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Building a Powerful Grounding Foundation

Jan 1, 2000 12:00 PM, By John Paschal, P.E., Editorial Consultant

At first glance, the deceptively simple and passive elements of a grounding system don't seem to do very much. But beyond this simple exterior lies the power of a sleeping giant.

If you've ever wondered how important a grounding system really is to the normal operation of a facility, just remember the alternatives: fires start from arcing faults; computers crash from spurious noise; data control systems shut down processes in error; human hearts "defib" and stop; motors burn up; arc-type lamps turn off (due to voltage imbalances caused by bad service bonding jumper terminations); voltage jacking occurs on ungrounded power systems or systems that have lost their grounding connection; and pipes spring leaks due to cathodic erosion.

Although generally hidden from view, grounding systems and their many functions make up the foundations upon which we build strong electrical systems.

Industrial environments. Let's look at a static grounding grid in an industrial plant energized through a high-voltage utility substation. Here, you'll see a variety of grounding electrode shapes. This almost completely hidden grounding system performs the following tasks:

• Minimizes the ground potential rise and coincident step and touch potentials that occur from high-voltage system zero sequence current flowing through the earth during utility system ground faults, such as insulator-string arc-over.

• Equalizes the DC potentials within the plant that build up from process flows.

• Limits the system-to-frame voltage for human safety and prevents overstress in phase-to-ground voltage.

• Provides an equipotential plane on which humans can stand and not be harmed during times of ground fault within the plant. (For instance, it equalizes the potential of a motor stator a maintenance person might touch during a ground fault and the surrounding earth on which the person stands. With no potential across the person's body, harmful current can't flow through the body.)

• Provides a ground reference plane to reference all of the instruments in the plant control system.

• Provides a secondary path through which ground fault current can flow back to the last transformer (or generator) ground point in the event of loss of the equipment grounding conductor path. (This provides increased assurance of tripping of overcurrent devices upon ground fault, and provides enhanced personnel safety from stray current flow, flash burns, and induced fires.)

• Provides an "earthing" point for lightning protection or lightning avoidance systems.

• Provides a cathodic protection current return path.

Commercial environments. In commercial installations, such as a typical high-rise office building, the grounding system and its function are similar to those in an industrial plant. However, the shapes of the grounding electrodes and grounding electrode conductors are different.

The structural steel columns within the building are suitable for lightning protection "down" conductors as well as for use as the grounding electrode system for each local transformer secondary on upper floors. In addition, these steel structures help attenuate magnetic noise coming from outside of the building. How? By forming a sort of "Faraday Cage" around the building contents.

In buildings containing radio transmitters with rooftop antennas, however, these same steel columns form a part of the radiating element/ground plane system. In this scenario, the column "cage" tends to cause high frequency noise within the building systems rather than attenuate it.

Nevertheless, building steel (in almost every type of structural steel building design) forms a very good low-impedance path for ground fault current. This path promotes rapid tripping of overcurrent devices; enhances personnel safety; and eliminates electrical noise of the type coming from arcing faults in high-impedance equipment grounding conductor paths.

Specialized environments. Suppose a building contains specialty systems, such as NEC Art. 645 Information Technology Equipment or Art. 517 Anesthetizing Locations. Yes, you rely on the grounding system to perform all of the functions previously listed. But, you also depend on it to minimize even low-voltage potential differences between any two conductive points in locations where you'll find delicate biological tissues or semiconductor devices.

Part of the methodology here depends on the capability of various equipment-grounding conductor forms (such as conduits) to absorb transmitted energy. These grounding conductors transform electromagnetic (EMF) waves into eddy currents and heat instead of letting the EMF "cut" system wires. Thus, they induce noise within the wires.

In fact, the functionality of digital systems requires the elimination of "noise" voltages since the equipment could erroneously interpret this as valid information. This is the reason we have specific modifications in grounding cable shields (terminate and ground on only one end), conduits (installing an insulating section with internal equipment-grounding conductor), or cables (providing many concentric wraps per foot of cable).

Finally, if these and similar steps are insufficient to guard against voltage transients, then the grounding-electrode system provides the equipotential plane to which you connect one side of transient surge suppressors to "short-to-ground" these unwanted voltages at wire terminations.

Remarkably, a grounding system within all these facilities reliably performs all of the above functions day in and day out while consuming zero watts of power.

Paschal is a consultant for Bechtel, Corp., Houston, and serves as EC&M Books Editor.

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