems are due to deficiencies in the wiring and more specifically to grounding,” which is from a book by highly respected authors in the 1990s. Hopefully, the design of grounding systems has improved significantly in the last 25 years. However, many of the same problems have resulted from changes—intentional or not—to the electrical infrastructure and the equipment powered by it that occurred after original construction. The evolution may have resulted in ground loops, EMI/RFI noise, loose connections, changes in the impedance of the grounding electrodes, and increased neutral and ground current, all of which can affect the power quality of the facility.
With the caveat that no changes to the grounding system should be made that conflict with the requirements of the latest version of the NEC , the following are a few of the most important things to look for and correct to improve the grounding system.
An equipotential ground plane for all interconnected equipment
All of the grounded equipment points within the system should have a very low impedance path to the grounding point so that there are no harmful voltages developed between such points. One way this can happen is if there are multiple connections from the neutral (grounded) conductor and the grounding conductor or multiple grounding electrodes made at various locations around the facility. This can result in ground loops, which are hazardous to equipment and people. Another is that numerous filters used to suppress electrical noise shunt the current into the grounding conductor, causing an increase in voltage at supposedly grounded points. Grounding of shielded cable that is interconnected between two independent sources is a problem, as is running a grounding conductor to building steel rather than back to the service entrance where all of the bonds should be made. The connections in the grounding system can become loose due to vibration, corrosion or because of changes in the earth around the electrodes.
Improper connections between insulated grounds and equipment safety grounds
Those orange receptacles, often referred to as isolated grounds, are common in data centers and areas with a significant amount of IT equipment. They are not isolated from ground; rather, they use an insulated grounding conductor. This conductor is used for a separate grounding system for such susceptible equipment and is not connected to the regular equipment grounding circuit that runs throughout the building. It minimizes the effects from high harmonic or noise currents that may be on the regular equipment grounding conductor on the equipment plugged into the orange receptacles. Their grounding conductor is often an insulated green conductor, which is treated like the other conductors that are kept electrically insulated from conduit and other grounded metal until the connections are made back at the service entrance. Ground is not ground, and these separate systems should not be combined just because it saves running wires back. In facilities that have susceptible equipment, but do not have an insulating grounding system, keeping the susceptible equipment on separate circuits from other equipment to the distribution panels is a similar though not as effective solution.
Performing routine maintenance
Periodic inspection and measurements can spot issues before PQ or safety problems result. Check for faulty, loose or resistive connections of the wires and the conduit and look for missing equipment grounds. Eliminate use of temporary extension cords as soon as possible. Verify that any surge-suppression devices are connected to a grounded circuit so that the currents from large voltage transients, such as lightning, are diverted from the loads that have an adequate, low- impedance current path. Measure the neutral and ground currents where possible and compare to previous measurements. Refer to NFPA 70B for additional measurements and maintenance procedures.
Grounding problems are real and cost millions of dollars in damage and lost revenue. A major internet connection facility had a significant portion of its capacity go down when the latest equipment went offline numerous times. Turns out, that rack of equipment didn’t have its grounding system connected to the single reference point like all the rest.
About The Author
BINGHAM, a contributing editor for power quality, can be reached at 908.499.5321.