The judicious use of high resistance grounding facilitates process continuity, reduces equipment damage, allows for predictive maintenance, reduces shock hazard and can minimize the impact of arc blast hazards.
by Andrew Cochran
by Andrew Cochran
Empirical data indicates that around 80 percent of all electrical interruptions are attributable to ground faults that are a result of an unintentional connection between system conductors and ground. The term grounding is commonly used in the electrical industry to mean both "equipment grounding" and "system grounding." System grounding means the intentional connection of a neutral or phase conductor, either solidly or through an impedance or resistance device, to ground from a source such as transformer, rotating machinery, solar photovoltaic, etc. Figure 1 illustrates the two types of grounding. 
System grounding, or the intentional connection of a phase or neutral conductor to earth, is for the purpose of controlling the voltage to earth, or ground, within predictable limits. It also provides for a flow of current that will facilitate overcurrent device operation during an unwanted connection between system conductors and ground (a ground fault).
The root cause of this unwanted connection is often a result of insulation breakdown. The energy released from the ground fault can lead to process interruptions, equipment damage and present a fire and explosion risk to personnel (see photo 1). Five to ten arc-flash explosions occur in electric equipment every day according to statistics compiled by CapSchell Inc, a Chicago-based research and consulting firm that specializes in preventing workplace injuries and deaths.
From 1992 to 1999 a single leading industrial insurance company reported 228 losses that were attributed to ground faults with a total cost of US $180 million.
The same insurance company has stated that they have evaluated the property and business interruption potential as a result of not having ground-fault protection and found it to be significant enough to warrant installation of ground-fault protection. In the winter issue of Pure Power, they went a step further and stated that, "using a high resistance ground system could knock 10% off the price of insurance."
And yet the majority of industrial facilities continue to operate without adequate ground-fault protection. These establishments typically operate an ungrounded or solidly grounded electrical distribution system, both of which have inherent disadvantages.
An ungrounded system is one in which there is no intentional connection between any of the system conductors and earth ground. However, in any system, a capacitive coupling exists between the system conductors and the adjacent grounded surfaces. Consequently, the "ungrounded system" is, in reality, a "capacitively grounded system" by virtue of the distributed leakage capacitance.
The reasoning behind the prevalence of ungrounded systems in many industrial facilities appears to be historical. Prior to the emergence of high resistance grounding in the late 1980s, the only choice when process continuity was desired was an ungrounded system that allowed for the controlled shutdown for fault repairs at a convenient time and this was of tremendous value to continuous manufacturing processes by reducing production losses, equipment damage and outages.
However experiences with multiple failures due to arcing ground faults has resulted in a change in philosophy over the use of ungrounded systems and this change is supported in by the Institute of Electrical and Electronic Engineers (IEEE) who in IEEE Standard 242-1986 Recommended Practice for the Protection and Coordination of Industrial and Commercial Power Systems 242-1986 section 7.2.5 offer the following perspective:
"Ungrounded systems offer no advantage over high-resistance grounded systems in terms of continuity of service and have the disadvantages of transient overvoltages, locating the first fault and burndowns from a second ground fault. For these reasons, they are being used less frequently today than high-resistance grounded systems."
There are many benefits from grounding the electrical distribution system including:
· Reduced magnitude of transient overvoltages
· Simplified ground fault location
· Improved system and equipment fault protection
· Reduced maintenance time and expense
· Greater safety for personnel
· Improved lightning protection
· Reduction in frequency of faults
System grounding, or the intentional connection of a phase or neutral conductor to earth, is for the purpose of controlling the voltage to earth, or ground, within predictable limits. It also provides for a flow of current that will facilitate overcurrent device operation during an unwanted connection between system conductors and ground (a ground fault).
The root cause of this unwanted connection is often a result of insulation breakdown. The energy released from the ground fault can lead to process interruptions, equipment damage and present a fire and explosion risk to personnel (see photo 1). Five to ten arc-flash explosions occur in electric equipment every day according to statistics compiled by CapSchell Inc, a Chicago-based research and consulting firm that specializes in preventing workplace injuries and deaths.
From 1992 to 1999 a single leading industrial insurance company reported 228 losses that were attributed to ground faults with a total cost of US $180 million.
The same insurance company has stated that they have evaluated the property and business interruption potential as a result of not having ground-fault protection and found it to be significant enough to warrant installation of ground-fault protection. In the winter issue of Pure Power, they went a step further and stated that, "using a high resistance ground system could knock 10% off the price of insurance."
And yet the majority of industrial facilities continue to operate without adequate ground-fault protection. These establishments typically operate an ungrounded or solidly grounded electrical distribution system, both of which have inherent disadvantages.
An ungrounded system is one in which there is no intentional connection between any of the system conductors and earth ground. However, in any system, a capacitive coupling exists between the system conductors and the adjacent grounded surfaces. Consequently, the "ungrounded system" is, in reality, a "capacitively grounded system" by virtue of the distributed leakage capacitance.
The reasoning behind the prevalence of ungrounded systems in many industrial facilities appears to be historical. Prior to the emergence of high resistance grounding in the late 1980s, the only choice when process continuity was desired was an ungrounded system that allowed for the controlled shutdown for fault repairs at a convenient time and this was of tremendous value to continuous manufacturing processes by reducing production losses, equipment damage and outages.
However experiences with multiple failures due to arcing ground faults has resulted in a change in philosophy over the use of ungrounded systems and this change is supported in by the Institute of Electrical and Electronic Engineers (IEEE) who in IEEE Standard 242-1986 Recommended Practice for the Protection and Coordination of Industrial and Commercial Power Systems 242-1986 section 7.2.5 offer the following perspective:"Ungrounded systems offer no advantage over high-resistance grounded systems in terms of continuity of service and have the disadvantages of transient overvoltages, locating the first fault and burndowns from a second ground fault. For these reasons, they are being used less frequently today than high-resistance grounded systems."
There are many benefits from grounding the electrical distribution system including:· Reduced magnitude of transient overvoltages
· Simplified ground fault location
· Improved system and equipment fault protection
· Reduced maintenance time and expense
· Greater safety for personnel
· Improved lightning protection
· Reduction in frequency of faults
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