Employing high-resistance grounding technology

Responding is Andrew Cochran, president, I-Gard Corp., Mississauga, Ontario, Canada.

When designing an electrical grounding system for an industrial facility, three basic choices are available – ungrounded, solidly grounded or resistance grounded.

When deciding which type of grounding system to specify, the two key deciding factors often are electrical reliability and electrical safety. The new language in NFPA 70E Annex O highlights a third factor – did you consider risk when designing the electrical grounding system?

Absent ground faults, any of the three options are reliable and safe but ground faults are a reality in any electrical system. So the question becomes: Does the grounding system choice affect the likelihood of experiencing an arc flash incident?

During a ground fault on an ungrounded system, the arcing nature “charges” the system capacitance. When the arc extinguishes, the charged system cannot dissipate the charge, so it holds it. When the arc re-strikes, more charge is added to the system. This continues until the insulation breaks down at the weakest point in the system.

The concern regarding the safety aspect of ungrounded systems when experiencing a ground fault is noted in IEEE 242-1986, “Recommended Practice for Protection and Coordination of Industrial and Commercial Power Systems, Section 7.2.5”:

“A second ground fault occurring before the first fault is cleared will result in a phase-to-ground-to-phase fault, usually arcing, with current magnitude large enough to do damage, but sometimes too small to activate overcurrent devices in time to prevent or minimize damage. Ungrounded systems offer no advantage over high-resistance grounded systems in terms of continuity of service and have the disadvantages of transient over-voltages, locating the first fault and burn-downs from a second ground fault.”

Under normal operating conditions, a solidly grounded system is safe and reliable; however, both criteria are impacted when the system is subject to a ground fault. A ground fault of sufficient magnitude will trip the over-current protection and isolate a process. An arcing fault may not be of sufficient magnitude to be detected by and trip the over-current device until the arc fully develops and it becomes destructive and possible deadly.

IEEE 141-1993, “Recommended Practice for Electrical Power Distribution for Industrial Plants, Section 7.2.4,” states that “The solidly grounded system has the highest probability of escalating into a phase-to-phase or three-phase arcing fault, particularly for the 480 and 600V systems. A safety hazard exists for solidly grounded systems from the severe flash, arc burning, and blast hazard.”

NFPA 70E states in Annex O that “A great majority of electrical faults are of the phase-to-ground type. High resistance grounding will insert an impedance in the ground return path and will typically limit the fault current to 10 amperes and below (at 5 kV nominal or below), leaving insufficient fault energy and thereby helping reduce the arc flash hazard level.”

IEEE 141-1993 “Recommended Practice for Electric Power Distribution for Industrial Plants, Section 7.2.2” states that “there is no arc flash hazard, as there is with solidly grounded systems, since the fault current is limited to approximately 5 A.”

By choosing high-resistance grounding on your electrical distribution system, you control the ground fault magnitude to the point where the vast majority of arc flash accidents simply never occur.