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Ground Fault Protection Ungrounded Systems to High Resistance Grounding Convers

Ground Fault Protection Ungrounded Systems to High Resistance Grounding Conversion Guide www.i-gard.com > the power to protect ABOUT I-Gard Our company I-Gard Inc., formerly known as IPC Resistors Inc., is growing from a resistor only company and to one that fully incorpo- rates our ground fault protection line into the company. The ground fault protection product line has provided an exciting vehicle for growth and the new name I-Gard better reﬂects our techni- cal and application focus. At the same time, we understand that our success in partnering in the global market achieved over the last 21 years has been built on designing and manufacturing high quality power resistors to meet our customer’s speciﬁc needs and that will not change. We look forward to being your technical partner in the future for all of your power resistor and ground fault requirements. 1 - I-Gard - The Leader in Ground Fault Protection TABLE OF CONTENTS SUBJECT PAGE 1. Power System Grounding .....................................................................................2 1.1 What Is Grounding? ........................................................................................2 1.2 What Is A Grounded System?..........................................................................2 1.3 Are There Different Types Of System Grounding? ...........................................2 1.4 What Is The Purpose Of System Grounding?..................................................2 1.5 What Is A Ground Fault? .................................................................................2 1.6 Why Are Ground Faults A Concern? ...............................................................2 2. Ungrounded Systems ............................................................................................3 2.1 What Is An Ungrounded System?....................................................................3 2.2 What Does IEEE Say About Ungrounded Systems? ......................................4 3. Resistance Grounded Systems..............................................................................4 3.1 Why Consider Grounding Your System?..........................................................4 3.2 What Is A Resistance Grounded System?.......................................................4 3.3 What Is A Low Resistance Grounded System? ...............................................4 3.4 What Is A High Resistance Grounded System? .............................................5 4. High Resistance Grounding ..................................................................................5 4.1 Why Consider High Resistance Grounding?....................................................5 4.2 Why Limit The Current Through Resistance Grounding? ................................5 4.3 What Are The Requirements For Sizing The Resistor? ...................................6 4.4 Measuring The System Capacitive Charging Current......................................6 4.5 Rule Of Thumb For System Charging Current.................................................8 4.6 Is There Any Performance Downside To Applying A 5A Resistor To A System That May Only Have 1A Of Charging Current? ..........................8 4.7 What Is The Probability That A 480 V Industrial System 4000 Kva Would Require More Than A 5A Resistor? ......................................8 4.8 What Are The Necessary Steps to Upgrade My System ................................8 2 - I-Gard - The Leader in Ground Fault Protection 1 POWER SYSTEM GROUNDING 1.1 WHAT IS GROUNDING? The term grounding is commonly used in the electrical industry to mean both “equip- ment grounding” and “system grounding”. “Equipment grounding” encompasses three things: (a) the bonding of all non-current carrying conductive parts of the electrical system together; (b) the connection of the equipment bonding system, via a bonding conductor, to the power system neutral, to provide a low impedance return path for ground fault current; and (c) the connection of the equipment bonding system to earth ground via a grounding conductor and grounding electrode. "System grounding" refers to the connection of the power system neutral to earth ground via a grounding conductor and grounding electrode. The bonding conductors carry ground fault current. Grounding conductors do not carry ground fault current. Figure 1 illustrates the two types of grounding. FIGURE 1 1.2 WHAT IS A GROUNDED Grounded System – a system with at least one conductor or point (usually the middle wire or neutral point of transformer or generator windings) is intentionally grounded, either solidly or through an impedance. IEEE Standard 142-1991 1.2 1.3 ARE THERE DIFFERENT The types of system grounding normally used in industrial and commercial power systems are: 1) Solid grounding 2) Low-resistance grounding 3) High-resistance grounding 4) Ungrounded 1.4 WHAT IS THE PURPOSE OF 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 allow detection of an unwanted connection between system conductors and ground [a ground fault]. SYSTEM GROUNDING? SYSTEM? TYPES OF SYSTEM GROUNDING? 2 3 - I-Gard - The Leader in Ground Fault Protection A CONCERN? 1.5 WHAT IS A GROUND FAULT? A Ground Fault is an unwanted connection between the system conductors and ground. 1.6 WHY ARE GROUND FAULTS Ground faults often go unnoticed and cause havoc on plant production processes. Shutting down power and damaging equipment, ground faults disrupt the ﬂow of products, leading to hours or even days of lost productivity. Undetected ground faults pose potential health and safety risks to personnel. Ground faults can lead to safety hazards such as equipment malfunctions, ﬁre and electric shock. During a ground fault condition, equipment can be damaged and processes shut down, seriously affecting the bottom line. UNGROUNDED SYSTEMS 2.1 WHAT IS AN UNGROUNDED An ungrounded system is one in which there is no intentional connection between the 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 capacitance. This is shown in Figure 2. FIGURE 2 Under normal operating conditions, this distributed capacitance causes no prob- lems. In fact, it is beneﬁcial, because it establishes, in effect, a neutral point for the system, as shown in Figure 3a. As a result, the phase conductors are stressed at only line-to-neutral voltage above ground. However, problems can arise under ground fault conditions. A ground fault on one line results in full line-to-line voltage appearing on the other two phases. Thus, a voltage 1.73 times the normal voltage is present on all insulation on the ungrounded phase, as shown in Figure 3b. This situation can often cause failures in older motors and transformers, due to insulation breakdown. FIGURE 3 A A B C C B Line-to-line voltage Each phase is at line-to-Neutral voltage above ground.. (a) (b) NORMAL OPERATION GROUND FAULT ON PHASE C Phase C is now at ground potential. Negligible fault current flows as there is no return path back to the source. Phase A and B are now at full line-to-line voltage above ground. Neutral point established by distributed capacitance. Voltage relationships. Phase A Conductor Phase C Conductor Phase B Conductor Delta Configuration SYSTEM? 3 4 - I-Gard - The Leader in Ground Fault Protection 2.2 WHAT DOES IEEE SAY ABOUT Ungrounded systems employ ground detectors to indicate a ground fault. These detectors show the existence of a ground on the system and identify the faulted phase, but do not locate the ground, which could be anywhere on the entire system. If this ground fault is intermittent or allowed to continue, the system could be subjected to possible severe over-voltages to ground, which can be as high as six or eight times phase voltage. This can puncture insulation and result in additional ground faults. A second ground fault occurring before the ﬁrst fault is cleared will result in a phase- to-ground-to-phase fault, usually arcing, with a current magnitude large enough to do damage, but sometimes too small to activate over-current 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. IEEE 242-1986 7.2.5 RESISTANCE GROUNDED SYSTEMS 3.1 WHY CONSIDER GROUNDING If the ground fault is intermittent (arcing, restriking or vibrating), then severe overvoltages can occur on an ungrounded system. The intermittent fault can cause the system voltage to ground to rise to six or eight times the phase-to-phase voltage leading to a breakdown of insulation on one of the unfaulted phases and the development of a phase-to-ground-to-phase fault. Overvoltages caused by intermittent faults, can be eliminated by grounding the system neutral through an impedance, which is generally a resistance, which limits the ground current to a value equal to or greater than the capacitive charging current of the system. The intentional connection of the neutral points of transformers, generators and rotating machinery to the earth ground network provides a reference point of zero volts. This protective measure offers many advantages over an ungrounded system, including: • Reduced magnitude of transient over-voltages • 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. 3.2 WHAT IS A RESISTANCE There are two broad categories of resistance grounding: low resistance and high resistance. In both types of grounding, the resistor is connected between the neutral of the transformer secondary and the earth ground. 3.3 WHAT IS A LOW RESISTANCE Low resistance grounding of the neutral limits the ground fault current to a high level (typically 50 amps or more) in order to operate protective fault clearing relays and UNGROUNDED SYSTEMS? YOUR SYSTEM? GROUNDED SYSTEM? GROUNDED SYSTEM? 4 5 - I-Gard - The Leader in Ground Fault Protection current transformers. These devices are then able to quickly clear the fault, usually within a few seconds. The importance of this fast response time is that it: • Limits damage to equipment, uploads/Geographie/ grounding-guide 1 .pdf