It isn't easy designing an effective grounding system. After all, grounding problems are often the result of very unpredictable events, and so, many power quality issues are rooted in inferior grounding practices. On October 9, a one-day symposium, hosted by Post Glover, Inc., Erlanger, Ky., offered a number of expert opinions on grounding systems both for industrial and commercial facilities.
It isn't easy designing an effective grounding system. After all, grounding problems are often the result of very unpredictable events, and so, many power quality issues are rooted in inferior grounding practices.
On October 9, a one-day symposium, hosted by Post Glover, Inc., Erlanger, Ky., offered a number of expert opinions on grounding systems both for industrial and commercial facilities. Discussion centered on three types of grounding systems: ungrounded, solidly grounded and high-resistance grounding (HRG).
The basic definition of power system grounding, says Jack Woodham, P.E., senior electrical engineer with Cincinnati-based Jedson Engineering, Inc., "is the grounding of the power system transformer winding connection." Woodham delivered the keynote presentation of the symposium: a tutorial on low-voltage—480 volts to 4,160 volts—applications for the three types of grounding mentioned above.
Basics of Grounding
There are generally three types of faults in industrial power systems:
Line-to-ground accounts for 98% of these failures.
Phase-to-phase faults are less than 1.5% of the total, and are usually the result of line-to-ground faults that aren't cleared.
Three-phase faults are less than 0.5% of all faults. Most of these are man-made; in other words, they are accidents caused by improper operating procedures.
In addition to these types, one must also define the arcing fault: an intermittent failure between phases or phase to ground. The arcing fault is a discontinuous current that alternately strikes, is extinguished and restrikes again. For solidly grounded systems, the arc currents are, in percent of bolted three-phase faulted: three-phase, 89%; line-line, 74%; and line-ground, 38%.
Ungrounded Pros and Cons
Ungrounded power systems have their advantages and disadvantages. A major advantage of this type of configuration, according to Woodham, is that "the system would run, even with a ground fault. And the probability of an arcing fault escalating to a major fault is very small."
Other benefits of an ungrounded power system would include the following:
Low value of current flow for a line-to-line ground fault—5 amps or less.
No flash hazard to personnel for accidental line-to-ground fault.
Continued operation on the occurrence of first line-to-ground fault.
Small probability of line-to-ground arcing fault escalating to line-line or three-phase fault.
But this type of system also comes with disadvantages. For one, it makes it difficult to locate a line-to-ground fault. Facility engineers would typically start shutting off equipment in an attempt to locate the fault through a time-consuming process of deduction. Consequently, there is the added disadvantage of system maintenance costs being higher, due to additional labor required for locating ground faults.
Another problem with an ungrounded system is that it does not control transient overvoltages. Added to this is the complication that a second ground fault on another phase will result in a phase-phase short circuit.
Some Solidly-Grounded Thoughts
This leads to a consideration of the next type of grounding system: a solidly-grounded system. Woodham argues right away "that generators are not built to be solidly grounded, even though this is done all the time."
In general, this type of system offers few advantages but several disadvantages. A line-to-ground short circuit in a solidly-grounded system creates a high current flowing through grounding wires, building steel, conduit and waterpipes. This creates a high potential for arcing, produces steam due to water in terminal boxes and results in a potential for disaster in a harzardous area.
There are, of course, some advantages to this type of system. For example, it controls transient overvoltage from neutral to ground, and it can make it easy to locate a fault. Also, a solidly-grounded system can be used to supply line-neutral loads.
However, the disadvantages are numerous: Severe flash hazard and equipment damage are much more likely. Add to this the increased costs of a main breaker that is needed, high values of fault current, and the problems can mount up. Also, single-phase fault escalation into three-phase fault is likely, and the system can create problems with the primary system.
All of these points add up to drawbacks that can far outweigh the benefits from a solidly-grounded system, which leads to a consideration of resistance grounding.
Nowadays, a discussion of resistance grounding means high resistance. Questioned about the difference between low-resistance and high-resistance grounding, Woodham responds, "Low-impedance grounding systems were rated for 400 amps; GE and Westinghouse hung onto these systems for a long time, but there were problems with them. There is a large company in the South that is converting to 5-amp."
High-resistance proponents argue that HRG provides: continued operation of a facility during fault; safety for personnel and equipment; easy locating of the fault; and ability to plan for repairs. In recent years, these systems have become quite sophisticated, offering digital high-resistance grounding with data communications, and display panels that provide users with neutral-ground voltage value, neutral amperage value, alarm beeper and LED indicator light showing system status.
In addition, programmable features include: neutral-ground voltage trip limit; neutral amperage trip limit; programmable alarm delay; alarm auto reset off/on; and dry alarm contacts for customer use.
Woodham, an outspoken proponent of HRG, claims that it offers all the advantages of the other two types of systems, without the disadvantages. According to Woodham, not only does HRG offer low value of fault current and no flash hazard, but it also controls transient overvoltage, eliminates damage to equipment, provides service continuity and has no impact on the primary system.
To sum up, Woodham offers the following advantages of HRG systems:
No shutdown when a ground fault occurs.
Quick identification of the problem.
Safe for personnel and equipment.
Offers all the advantages of ungrounded and solidly-grounded systems.
No known disadvantages.
However, complications in implementing an HRG system can arise when converting an existing facility to this type of system.
"One has to be careful about installing HRG in an existing system," says Mike Mossman, vice president of CCG Facilities Integration Inc., Baltimore. "You don't always know what is already there." Mike Votaw, vice president of Votaw Electric, Fort Wayne, Ind., concurs: "If you don't have to worry about converting an existing grounding system, it is less complicated."
But Woodham dismisses the difficulties of retrofitting existing facilities with HRG. "We've converted numerous—100 systems—with Proctor & Gamble in the United States. We've converted many 2,400-volt systems and are working on converting a pulp plant in the Philippines from solidly-grounded to HRG."
The User Perspective
Clifford Normand with International Paper sits on the IEEE Pulp & Paper Safety Committee. Normand presented valuable input regarding how codes and standards currently view the HRG systems. He provided a case study where two electricians were severely burned, one fatally, while testing for voltage in a motor starter. As one held the multimeter, the second applied test prods to energized terminals. An unexpected movement by one of the electricians caused a test lead banana plug to separate from the multimeter jack. The banana plug, energized from the test circuit, contacted the grounded metal enclosure of the motor control center and initiated a high-energy electric arc.
In 1996 the IEEE PCIC Safety Subcommittee conducted a study to improve understanding of how personnel are exposed to electrical hazards in an industrial setting. The purpose of this study was to quantify current, sound, temperature and pressure levels associated with arcing faults. It documents that even when equipment is properly applied—following current standards and installed according to the manufacturer and third party listing and labeling requirements—personnel still risk exposure to arc flash hazard. In other words, even when the employee does everything right, there is still a risk. "Meeting code at a facility is not enough to prevent incidents and injuries," says Normand.
CCG's Mossman agrees. "It has to be remembered that HRG is not going to prevent lethal voltages from appearing. It is there to prevent massive damage to equipment."
Normand points out that even though HRG systems reduce flash hazard in a phase-to-ground fault, National Fire Protection Association (NFPA) Standard 70E does not recognize high-resistance grounding and requires the use of flash protection when working on live electrical parts, including voltage testing, in the event of a phase-to-phase fault.
Normand offers several precautions for using HRG systems. "Detect ground faults and remove them quickly," says Normand. "Sustained ground faults are unsafe, regardless of the protection provided. The National Electrical Code (NEC) allows the use of high-resistance under certain conditions (see Article 250-36), but NEC states that the conditions of maintenance and supervision must ensure that only qualified persons will service the installation.
"Several of our newer paper mills were fitted with a high-resistance grounding system on low-voltage substations," explains Normand. "One mill converted all of their solidly-grounded low-voltage subs to HRG." Normand went on to describe how another mill with a 600-volt delta-connected substation installed HRG, enabling them to locate a persistent ground that they has not been able to find—even after a couple of years.
Normand does offer some further concerns and recommendations about implementing HRG systems. Major concerns are failure to locate grounds in a timely manner, failure to install suitable alarms in a manned location and lack of training for personnel. His main recommendations are that HRG systems can be installed on all low-voltage substations, but personnel must follow NFPA 70E, Standard for Electrical Requirements for Employee Workplaces, 2000 Edition.
The bottomline question for Normand is: How important is service continuity for the electrical system? If the answer is "not very," then a solid-grounded system is fine. If service continuity is extremely important for the system, then high-resistance grounding should be the choice.
Monitoring the System
The importance of trained personnel to support an HRG system brings up another issue: remote monitoring of the system. Mike Votaw of Votaw Electric delivered a presentation on remote monitoring via the Internet. According to Votaw, there are a number of business objectives here. Early warning of situations that threaten production objectives is foremost here. But it also involves opportunities to improve process and verify implementation effectiveness. Also, Internet monitoring can provide several opportunities for energy reduction.
Building a facility information acquisition system would call for incorporating several elements into the system:
Archiving storage device
Data generators (transducers, PLCs, and intelligent devices)
The equipment exists to digitally monitor HRG systems, equipment that offers remote monitoring, setup and control, as well as time-stamped logging.
What to Consider
In the end, the selection of a ground system depends on some basic considerations. The parameters to consider would include limitation of ground fault currents; limitation of transient overvoltages; selectivity and sensitivity of protective relaying; and surge voltage protection with surge arresters. Beyond these concerns, however, are the ultimate issues—the protection of personnel and equipment from ground-fault disasters.
From Pure Power, Winter 2002
AOL Goes With HRG
While high-resistance grounding is not new—it is allowed under the National Electrical Code (NEC) 250-36—it has traditionally been used in industrial facilities with medium-voltage systems, where a shutdown due to ground fault would be hazardous to the production process.
Nowadays, however, all types of mission-critical facilities are relying on HRG. In fact, the preeminent data company—America Online—has chosen this technology. Baltimore-based CCG Facilities Integration Inc. became acquainted with AOL in 1995, and has since designed HRG systems for two AOL facilities in Virginia—the Dulles Technology Center and the Manassas Technology Center.
Dulles encompasses a total 180,000 sq. ft. with 93,000 sq. ft. of raised floor—a critical load up to 60 watts per sq. ft. It is served by dual, redundant 35-kV utility services, 10 MVA each, with six 480-volt service entrances. Backup power is provided by two diesel generator plants that together can deliver 18 MW of power. In addition, three static UPS systems, 3 MW each, insure even greater system redundancy.
The Manassas Center is larger but similar: a total 230,000 sq. ft. with 95,000 sq. ft. of raised floor area. Dual, redundant 25-kV utility service, 14 MVA each, are served from an adjacent utility company switchyard. There are eight 600-volt service entrance feeders, backed up by three diesel generators and three UPS. Critical load in this facility is up to 92 watts per sq. ft.
Engineers chose a high-resistance grounding system for several reasons, including: its compatibility with TVSS and UPS systems; compatibility with solid-state drives; and special controls for line-interactive UPS.
According to Scott Davis, chief engineer of data center operations at AOL, Inc., "The application of HRG within the electrical system's at AOL's technology centers has given us a more robust and reliable facility to support our business mission."
The Insurer's Perspective on Grounding
Generally speaking, the terms of insuring an industrial facility include the following:
Peril is the way in which the insurer categorizes the cause of a loss: i.e., flood, fire, electrical failure.
Deficiency is the meaningful deviation between current conditions and recommended standards.
Likelihood is the probability of an event based on experience.
Hazard is an event that may happen as a result of a deficiency.
Risk is a total business impact related to a hazard.
Exposure is unacceptable risk.
The big questions are: What drives losses? Where does one focus loss prevention efforts?
"We tell our customers to first determine whether they have a resistance ground system or ground fault detection," says Ken Tate, an electrical loss prevention specialist with FM Global. "Determine the type of grounding system you have, and then, consider conversion from an ungrounded system to a resistance grounded system."
FM Global also recommends that its clients update short-circuit and coordination studies to take into consideration ground-fault coordination between ground-fault devices and phase-fault devices. Additionally, it is important to perform functional tests of circuit breakers and GFP. Only in this way can facility managers achieve a level of comfortable risk management. "Using a high-resistance ground system could knock 10% off the price of insurance," says Tate.