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Neutral Grounding Resistors Technical Information NEUTRAL GROUNDING RESISTORS TECHNICAL INFORMATION 1369 Cox Avenue Erlanger, KY 41018 USA Phone: 859-283-0778 Toll-Free: 800-537-6144 Fax: 859-283-2978 Web: www.postglover.com Serving the Electrical Industry Since 1892 Quality System Certified to ISO 9001 With over 130 years of combined industrial and utility experience, Post Glover delivers the industry’s strongest, broadest and most technologically advanced products available. Experienced Post Glover has grown into the world’s largest power resistor company, based on its industry leading positions in grounding solutions and dynamic braking resistors. Post Glover can be trusted to deliver cost- effective, reliable products to the marketplace. Efficient Post Glover’s factory in Erlanger, Kentucky integrates computer aided design and manufacturing with the industry’s strongest engineering team for greater manufacturing capabilities and efficiencies. Post Glover continues to improve through Kaizen events, third party certifications and regular audits of our internal practices. Focused Service The industry’s most experienced sales and engineering team and largest independent sales representative network insure a timely and accurate, same day response to your typical and complex applications. With 16 engineers on staff, we are poised to answer your product and application questions. Certified Products Post Glover prides itself on designing and manufacturing in accordance with all applicable standards, be they IEEE, ANSI, NEMA or IEC. Taking safety one step further, we offer the only UL listed high resistance grounding unit in the industry, as well as UL and CSA offerings in low resistance grounding resistors and dynamic braking resistors. Table of Contents Grounding of Industrial Power Systems ...................................................4 Definition of Grounding ................................................................................4 Characteristics of Ungrounded Systems .....................................................4 System Neutral Grounding ................................................................................5 Importance ...................................................................................................5 Solid Grounding ...........................................................................................5 Resistance Grounding .................................................................................6 • Low Resistance ...................................................................................6 • High Resistance...................................................................................6 Grounding Recap .....................................................................................................7 Comparative Performance Rating Table .....................................................7 Rating & Testing Neutral Grounding Resistors ....................................8 IEEE-32 Standards ......................................................................................8 Time Rating ................................................................................................. 8 Tests ............................................................................................................8 CSA Standards ............................................................................................9 Selection of Neutral Grounding Resistors............................................ 10 Factors to Consider ................................................................................... 10 The Selection Process ............................................................................... 10 Other Methods of Grounding ........................................................................ 12 Single Phase Transformer & Loading Resistor .......................................... 12 Grounding Transformers ............................................................................ 12 • Zigzag ................................................................................................ 13 • Wye-Delta .......................................................................................... 14 • Alternate Wye-Delta .......................................................................... 14 Specifications ......................................................................................................... 15 Neutral Grounding Resistors ..................................................................... 15 • High Voltage, Low Resistance ........................................................... 15 • Low or Medium Voltage, High Resistance ......................................... 16 Zigzag Grounding Transformers ................................................................ 17 Glossary of Terms ................................................................................................... 18 3 PGR Document #NG112-06 Grounding of Industrial Power Systems Definition of Grounding FIGURE 2 Phase A conductors The term grounding is commonly used in the electrical Phase B conductors industry to mean both “equipment grounding” and Phase C conductors “system grounding”. “Equipment grounding” means the connection of earth ground to non-current carrying conductive materials such as conduit, cable trays, Delta configuration junction boxes, enclosures and motor frames. “System grounding” means the deliberate connection of earth ground to the neutral points of current carrying the normal voltage is present on all insulation in the conductors such as the neutral point of a circuit, a system, as shown in Figure 3b. This situation can often transformer, rotating machinery, or a system, either cause failures in older motors and transformers, due to solidly or with a current limiting device. Figure 1 insulation breakdown. illustrates the two types of grounding. The interaction between the faulted system and FIGURE 1 its distributed capacitance may cause transient Metal enclosures overvoltages (several times normal) to appear from line to ground during normal switching of a circuit having To a line to ground fault (short). These overvoltages Source Neutral load may cause insulation failures at points other than the Bonding jumper original fault. In addition, a second fault on another SYSTEM phase may occur before the first fault can be cleared. Transformer bank GROUNDING This can result in very high line to line fault currents, equipment damage and disruption of both circuits. EQUIPMENT GROUNDING In addition to the cost of equipment damage, ungrounded systems present fault locating problems. Characteristics of Ungrounded Systems This involves a tedious process of trial and error, first isolating the correct feeder, then the branch, and finally An ungrounded system is one in which there is no the equipment at fault. The result is unnecessarily intentional connection between the conductors and lengthy and expensive downtime. earth ground. However, in any system, a capacitive coupling exists between the system conductors and Despite the drawbacks of an ungrounded system, the adjacent grounded surfaces. Consequently, the it does have one main advantage. The circuit may “ungrounded system” is, in reality, a “capacitively continue in operation after the first ground fault, grounded system” by virtue of the distributed assuming it remains as a single fault. This permits capacitance. This is shown in Figure 2. Under normal continued production, until a convenient shutdown can operating conditions, this distributed capacitance be scheduled for maintenance. causes no problems. In fact, it is beneficial, because it establishes, in Phase A and B are now at full effect, a neutral point for the system, FIGURE 3 line-to-line voltage above ground as shown in Figure 3a. As a result, the A B phase conductors are stressed at only Line-to-line voltage line-to-neutral voltage above ground. A B Each phase is at C However, problems can arise under line-to-neutral Neutral point Phase C is now at ground voltage above ground established potential. Virtually no fault current ground fault conditions. A ground C by distribution flows as there is no return fault on one line results in full line-to- capacitance path back to the source line voltage appearing throughout the (a) NORMAL OPERATION (b) GROUND FAULT ON PHASE C system. Thus, a voltage 1.73 times Voltage relationships 4 PGR Document #NG112-06 System Neutral Grounding Importance Advantages of Grounded Neutral Systems This section is devoted to the proven benefits of Resistance grounding is by far the most effective and proper system grounding, and in particular, the added preferred method. It solves the problem of transient advantages of resistance (current limited) grounding. overvoltages, thereby reducing equipment damage. It accomplishes this by allowing the magnitude of the The intentional connection of the neutral points of fault current to be predetermined by a simple ohms law transformers, generators and rotating machinery to calculation (see Table 1). Thus the fault current can be the earth ground network provides a reference point limited, in order to prevent equipment damage. of zero volts. This protective measure offers many advantages over an ungrounded system, including: Table 1 E I = Where: I = Limit of Fault Current • Reduced magnitude of transient overvoltages R • Simplified ground fault location E = Line-to-Neutral Voltage • Improved system and equipment fault of System protection R = Ohmic Value of Neutral • Reduced maintenance time and expense Grounding
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