CHAPTER 15 GROUNDING REQUIREMENTS FOR ELECTRICAL EQUIPMENT

Transcription

1 CHAPTER 15 GROUNDING REQUIREMENTS FOR ELECTRICAL EQUIPMENT A. General In a hazardous location grounding of an electrical power system and bonding of enclosures of circuits and electrical equipment in the power system is essential. Power systems which are not grounded are highly susceptible to overvoltage during a phase to ground fault. These overvoltages may produce arcs or sparks which reduce the safe conditions of the hazardous location because of an increase in explosion hazard. A phase to ground fault will generally connect an inductive reactance, such as an inductive coil or motor winding, to ground causing an overvoltage in an ungrounded power system. If an inductive reactance should accidentally be connected to earth, the reactance of the inductive element is in series with the capacitive reactance of the power system. It must be born in mind that every element in an electrical power system contains capacitance, no matter how small. This capacitance can be considered a reactance which couples the wiring of the power system to earth. Because of the capacitance coupling, the neutral of the power system cannot be considered truly divorced from earth when not purposely grounded to earth. As a result of this condition the accidental connection of an inductive reactance to ground may be the cause of serious overvoltage in the power system. When the inductive and capacitive reactance is the same, the voltage across both reactances are also the same and may become extremely high, as much as 10 times normal or more. It is the ratio of the inductive reactance to earth to the total capacitive reactance of the system to earth which controls the degree of the overvoltage. The highest overvoltage will occur when there is a large capacitance in the system and both reactances are the same. Therefore, if the neutral of a power system is not purposely grounded to earth, it must be recognized that a phase to ground fault may produce serious overvoltages. The danger of the overvoltage is that it puts insulation of circuits and electrical equipment under too much stress which will cause the insulation to break down. If the insulation breaks down a small current will flow from the point of failure to earth accompanied by arcs or sparks. Dangerous arcs or sparks as a result of ungrounded neutrals can be completely eliminated by suppressing

2 the overvoltage when a relatively high resistance is connected between the electrical system neutral and earth. A ground resistor of about the same ohmic value as the total charging capacitive reactance to earth is generally sufficient to completely eliminate a dangerous overvoltage. When the neutral of the power system is properly grounded (i.e., solidly or by low or high resistance), arcs or sparks can still occur when there is an insulation failure. It is not so that arcs or sparks will only occur when the power system is ungrounded. They may also appear under insulation failure when the power system is grounded. With the power system properly grounded the system insulation is not under high stress but will normally cause a large current to flow if the insulation breaks down. In both cases, in ungrounded and grounded power systems, a current will flow. This current may produce dangerous arcs or sparks along the path to ground. For example, arcing or sparking may occur along the conduit path of an electric motor when the conduit lacks sufficient continuity. The electric motor shown in Fig. 1-25, for example, receives its power through wiring enclosed in a metal conduit. If this metal conduit should be the sole external ground return path, current will flow along this path as a result of insulation failure when the motor enclosure becomes unintentionally energized. During the flow of the fault current a substantial potential difference could exist between the motor housing and earth. If the external return path lacks sufficient continuity, sparks or arcs will occur at the location where continuity is lacking. For example, if there should be a separation in the conduit run to the motor, a potential difference caused by an insulation failure will appear at the separated elements and arcs or sparks will be produced at this location. A small nonvisible separation could exist, for example, in the conduit union marked with an "a" in Fig This separation could easily exist if the two parts of the union are not completely tightened, or when the union is not free from dirt, grease or corrosion. When an arc or spark does appear in the separated elements of the union, and most likely they will under sufficient voltage stress, they can easily ignite any flammable gas or vapor in the immediate vicinity of the electric motor. Arcs or sparks may also be produced between the two metal pipes at location "b" as shown in Fig One pipe is in direct contact with the electric motor enclosure, and the other pipe is in direct contact with earth. Both pipes are separated from each other by a gap of high resistance. During an insulation failure the gap allows a potential difference to exist between the pipes. Arcs or sparks are generally of sufficient energy to initiate an explosion when the faulty electrical equipment is inductive and surrounded by a flammable gas or vapor. The minimum sparking energy required to ignite hydrocarbon-air mixtures ranges from approximately to 0.3 millijoules. A hydrogen gas-air mixture,

3 for example, can be ignited by a spark with an energy as low as millijoules (One joule is the electrical energy in terms of 1.0 Volt x 1.0 Amp per second.) When the neutral of the power system is grounded, dangerous arcs or sparks can also be completely eliminated by applying two grounds: 1) an internal or external grounding conductor running in close proximity with the phase conductors between the electrical equipment housing and the neutral of the power supplying equipment, and 2) by bonding the housing of the electrical equipment to a supplementary grounding system which also must be connected to the grounded neutral of the power supply equipment. This supplementary grounding system has a dual function. It eliminates arcs or sparks and it eliminates shock hazards when a fault current is flowing as a result of an insulation failure. The supplementary ground alone, without an internal or external grounding conductor is not permitted. The power supplying conduit to the electric motor as shown in Fig which is partially buried in earth is generally inadequate to function as a supplementary ground. Proper grounding of circuits and electric equipment in a hazardous location is, therefore, of vital importance. Consequently, the recommended grounding practice for a hazardous location, is not only grounding of the system neutral, at the power source, but also by using an external or internal grounding conductor in combination with a supplementary grounding system. B. Internal and External Grounding Conductors There are two types of grounding conductors: an internal grounding conductor and an external grounding conductor. Both types are required for carrying phase to ground fault currents from an unintentionally energized circuit or equipment enclosure to the neutral of the electrical power source. An internal grounding conductor may consist of a copper wire, solid or stranded, insulated or bare. An external grounding conductor usually consists of rigid metal conduit, electrical metallic tubing, flexible metal conduit approved for the purpose, a cable tray, armor of type AC cables, or other raceway approved for carrying ground fault currents. Both types of grounding conductors are applied for bonding and grounding enclosures for circuits and electrical equipment. Only one of the types is normally used for grounding purposes. If an internal grounding conductor is used for grounding electrical equipment it is normally colored green, insulated or bare. External grounding conductors consisting of rigid metal conduits are usually less reliable than internal copper grounding conductors. The reason for this is that conduit joints may be of poor workmanship causing high resistance or preventing continuity as explained before. Therefore, external grounding conductors may allow arcs or sparks to occur under fault conditions. Arcing may start between the threads of the joints at certain

4 UNION (a) VERTICAL PUMP FIG SUPPLEMENTARY GROUNDING OF ELECTRICAL EQUIPMENT

5 current levels when the joints are not completely tightened or when they are not sufficiently clean. These joints may not only produce arcs or sparks but also a stream of molten metal during heavy fault conditions. External grounding conductors consisting of aluminum conduits are more suitable for a fault return path because the probability of producing arcs between the threads is much less. The reason is that the softer the material, the more the threads tend to deform. Use of aluminum will therefore ensure a better electrical continuity when the couplings are tightened. However, external grounding conductors and fittings made of aluminum shall not be used in earth or concrete when subject to corrosive conditions. Where general purpose enclosures are used in hazardous locations the ground return path may become even more unreliable because metal threaded conduits may be used in conjunction with ordinary locknuts and bushings. Ordinary locknuts may be used, but only if bonding jumpers are applied between the enclosure and the raceway. Bonding jumpers could be deleted if both locknuts are of the carving type. These locknuts, when applied to rigid steel conduits entering a general purpose enclosure, will carve into the metal of the enclosure and will provide a low impedance between raceway and enclosure. However, the application of carving type locknuts must be considered unreliable because their application depends entirely on proper workmanship. When this is lacking, arcing and sparking may occur between raceway and enclosure under fault condition. Bonding jumpers may be deleted if the bushings are of the bonding type in which a jumper must be applied between the bushing and a ground terminal in the general purpose enclosures. An external grounding conductor, for example, is the vertical conduit to the electric motor in Fig Whether an internal or external grounding conductor is used, they are required to run in close proximity with their phase conductors. This is to minimize the impedance of the ground return path. The impedance of the phase conductors and of the grounding conductors depends greatly on the size of the conductors but mostly on the distance between the individual conductors. A low impedance of the ground return path is important because it allows fast tripping of the overcurrent devices under fault condition and it will shorten the life span of arcs or sparks if they do appear under a phase to ground fault. The tripping time of overcurrent devices is dependent on the magnitude of the phase to ground fault current which in turn is a function of the impedance of the ground fault return path. If the impedance of the grounding conductor is high, the higher impedance will reduce the fault current to a lower magnitude resulting in a longer tripping time. The arrangement in Fig. 1-26, for example, is in violation of the requirements for fast tripping. The fault current in the lamp is required to follow the same route as the current in the supply conductors. But, instead, the fault current will flow through the metallic return path as shown by arrows in Fig

6 BRANCH CIRCUITS W LAMP PHASE-TO-GROUND FAULT LIGHTING PANEL PANEL BOLTED TO AND IN METALLIC CONTACT WITH GROUNDED STEEL STRUCTURE METAL FLOOR PVC PIPE PVC PIPE SERVICE SWITCH CONCRETE FLOOR GROUND GRID OR GROUND ELECTRODE FIG INCORRECTGROUNDING METHOD

7 Since the metallic return path has a much higher impedance, the magnitude of the fault current will be smaller, resulting in a longer tripping time. With the equipment ground not kept physically close to the supply conductors, the impedance of the fault circuit will have a greater inductive reactance and a greater AC resistance due to a smaller mutual cancellation of the magnetic fields around the conductors, resulting in a greater voltage to ground while the circuit overcurrent devices will operate slower because of the smaller current. Therefore, the steel framework of a building that is constructed without regard for a low impedance for the flow of fault current, does not comply with the fast tripping requirements when it is used as the sole grounding conductor. Internal and external grounding conductors are shown in Fig The power supplying equipment shown in Fig represents a "service supplied AC system" which requires two system grounds: one grounding conductor is to be connected from the transformer neutral to an electrode in earth, the other grounding conductor is to be connected from the neutral in the service panel to another electrode in the earth. This system shown in Fig is not a "separately derived system." If the power system should consist of a separately derived system, then only a single ground connection is required either at the transformer neutral or at the service panel depending on whether the service panel is provided with a main disconnecting means or not. If not, the grounding connection can only be made at the transformer even when the service panel is provided with individual branch overcurrent devices. C. Supplementary Ground System The basic concept for applying supplementary grounding in a hazardous location is to reduce the potential differences between the electrical equipment and earth during a phase to ground fault. Reducing the potential differences is accomplished by bonding the enclosures of circuits and electrical equipment to the supplementary ground system by means of a bonding jumper "c" as shown in Fig A supplementary ground system may consist of the following: (1) A ground grid system of copper conductors buried in earth 2 1/2 feet or more deep, each conductor not smaller than 1/0 AWG. (2) A single bare copper conductor sized 1/0 AWG. minimum buried in earth at least 2 1/2 feet deep and looped around the electrical equipment. (3) The metal frame of a medium-sized building with the building columns thermally welded to a copper grounding conductor looped around the building. The ground loop is required to be buried a minimum of 18" below the finished

8 grade. If a water pipe is available, the loop must be connected to the water pipe if it is of sufficient length (10 feet or more). Underground metal gas pipes and aluminum electrodes are prohibited. The size of the ground loop is determined by the magnitude of the current and the time of the current flow based on an allowable maximum temperature. The following equation may be used in determining the size of the ground loop: For an initial temperature of 25 0 C and a final temperature of 25O 0 C, the minimum size of the ground loop will be: CM = I 5 V(f). Where I s is the RMS short circuit current and t is the tripping time in seconds. If, for example, I 5 = 25,000 amps and t = 0.57 seconds, then CM = x 25,000 V(0.57) = 211,600 CM or 4/0 AWG. (4) The metal frame of a large building provided with a network of copper conductors underneath the foundation of the building. With the grounding network supplemented by galvanized or copper-coated grounding rods of at least 8 feet in length and 3/4" in diameter. Electrical equipment located in these buildings must be grounded to the building structure by means of ground leads or by bolting or welding the electrical equipment to the steel frame of the building. There are practical limits which will determine the minimum and maximum size of the supplementary grounding system. For mechanical strength, the buried conductors shall not be smaller than 1/0 AWG but it is not necessary to exceed 500 MCM. A supplementary grounding system is not permitted to be used in lieu of internal or external grounding conductors. They may only be used for supplementary protection. Where a metal sleeve is used for protection of the grounding conductor to the supplementary ground system, the sleeve must be bonded at both ends to the grounding conductor. The supplementary ground system must also be connected to the neutral of the power supplying equipment as shown in Fig

9 EXTERNALGROUND POWER SOURCE SERVICE BRANCH LOAD EQUIPMENT NEUTRAL BOND BOND GROUNDING CONDUCTOR INTERNAL GROUND BOND X: CONNECTION FOR EXTERNAL GROUND SUPPLEMENTARY GROUND FIGURE INTERNAL/EXTERNAL GROUND WITH SUPPLEMENTARY GROUND