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ent to ground: the larger the fault curient, the lower the resis- tance. For large substations and generating stations, the resistance of the system grounding grid should not exceed 1 ohm. For smaller substations and for industrial plants, a resis- ‘tance of less than 5 ohms should be obtained, if p...

ent to ground: the larger the fault curient, the lower the resis- tance. For large substations and generating stations, the resistance of the system grounding grid should not exceed 1 ohm. For smaller substations and for industrial plants, a resis- ‘tance of less than 5 ohms should be obtained, if practicable. NFPA 70 approves the use of a single-made electrode if its resistance does not exceed 25 ohms. 5.6.2 Grounding Electrodes Driven ground rods, 5/8 to 1 in. in diameter, and 8 or 10 ft long, are the most common type of grounding electrodes; however, a single ground rod is not adequate when relatively low resistance is required. A single 3/4-in. x 10-ft ground rod will have a resistance to ground of over 6 ohms, even in soil of low resistivity (2,000 ohm-cm). A number of rods con- nected by buried cable can be used to obtain lower resistance, and longer rods can be used where soil conditions permit; however, because of mutual effects, ground resistance does not decrease in direct proportion to the number or length of rods. Buried metallic piping or other existing underground metallic structures, including concrete-encased electrodes, for example, rebar in concrete foundations (see NFPA 70, Article 250), are also frequently used as grounding electrodes. Ground mats consisting of buried cables with or without ground rods at cable intersections commonly form a portion of the grounding electrode system used at substations. (See IEEE Std 142 for additional information.) 5.6.3 Step and Touch Potentials Where current flows into the soil from a grounding elec- trode, potential gradients are created in the soil. The ground- ing configuration should ensure that the potential gradients will not create a hazard to personnel standing or walking on the ground in the vicinity of a grounding electrode, or touch- ing a grounded structure carrying ground current. (See IEEE Handbook For Electrical Engineers? 5.6.5 Corrosion Problems Copper is commonly used for grounding system grids because of its resistance to corrosion and high conductivity. Because of the galvanic,couple between copper and steel, an extensive copper grounding system grid may accelerate cor- rosion of steel piping and other buried structures that are connected to the system. Under this condition, galvanized steel ground rods and insulated or coated copper conductors could be used, but care must be taken to ensure that the grounding electrodes do not con-ode and reduce their effec- tiveness; and that the use of insulated or coated conductors does not prevent the overall grounding system from main- taining safe step and touch potentials. Cathodic protection of the buried steel subject to corrosion should be considered to alleviate this problem. 5.7 LIGHTNING PROTECTION 5.7.1 General Lightning is a very large’ electrical discharge in the atmo- sphere between the earth and a charged cloud or between two oppositely charged clouds. The energy in a lightning stroke can readily ignite flammable vapors; and damage to equipment and structures can result from the flow of lightning discharge cur- rent through any resistance in its path. Lightning protection systems use air terminals (rods, masts, or overhead ground wires) to intercept lightning strokes and to divert the lightning current to ground through circuits of low electrical impedance. 5.7.2 Zone of Protection The zone of protection of an air terminal is defined by a circular arc concave upward, passing through the tip of the air terminal and tangent to the ground plane. For complete pro- tection, the radius of the arc must be less than the striking dis- tance of the lightning stroke. In practice, it is conservative to use a radius of 30 m (100 fi). For air terminals less than 15 m (50 ft) above the ground, the zone of protection may be assumed to be a cone with its apex at the top of the air tenni- na1 and a base radius equal to the air terminal height. All structures completely within the zone of protection may be considered essentially immune from direct lightning strokes. (See NFPA 780 for additional information.) Std 80 for additional information.) 5.7.3 Need for Protection 5.6.4 Ground Resistance Measurement A number of factors should be taken into consideration when deciding whether or not lightning protection devices are h many installations, it is necessary to measure the resis- The major factors to be are as follows~ tance to earth of the grounding. system to determine if the actual value of this re&tance within design limits. Meth- a. The frequency and severity of thunderstorms. ods for measuring ground network resistance are discussed b. Personnel hazards. c. Inherent self-protection of equipment. d. Value or nature of the structure or contents and of other structures that might be involved if lightning caused a fire or explosion. e. Possible operating loss caused by plant shutdowns. 5.7.4 Protected Equipment 5.7.4.1 Steel Structures, Tanks, Vessels, and Stacks Ordinary steel structures, process columns, vessels, steel storage tanks, and steel stacks of a petroleum processing plant or similar installation will not be appreciably damaged by direct lightning strokes. However, it is necessary to ground the taller structures adequately to prevent possible damage to their reinforced concrete foundations, and to provide a zone of protection for electrical apparatus and other equipment in the immediate area. (See MI W 2003 and NFPA 780 for additional information.) 5.7.4.2 Electric Power Distribution Systems Electric power distribution systems should be protected against lightning strokes to avoid damage to equipment, a plant shutdown, and personnel shock hazards. Overhead lines can be shielded from lightning strokes by the installa- tion of overhead ground (static shield) wires that provide a triangle of protection for the phase conductors. Similarly, substations and outdoor switching equipment can be shielded by lightning towers or overhead static shield wires, but these shielding devices must be connected to an ade- quate grounding system to be effective. Aerial cable nor- mally will be protected by its messenger cable if the messenger is adequately grounded at intervals defined in ANSMEEE C2. If the cable has a metallic sheath or armor, the sheath or armor should be bonded to the messenger cable at each grounding point. Feeders consisting of cables in metallic conduit are essentially self-protecting; but con- duits and metal sheaths should be properly grounded and bonded to the equipment at each end. 5.7.5 Surge Arresters When electrical equipment is connected to an electric power distribution system that is exposed to direct lightning strokes, or to voltage surges caused by indirect lightning strokes, the electrical equipment should be protected by suit- able surge arresters. Arresters have the ability not only to pass essentially no current at line voltages but also to pass ,very high current at surge voltages with little voltage drop. This protection through surge arresters would be in addition to the types of shielding outlined in 5.7.4.2. (See IEEE Stds 141 and 242 for additional information.) Arresters should be installed as close as possible to the equipment to be protected. They are recommended as follows: a. At both high- and low-voltage terminals of distribution, and power transformers with open bushings. b. At the junction of a transfolmer feeder cable and open- wire line for completely enclosed transfolmers. Depending on the cable length and the arrester rating, surge arresters may be required at the transformer terminals as well. c. On open-wire lines, at each point where a cable junction is made. d. At the terminals of dry-type transformers when fed from an exposed line. e. At the terminals of important motors fed from an exposed line or supplied by a transformer fed from an exposed line. f. On the secondary side of a transformer fed from an exposed line, for the protection of a group of motors (usu- ally combined with surge capacitors at the motor terminals). Arresters installed on systems connected to utility power should be coordinated with the utility. 5.7.6 Instrument Lightning Protection Process instrument and control systems, remote tank gaug- ing systems, and other similar low-energy systems can be damaged by lightning-induced transients even though they are protected from direct lightning strokes. Protection against such transients can be provided by combinations of series resistors with Zener diodes, metal oxide varistors, or other devices to bypass voltage surges to ground. Most equipment suppliers can recommend methods of transient suppression to protect their equipment; these recommendations should be followed. To be effective, the protective devices must be con- nected to an adequate grounding system. 5.7.7 Surge Capacitors Surge capacitors are,used to reduce the rate-of-rise of volt- age surges to protect AC rotating machines and other equip- ment having low electrical impulse or turn-to-turn insulation strength. They are usually applied in conjunction with surge arresters and connected line-to-ground. The capacitor voltage rating must match the system voltage and be designed for surge protection applications. The connection leads between the capacitor and each phase and between the capacitors and ground must be as short as possible. 6.1 PURPOSE Many manufacturers offer severe-duty type motors that pelform well in petroleum facility atmospheres (see IEEE motors and controllers to meet the varied demands of the and having Class F insulation with class B rise. petroleum industry. It highlights considerations which must The motor would normally have a factor, and if be addressed in with Stds 541, 546, and operated at 1.0 service factor (full-load nameplate rating), it IEEE Std 84 l. will experience extended insulation and bearing life because 6.2 SCOPE of a lower operating temperature. This section as a guide for and Std 841). This standard requires Severe duty features, high Because of its broad application, the material presented in this section will be general in nature and reflect current petro- 6.4 RELATIVE LOCATIONS OF MOTORS AND CONTROLLERS leum industry practice. Industrial motors and controllers are manufactured in accordance with applicable standards pub- lished by EEE, NEMA, ANSI, and API. When more specific or detailed information is required, the equipment manufac- turer should be consulted. Most driven equipment is constant speed. Three-phase AC motors are well suited to these applications. DC motors are not common in petroleum facilities because additional requirements would be necessary for their installation in clas- sified locations. The hgh equipment and maintenance costs of DC motors and controls compared with three-phase AC motors also make the DC equipment unattractive. It is common practice to use magnetically operated con- trollers and to install them remotely from the motors. These remotely mounted controllers will be group-mounted in one or more assemblies, usually motor control centers or switch racks. It is generally not practical to locate the controllers adjacent to the motors in a typical facility. 6.5 FREQUENCIES A frequency of 60 Hz is recognized as the preferred stan- dard for all AC systems and equipment in North America. Standard motors are also available for operation at frequen- cies of 25 Hz and 50 Hz. 6.3 MOTOR RATING AND EFFICIENCY 6.6 STANDARD VOLTAGE FOR MOTORS Motors have been rerated at various times, usually result- ing in smaller frame sizes for given horsepower ratings. The last rerate program resulted in the NEMA T-frame series (143T through 445T, approximately 3/4 HP through 250 HP). These motors are rated 200 V, 230 V, 460 V, and 575 V. Class B insulation is the minimum insulation used, but Class F insulation is normally specified. Individual manufacturers should be consulted for frame and horsepower assignments. Since standard-efficiency T-frame motors may operate at higher insulation and bearing temperatures, it is recommended that care be exercised in sizing their associated loads. They should also be operated as near to rated voltage and frequency as possible. Most manufacturers now offer high- and premium-effi- ciency motors at an increase in price. Where motors run con- tinuously or for long periods of time, the reduction in power cost will usually justify the extra cost of the high-efficiency motors. The justification is based on power cost and rate of return required for the additional investment. This will vary with different companies and types of projects. Section 3 pro- vides assistance with economic evaluation. The U.S. Energy Policy Act of 1992 (implemented October 24, 1997) effec- tively removes standard-efficiency, horizontal-footed motors United States. Similar action has taken place in Canada. i rated through 200 HP from the new motor market within the I Most of the motors used in the industry have voltage rat- ings as indicated in 6.7. In addition to the ratings listed, other standard ratings are available aqd are sometimes used. For information regarding standard motor ratings, the user should refer to catalogs and other data available from manufacturers. Additional information may be obtained from current ANSI and NEMA standards. 6.7 MOTOR VOLTAGE SELECTION 6.7.1 Single-phase Motors Single-phase motors driving fixed equipment usually are rated to operate at 115 V or 230 V. For portable motors, 115 V is generally preferred, except where there is reason to use equipment designed for some lower voltage, such as 32 V. Because single-phase units frequently employ potentially sparking mechanisms, care should be exercised in the appli- cation of this type of motor in classified areas. 6.7.2 Three-phase Motors Depending upon local power system utilization practice, either 460-V or 575-V ratings are preferred for 60 Hz low- voltage service (less than 600 V). Motors with a rating of 460V or 575 V have a voltage tolerance of flO% (per NEMA MG 1) and are generally supplied from a 480-V or 600-V power system, respectively. Motors with ratings of 200 V, 208 V, and 230 V are generally not used except in instances where power is readily and economically available at the related service voltages, and where 480-V or 600-V service would entail undue expense. For service in excess of 600 V, the preferred rated voltages for induction motors are 2,300 V and 4,000 V for motors up to 5,000 HP. For larges motors, the preferred rated motor volt- ages are 4,000 V, 6,600 V, and 13,200 V; one of these voltages must be selected to suit each specific application. Synchro- nous motors usually have nameplate voltage ratings that are identical to the service voltage of the system to which they are connected. 6.7.3 Voltage Breakpoint The economic breakpoint between the installation of the low-voltage motors (600 V class) and the medium-voltage motoss (2,300 V and higher) is usually in the range of 250 HP- 300 W. Motors with ratings of 2,300 V and 4,000 V are used for sizes up to 5,000 HP. The choice between 2,300V and 4,000 V will depend on the economics of the individual plant under consideration. For motors above 5,000 HF’, the economic breakpoint may dictate ‘the use of 4,000 V or 6,600 V, with or without captive transformers, os even 13,200 V. The economic breakpoint will vary depending on the local’ conditions and the relative number of large and small motors to be served at the location under consideration. If an economic breakpoint has not already been established, it is recommended that an engineering analysis be made before an installation is begun. This analysis will determine the economic dividing line, taking into account the cost of necessary transformers, control- lers, breakers, and all other applicable elements. After the breakpoint has been established and has been used as a guide for making installations at a particular plant or location, it is recommended that the economics be restudied regularly to make certain the previously established dividing line still holds. Allowance should be made for the value of maintaining interchangeability between motors of the same ratings and types. 6.7.4 Supply Voltage Supply voltage and frequency at the motor terminals should be maintained within the limits of NEMA MG l. 6.8 TEMPERATURE AND ALTITUDE CONSIDERATIONS IN MOTOR APPLICATIONS 6.8.1 Normal Temperature Operation Motors of standard design and construction are suitable for operation at their standard ratings, provided the ambient temperature does not exceed 40°C (104°F); however, for conditions where higher ambient temperatures prevail con- ~~ ~ ~~ tinuously os for extended periods, the continuous duty rat- ing of the motors should be reduced by some amount based on the operating ambient temperature. Refer to NEMA MG 1 if the motor is to be specified at ambient air tempera- ture exceeding 40°C (104°F). 6.8.2 High Temperature Operation Motors that are to be installed where the ambient temper- ature will normally exceed 40°C (104°F) should be consid- ered as special. They should be able to provide dependable service at the expected ambient temperature; this includes fulfilling the requirements for satisfactory lubrication at abnormally high temperatures. Motors are available with design ambient temperatures nameplated higher than 40°C (104“F), usually 45°C (113°F) and 50°C (122°F). 6.8.3 Low Temperature Operation Whese ambient temperatures of less than 10°C (50°F) will be encountered for extended periods, consideration should be given to requirements for lubrication at low temperatures. The motor manufacturer, and API Stds 541 and 546 should be con- sulted for other low-temperatuse considerations. In many instances, a low-temperature grease is suitable. For tempera- tures less than -20°C (IlOF), special material and machining may be required. Close coordination with the equipment manu- facturer is suggested for”Arctic Duty" service. 6.8.4 Elevation Motors of standard design are suitable for installation at ele- vations up to 1,OOO meters (3,300 feet). Applications above this elevation will result in increased heating and will require derat- .hg of the standard motors, or special design and manufactur- ing. The operating elevation should be specified so that manufacturers can make the necessary allowances for applica- tions above 1,000 meters (3,300 feet). When operated at eleva- tions above 1,OOO meters (3,300 feet), the specific rating and altitude should be stamped on the nameplate. 6.9 OTHER CONDITIONS AFFECTING DESIGN AND APPLICATION When motors are subjected to unusual conditions and there is doubt about the specifications when ordering, the manufac- turer should be advised of the unusual conditions to be met, especially when the motors aie to be used under the condi- tions shown in Table l. 6.10 TYPES OF MOTOR CONSTRUCTION 6.10.1 UsualTypes Most of the motors used in petroleum facilities are of the three-phase, squirrel-cage induction .type. Other types of motors have special applications or economic advantages; their uses are described in the following paragraphs. 6.1 0.2 Fractional Horsepower Motors It is a common and convenient practice to use single-phase motors for all ratings up to a fixed size, such as ‘/2 horse- power or 1 horsepower, and to use three-phase motors for higher horsepower ratings. When a three-phase, low-voltage supply is readily available, there may be an economical advantage to using small three-phase motors. Three-phase motors are advantageous from a maintenance and safety standpoint because they contain no contact-making device. An engineering analysis to determine if small three-phase motors can be used should be made for each application when the answer is not obvious. The difference in the cost of sup- plying current to the motors of the two types, when consid- ered with other cost factors, is often sufficient to determine which installation should be made. 6.10.3 Synchronous Motors Generally, synchronous motors are considered for large- horsepower and slow-speed applications where power factor improvements are justified and where other characteristics suit the applications. Low-speed engine-type synchronous motors are well suited for use as drives for slow-speed equip- ment such as reciprocating compressors and pumps. Synchro- nous motors often are used instead of induction motors, par&icularly at speeds less than 514 rpm where it is practical to avoid the use of gears or other speed-reducing equipment. High-speed synchronous motors are well adapted for use as drives for large rotating equipment such as fans, blowers, and centrifugal pumps. When the resulting improvement in power factor or efficiency will yield a satisfactoly rate of return on the additional investment required, synchronous motors are prefelred over squirrel-cage induction motors. A 1.0 power factor synchronous motor is usually the most efficient selection; however, it will have a lower pullout torque than a leading power factor synchronous motor. This may be a significant consideration if system voltage dips are expected during operation. Brushless synchronous motors are now used extensively in’ petroleum facilities. For excitation, the brushless system uses an AC exciter with shaft-mounted diode rectification. The AC exciter, in turn, receives its excitation and control from a small rectifier assembly and rheostat fed from the Table 1-Conditions Affecting Motor Design Conditions Generally Applied Types Exposed to chemical fumes. Operated in damp places. Driven at speeds in excess of rated speed. Exposed to steam. Operated in poorly ventilated spaces. Operated in Class I locations. Exposed to temperatures under 10°C (50°F) or over 40°C (104°F). Exposed to oil vapor. Exposed to salt air. Exposed to the weather. Exposed to abnormal shock or vibration from external sources. Where departure from rated voltage exceeds the limits specified in NEMA MG 1. Applications where parallel operation of motor-driven generator is required or similar applications where two or more motors need to be matched according to speed-torque characteristics. i Unbalanced supply voltage. I Operated at elevations geater than 1,OOO m (3,300 ft) above sea level. I Other unusual conditions, such as extended period of idleness, spe- cial torque requirements, or unusual operating duty. Adjustable speed applications. Should use a chemical-type motor. Should use additional impregnation or sealed insulation system and space heater within the motor enclosure and main terminal box. Consult manufacturer. Should be totally enclosed. Oversize the motor. For large motors, consid