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ulation within its rating by increasing the effective cooling of the transformer. This can be accomplished with forced-air cooling. Where load growth is anticipated, consid- eration should be given to providing fans for forced-air cool- ing; or to providing the brackets, temperature switch, and wiri...

ulation within its rating by increasing the effective cooling of the transformer. This can be accomplished with forced-air cooling. Where load growth is anticipated, consid- eration should be given to providing fans for forced-air cool- ing; or to providing the brackets, temperature switch, and wiring necessary to accommodate the future addition of fans. Forced-air cooling of a transformer also allows increased capacity while holding the impedance to a value that permits the use of secondary switching equipment of lesser intermpt- ing capacity. Forced-air-cooled transformers should be con- sidered when automatic bus transfer is provided between the secondaries of two transformers. With this arrangement, the transformer may be more economically loaded under normal operation. Forced-air cooling also permits carrying additional load, (up to 2?? on’large transformers) without exceeding the specified temperature limits. 4.1 1 S.7 Forced-Oil Cooling Forced-oil cooling is another method of increasing trans- ’- former capacity because it reduces oil temperature. The pumping and circulating of oil is common in the design of larger (above 8 MVA) power transformers. 4.1 1.6 Loading A properly designed electrical system will seldom require the emergency loading of transformers. System growth may, however, make such loading necessary, in which case IEEE C57.91 should be consulted. 4.12 OVERHEAD ELECTRIC POWER DISTRIBUTION 4.12.1 General The use of open conductors, pre-assembled or field-spun aerial cable, and spacer cable supported by poles or struc- tures for distribution systems outside of process limits, util- ity areas, and operational areas should be subject to engineering approval. Overhead electrical distribution sys- tems should be designed and installed in accordance with the requirements of NFPA 70, ANSUEEE C2, and applica- ble state and local codes. 4.12.2 Materials Most pole-line materials conform to the standards and sug- gested specifications of the Edison Electric Institute (EEI)16 or I ~ 16Edison Electric Institute, 701 Pennsylvania Avenue, Washington, D.C. 20004. are similar in design, material, and workmanship. Special structures, such as A-Frame or H-Frame structures, may be required for the support of a line or a group of lines whose loading is in excess of that which can be safely or economically supported on single poles or other simple structures. 4.12.3 Aerial Cable Aerial cable provides an alternative to open conductor dis- hibution. Aerial cable is available in single- and three-con- ductor types, shielded or nonshielded; and aerial cable with a self-supporting synthetic jacket is preferred. Messengers of self-supporting cable should be grounded at frequent inter- vals. Surge arresters should be installed at terminal poles where aerial cable is connected to open conductors. 4.12.4 Metal-Clad Cable Metal-clad (Type MC) cable supported by a messenger may be used as an alternative to aerial cable. Metal-clad cable consists of one or more conductors with necessary insulation, shielding, and fillers over which a suitable metallic sheath is applied. A jacket is normally supplied over the sheath. Gener- ally, this sheath should not be relied on as a grounding con- ductor; a grounding conductor should be installed in the cable interstices during manufacture. 4.12.5 Accessibility All parts of the overhead distribution system that must be examined or adjusted during normal operation should be readily accessible to authorized personnel. Provisions should be made to ensure adequate climbing spaces, working spaces, working facilities, and clearances between conductors. The electrical clearances must be established in accordance with ANSIAEEE C2 and any applicable state and local codes. 4.12.6 Isolation and Guarding To provide for the safety of employees not authorized to approach conductors and other current-carrying parts of elec- trical supply lines, the arrangement of live parts must ensure adequate clearance to ground, or guards should be installed to isolate these parts effectively from accidental contact. 4.12.7 Grounding of Circuits and Equipment Grounding of circuits and equipment should conform to ANSUEEE C2 and any applicable state and local codes. Metallic sheaths, conduits, metal supports, fixtures, frames, cases, and other similar noncurrent carrying parts should be properly grounded. A temporary ground for maintenance pur- poses should consist of a secure mechanical connection to a buried metallic structure or driven ground rod. Resistance of such a ground should limit touch and step potentials to acceptable levels in accordance with IEEE 80. 4.12.8 Clearances The clearances specified for conductors in ANSVIEEE C2, Section 23, are a minimum recommendation. Any applicable local and state requirements must also be considered. 4.12.9 Location 4.12.9.1 Routing The recommended routing for overhead lines is along facility roads or streets. When lines must be located in tank areas or other locations that are not reached by roadways, it is recommended that the lines be routed along earthen fire- walls, the toe of dikes, or other logical routes. The location of overhead lines should comply with the requirements of applicable fire codes; and wherever possible, lines should be routed so as to minimize exposure to damage from fires originating in equipment or structures along their routes. Lines should not be run in areas where interference with crane booms and similar apparatus is likely during normal plant operation or routine maintenance. 4.1 2.9.2 Water Spray Exposure Overhead lines should be located far enough from cooling towers, spray ponds, and other sources of water spray to avoid fouling their insulation and corroding their metal parts. When this is not practical, the lines should be designed and con- structed to withstand the particular type of exposure to which they will be subjected. This may require overinsulation of the exposed sections of lines, the use of materials to withstand the corrosive effects of the spray, or other measures. 4.12.9.3 Petrochemical Exposure When overhead lines are exposed to petrochemicals or other similar contaminants, the lines should be designed to withstand the effects of such contaminants. Specially approved silicon-type grease on the insulators, bushings, and similar items provides a strong deterrent to current leakage and insulator flashover. Silicon tends to absorb the foreign matter deposited on the dielectric material and continually provides a nonconducting, water-repellent exterior seal for the equipment. 4.12.9.4 Lines Adjacent to an NFPA 70 Defined Class I Location When installed adjacent to or traversing Class I locations, overhead lines should be placed so that the current-canying components will be outside the space that may contain flam- mable gases or vapors (see API Rp 500). Conventional over- head-line construction normally meets this requirement ,,. because of the isolation naturally afforded by the horizontal distance from, or elevation above, the classified location. 5.1 PURPOSE The following grounding practice is recommended: This section provides a guide to the general principles of grounding and lightning protection as they apply to petro- leum processing plants. 5.2 SCOPE This section is limited to the consideration of grounding practices in the following categories: a. System grounding: The protection of electrical equipment and the reliability of an electrical system. b. Equipment grounding: The protection of personnel against electric shock. c. Lightning protection against the hazards of fire and explo- sion, as well as damage to electrical equipment, caused by lightning. 5.3 STATIC ELECTRICITY AND STRAY CURRENTS The application of bonding and grounding for protection against the effects of static electricity and stray currents (such as currents associated with cathodic protection) is not covered in this section. These important subjects are discussed in API RP 2003 and NFPA 77. 5.4 SYSTEM GROUNDING 5.4.1 General Electric power distribution system grounding is concerned with the nature and location of an intentional conductive con- nection between the neutral (or derived neutral) of the system and the ground (earth). The common classifications of grounding methods used in industrial plant power distribution systems are as follows: a. Ungrounded. b. Low-resistance grounded. c. High-resistance grounded. d. Reactance grounded. e. Solidly grounded. The nature of system grounding significantly affects the magnitude of line-to-ground voltages under both steady-state and transient conditions. Without system grounding, severe overvoltages can occur; reducing insulation life and present- ing a hazard to personnel. System grounding can control these overvoltages to acceptable levels. Further, NFPA 70 requires that certain systems be solidly grounded. For these reasons, some type of system grounding is gener- ally recommended. a. Systems rated at less than or equal to 480 V that supply phase-to-neutral loads must be solidly grounded. These include 120/240-V, single-phase, three-wire systems; 208Y/ 120-V, three-phase, four-wire systems; and 480Y/277-V, three-phase, four-wire systems. b. Low-voltage (480-V and 600-V), three-wire systems should be either high-resistance grounded or solidly grounded. c. All other plant distribution systems may be resistance grounded. These include 2,400 V through 34,500-V, three- phase, three-wire systems. (Open. wire distribution may require solid grounding.) A full discussion of the relative merits of the various sys- tems is not within the scope of this section, but a brief sum- mary of the principal features is included. A more extensive discussion of the subject can be found in IEEE Std 142. 5.4.2 Ungrounded System In an ungrounded system, there is no intentional connec- tion to ground, but the system is capacitively grounded because of the capacitance coupling to ground of every ener- gized conductor. The operating advantage of this system is that a single line-to-ground fault will not result in a trip-out of the circuit because there is only a minor charging current flowing to ground. During such a fault, the other phases will be subject to line-to-ground voltages equal to the full line-to- line voltage; therefore, insulation for equipment used in such a system must be properly rated for this condition. Further, because of the capacitance coupling to ground, the ungrounded system is subject to overvoltages (five times nor- mal or more) as a result of an intermittent-contact ground fault (arcing ground) or a high inductive-reactance connected from one phase to ground. The advantage of the ungrounded system will be lost if the ground is allowed to persist until a second ground occurs. A second ground would cause an outage if it is on another phase. An adequate ground detection system, along with a program for removing grounds, is essential for satisfactory operation of an ungrounded system. 5.4.3 Grounded Systems Resistance grounding employs a resistor connected between the system neutral and ground. This resistor is in parallel with the total system-to-ground capacitive reactance. The high-resistance grounded system employs a resistance value equal to or slightly less than the total system-to-ground capacitive reactance. (The size of the resistor is normally expressed in amperes.) This will limit the ground-fault current to a few amperes and will eliminate the high transient overvolt- ages that can be created by an inductive reactance connected ftom one phase to ground or from an intermittent-contact ground fault. The high-resistance grounded system also pro- vides a convenient means for detection of and alam on a ground fault and facilitates the use of equipment which can determine the fault location without electrical system shut- down. In addition, this system is similar to the ungrounded sys- tem in that it can continue operation with a single line-to- ground fault if the maximum fault current (and total system-to- ground capacitive charging current) is limited to less than 10 amps (see IEEE 142 for additional information). The ground fault, once detected, should be cleared. as soon as possible because the system is not designed to operate with the ground fault condition indefinitely. The low-resistance grounded system uses a value of resis- tance that is sized to give a ground-fault current value suit- able for relaying purposes (see IEEE 32 for resistor time rating). Typical current values will range from 200 amps on ’ systems using sensitive ,window-type current transformer ground-sensor relaying to 2,000 amps on the larger systems using ground-responsive relays connected in current trans- former residual circuits. This system provides a controlled value of ground-fault current and eliminates the overvoltage problems of the ungrounded system, but the action of a three phase circuit-switching device is required to clear a single line-to-ground fault. Installation of resistance-grounded systems requires that equipment basic impulse levels as well as the application of surge arresters be reviewed carefully. The solidly grounded system gives the greatest control of overvoltages but develops the highest ground-fault currents. These high currents may cause damage in equipment and may create other shock-hazard problems for personnel if equipment grounding is inadequate. However, the high magnitude ground current may be desirable to ensure effec- tive operation of phase-overcurrent trips or interrupters. Cable shields must be sized to carry the available ground- fault current for the duration of the’fault without exceeding cable thermal limitations. Reactance-grounded systems are not ordinarily employed in industrial power systems and will not be discussed here. 5.5 EQUIPMENT GROUNDING 5.5.1 Purpose Equipment grounding accomplishes the following: a. Ensures that all of the parts of a structure or an equipment enclosure are not at a voltage above ground that would be dangerous to personnel. Adequate ground connections and devices should to ensure that abnormal conditions, such as ground faults or lightning strokes, will not raise the potential of the structure or enclosure to a dangerous level. b. Provides an effective path over which fault currents involving ground can flow without sparking or overheating to avoid ignition of combustible atmospheres or materials. 5.5.2 Grounded Equipment The metal framework of all buildings and structures hous- ing or supporting electrical equipment and all noncurrent- carrying metal parts of electrical equipment and devices should be grounded by connection to a grounding system. In general, equipment grounding conductors should be con- nected as directly as practicable to the electrical system ground. Routing the grounding conductors as close as practi- cable to supply conductors will minimize the voltage drop under fault conditions. 5.5.3 Equipment Grounding System The principal requirement of an equipment grounding sys- tem is to maintain the resistance to earth of structures and equipment enclosures at the lowest practicable value. With an adequate system, the potential to ground during fault condi- tions will not be dangerous to personnel (because of equaliz- ing of potentials) and equipment, and protective devices will operate properly. ways. The grounding system for a large or complex plant may involve an extensive network of equipment enclosures and structure ground grids interconnected by cables to provide an overall plant-grounding system. Specific requirements for grounding systems are given in NFPA 70, and detailed infor- matiòn is included in IEEE Std 142. 5.5.4 Specific Grounding Applications 5.5.4.1 Structures and Process Equipment Grounding-system connections may be made in various .’ Steel building frameworks, switchgear structures, and similar installations should be grounded at several points (at least two per structure) with substantial connection to the grounding system grid. Tanks, vessels, stacks, exchangers, and similar equipment not directly supported by or bolted to a grounded supporting structure should be grounded using a minimum of two connections to the grounding system grid. Special attention should be paid to piping systems to assure the pipe is adequately grounded. Inadequate grounding could result in a difference of potential if for example the pipe was separated at a flange to replace a gasket. This could result in arcing, sparking, or a shocking an employee performing the work. 5.5.4.2 Motors and Generators Motor and generator enclosures should be connected to the overall plant grounding system. This connection is accom- plished with a mechanically and electrically continuous equipment-grounding conductor that is routed with the phase conductors of the machine. This may be a conductor run with phase conductors inside a conduit, a continuous-threaded rigid conduit system, a cable tray system, or another NFF’A- 70-approved method. In any case, the grounding connection must provide a low-impedance circuit from the machine enclosure back to the electrical system ground. Where con- duit or trays are used, joints must be made up tightly, and bonding jumpers should be installed at expansion joints and similar locations. The bonding jumpers should be inspected periodically to insure a low impedance connection. Supplemental grounding protection should be provided by connecting an additional grounding conductor from each machine to the local grounding system grid. The purpose of this connection is to equalize potentials in the immediate vicinity of each machine. 5.5.4.3 Metallic-Sheathed and Metallic-Shielded Cables The metallic sheath and metallic shield (if applicable) of any power cable should be continuous over the entire run and should be grounded at each end. Grounding of the shield at both ends may require the cable to be derated due to circulating currents. Grounding at one end is permissible if a 25-V gradi- ent is not exceeded (see IEEE Std 422 for method of estimating shield voltages). If any metallic-sheathed or metallic-shielded cables are spliced, care must be taken to obtain continuity as well as an effective physical connection with the metallic sheath or shield at the splice. Where metallic armor is used over metallic sheath, sheath and armor should be bonded together and connected to the ground system at each end of the cable and at any accessible splices. The metallic sheath on metal-clad cable may also be used as an equipment- grounding conductor if the sheath is a continuous corrugated tube. However, a sepa- rate grounding conductor installed in the cable interstices dur- ing manufacture is recommended. The distinctions between sheaths, armoring, and shields can be obscure. An overall welded metal covering is referred to as a sheath but it may act as an armor and a shield (see IEEE Std 100 for additional infor- mation). Adjustable speed drive applications may require that one end of the cable metallic shield remain ungrounded to pre- vent common mode voltages and circulating ground currents. 5.5.4.4 Conductor Enclosures NFPA 70 requires that exposed metallic noncurrent-carry- ing enclosures of electrical devices be grounded. This includes conduit, wireways, and similar wiring materials. Where the continuity of the enclosure is assured by its con- struction, a grounding connection at its termination points will be adequate. If continuity is not assured by the construc- tion, care must be taken to provide adequate connections of all sections to the grounding system grid. 5.5.4.5 Enclosures for Electrical Equipment Switchgear, control centers, and similar electrical equip- ment should include a ground bus. Where the equipment con- sists of a lineup of two or more sections, two grounding connections to the grounding system grid, one on each end of the ground bus, are recommended. 5.5.4.6 Fences Metal fences and gates enclosing electrical equipment or substations must be connected to the grounding system grid. A number of factors are involved here, including the resis- tance to ground of the substation grounding system, the dis- tance of the fence from grounding electrodes, and voltage gradients in the soil. (For additional information, see IEEE Std 80 and Std 142.) 5.5.5 Portable Electrical Equipment This paragraph is limited to consideration of portable elec- trical equipment operating at less than or equal to 600 volts; portable equipment operating at higher voltages is applied infrequently and requires special consideration. IEEE Std 142 provides information on portable electrical equipment operat- ing at higher voltages. Portable electrical equipment poses one of the greatest potential hazards to personnel, so it is mandatory that the enclosures of portable equipment of any type be maintained at ground potential or be protected by an approved system of double insulation. Portable electrical equipment that is without double insula- tion and is operating above 50 V must be provided with a cord containing a separate grounding conductor which terminates in a grounding-type plug that is used with a matching receptacle. The grounding contact of the receptacle must be properly tied to a grounding system. NFPA 70 requires ground-fault circuit interrupters for all 125 V single-phase 15 amp and 20 amp receptacle outlets on temporary wiring used for maintenance or construction. An assured grounding program is an acceptable alternative to ground-fault circuit interrupters (for additional information see NFPA 70, 305-6). The ground-fault circuit interrupter is for personnel safety and is not to be confused with ground-fault protection of equipment requirements for items such as electrical resistance heating elements. 5.5.6 Instrument Grounding Special considerations apply to instrument grounding. All grounding systems should be tied together in accordance with NFPA 70. In general, a power supply, equipment, and cable shields should be brought to a single point on the overall plant grounding system (procedures are necessary to allow for safe troubleshooting that may require a momentary separation of the tie to the overall plant grounding system). (See 9.8 and IEEE Stds 518 and 1100 for additional information.) 5.6.1 Acceptable Ground Resistance Ideally, a ground connection would have zero resistance, but this is impossible. The resistance of a ground connection is a function of soil resistivity and the geometry of the grounding system. In soils of high resistivity, extensive arrangements may be required to obtain an acceptable low- resistance ground. The allowable resistance varies inversely with the fault cur- r