In the latest edition of “Electrical Power Systems Quality” by Dugan, McGranaghan, Santoso and Beaty, the authors point out that “it is commonly stated at power quality conferences and in journals that 80 percent of all power quality problems reported by customers are related to wiring and grounding within a facility.”
They go on to explain that “while this may be an exaggeration, many power quality problems are solved by simply tightening a loose connection or replacing a corroded conductor. Therefore, an evaluation of wiring and grounding practices is a necessary first step when evaluating power quality problems in general.”
A review of some of the key terms found in Article 250 of the National Electric Code is a good place to begin. A grounding system has several key components: the connection to earth (often the grounding electrodes); the grounding conductor (typically called the green or safety wire); the bonding jumper that connects the grounding conductor to the grounded conductor (often referred to as the neutral); and, the connection of the equipment connected to the grounding system. A few of the definitions from the NEC are listed below, to ensure that we are all speaking the same language.
o Bonding Jumper. A reliable conductor to ensure the required electrical conductivity between metal parts required to be electrically connected.
o Grounding Conductor. A conductor used to connect equipment or the grounded circuit of a wiring system to a grounding electrode or electrodes.
o Grounding Conductor, Equipment. The conductor used to connect the noncurrent-carrying metal parts of equipment, raceways, and other enclosures to the system grounded conductor, the grounding electrode conductor, or both, at the service equipment or at the source of a separately derived system.
o Grounded Conductor. A system or circuit conductor that is intentionally grounded.
o Grounding Electrode Conductor. The conductor used to connect the grounding electrode to the equipment grounding conductor, to the grounded conductor, or to both, at the service equipment or at the source of a separately derived system.
o Grounding Electrode. A grounding electrode is a conductive body deliberately inserted into earth to make to electrical connection to earth.
Some of the power quality and safety related things to look for in the wiring and grounding include: bad connections, missing grounding (safety) conductor, multiple bonds of grounding-to-grounded conductor (neutral-to-ground connections), ungrounded equipment, additional ground rods, ground loops, and insufficient size of the grounded (neutral) conductor.
Bad or loose connections may result in intermittent problems, such as when the vibration from a truck going past or a large motor starting up causes the circuit to be opened, as shown in Figure 1.
This can be accompanied by excessive heat and arcing at the connection point, which would cause the problem to worsen over time. The heating and cooling cycle of the daily load changes can open the connection further. The arcing can cause carbon buildup and pitting that increases the connection impedance, which further increases the heating of the connection.
Often, it just takes a periodic tightening of connections (safely done, of course). If the damage is substantial, the connector and the wire might have to be replaced.
Without a connection between the equipment grounding conductor and grounding (safety) conductor, a fault in the equipment from the phase conductor to the enclosure can elevate the touch potential of the enclosure and would result in a very hazardous condition that would not trip the breaker or fuse.
Multiple bonds of the neutral (grounded) and grounding conductor provide multiple paths for the return current to flow. This can cause an elevated touch potential, as well as mis-operation of protective devices and equipment.
Isolated grounds are a bit of a misnomer. They should be considered “insulated” grounds, where the grounding conductor is encased in a insulating material, as the phase conductors are.
The grounding conductor is separately run to equipment requiring “clean” ground connections, and properly bonded. Having multiple grounding electrodes for isolated grounds provides additional paths for lightning stoke currents to flow and may disrupt the equipotential grounding system.
Rather than the entire system elevating in potential, different circuits will be at different potentials, which may be a problem for interconnected equipment. Similar results (though less dramatic) occur with ground loops, such as connecting the grounds or shields of communication cables between two pieces of equipment that have different power system ground paths.
The potential difference in the two grounds causes current to flow in this group loop path, with possible noise and/or component failures.
Whether the percentage of PQ problems that can be traced to the wiring and grounding is 60 percent, 70 percent, or 80 percent, they still occur frequently enough to take corrective action, which is often no more costly than turning a screwdriver.
Taking care of these problems first takes away some of the difficulty when trying to solve the more complicated problems. EC
BINGHAM, a contributing editor for power quality, can be reached at 732.287.3680.