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When designing or selecting an electrical grounding system for industrial operation for voltages of 5 kV and below there are three basic choices:
When deciding which type of grounding system to specify there is a need to consider:
Under normal conditions any of the three grounding methods are reliable, free from electrical risks and have similar operating costs but ground faults are a reality in any electrical system and so the question becomes how does the grounding system decision affects reliability, risk and costs?
The main advantage of Resistance Grounded systems is the ability to continue to use the system with a single fault present. It is, therefore, very important that the first ground fault should be located and removed as soon as possible to prevent unnecessary trip-outs before a second fault develops. Ground fault locating devices are available and may be incorporated in High Resistance Grounding schemes. One may even use the traditional method of tripping breakers in sequence to see when the fault disappears, but this defeats one of the principal advantages of the High Resistance Grounding, i.e. power continuity and the ability to locate a ground fault without shutting down the system.
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Solidly Grounded
High Resistance GroundedThe vast majority of industrial facilities in North America operate with a solidly grounded electrical system, which inherently creates two issues: process interruptions when there is a ground fault, and less frequently, but more seriously, arcing ground faults that can destroy equipment, and severely injure or kill employees.
The two most common issues with solidly grounded systems are that since ground faults result in process interruptions, ground fault trip settings are often set to the maximum level which can cause an arcing fault to go undetected until dangerous and destructive. HRG technology resolves both of these issues, by eliminating the need to interrupt your processes due to a single ground fault, and ensuring reduced risk of the likelihood of an arc flash by 95%.
IEEE Standard 141-1993
The solidly grounded system has the highest probability of escalating into a phase-to-phase or three-phase arcing faults, particularly for the 480V and 600V systems. A safety hazard exists for solidly grounded systems from the severe flash, arc burning and blast hazard from any phase-to-ground fault.
Ungrounded
High Resistance GroundedA substantial number of continuous process facilities in North America operate an Ungrounded electrical system, which is also subject to two issues: inability to locate the fault without interrupting the process, and uncontrolled transient over-voltages damaging equipment and creating the risk of a second ground fault occurring which interrupts processes and damages equipment. Both issues are resolved by upgrading to HRG technology.
HRG technology manages both issues. HRG with optional pulsing empowers personnel to trace and locate the fault without interrupting any process. Furthermore, HRG technology controls and limits over-voltages, thereby reducing the impact on equipment and extending the operational life of motors and drives.
Over-voltages caused by intermittent fault can be eliminated by grounding the system neutral through an impedance, which is generally a resistance that limits the ground current to a value equal to or greater than the capacitive charging current of the system. This can be achieved on a wye-connected system by a neutral grounding resistor, connected between the wye-point and ground, a step down transformer may be used for medium voltage systems to allow the use of a low voltage resistor. On a delta-connected system, an artificial neutral is required since no star point exists. This can be achieved by use of a Zig-Zag grounding transformer as shown, or alternatively, three single phase transformers can be connected to the system and ground to provide the ground path, with secondary terminated by a current limiting resistor (NGR).
IEEE Standard 242-2001 Clause 8.2.5
Ungrounded systems offer no advantage over High Resistance Grounded systems in terms of continuity of service and have the disadvantages of transient over-voltages, difficulty in locating the first ground fault, and burndowns from a second ground fault. For these reasons, they are being used less frequently today than High Resistance Grounded systems, and existing Ungrounded systems are often converted to High Resistance Grounded systems by Resistance Grounding the neutral if it exists or, if the system is fed from a delta source, creating a neutral point with a Zig-Zag or other transformer and then resistance grounding it.
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Electrical hazards, such as ground faults and arc flashes, impact industry safety every day by interrupting processes and damaging equipment, thereby posing a safety risk to personnel. There is a solution. The proven solution, High Resistance Grounding (HRG), ensures:
HRG is a technology that is recognized by NFPA 70E, CSA Z462 and the National Electrical Code, High Resistance Grounding reduces the likelihood of the arc flash hazard, protects against ground faults and ensures process continuity.
NFPA 70E states in Annex O Safety-Related Design requirements:
“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 A and below (at 5 kV nominal or below), leaving insufficient fault energy and thereby helping reduce the arc flash hazard level.”
IEEE141-1993 Recommended Practice for Electric Power Distribution for Industrial Plants Section 7.2.2:
“There is no arc flash hazard, as there is with Solidly Grounded systems, since the fault current is limited to approximately 5 A.”
Fig.4 Sleuth HRG systemBy 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.
Not only does HRG technology allow for process continuity during a single phase to ground fault thereby avoiding unnecessary process interruptions, there is evidence that converting to HRG will reduce equipment repair costs.
Webinar Recording & Presentation: Why HRG is superior to solidly grounded technology for industrial facilities? (May 12th, 2020 Webinar)
