منتدي ممدوح عزت موسي MAMDOUH EZAT MOUSA FORUM

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    design of electrical works in building

    شاطر

    ممدوح عزت موسي
    مدير عام المنتدي
    المشرف العام
    مشرف منتدي القصه
    مشرف منتدي العلوم الهندسيه
    مدير عام المنتدي  المشرف العاممشرف منتدي القصهمشرف منتدي العلوم الهندسيه

    عدد المساهمات : 350
    التميز : 9
    تاريخ التسجيل : 13/05/2010

    design of electrical works in building

    مُساهمة من طرف ممدوح عزت موسي في السبت سبتمبر 18, 2010 11:17 am

    Introduction:
    This report is intended as an outline defining the scope of electrical works, and to point out the essential guidelines and criteria which will be used in the preparation of the final electrical design drawings and specifications.

     System Description:

    The scope of electrical installations in the Building includes the following:
    • Low voltage network
    • Lighting
    • Small power
    • Data network
    • Telephone network
    • Earthing System

     Basic Design Criteria:
    • The design, selection and sizing of electrical equipment is affected by many factors and installation conditions such as ambient temperature, altitude, load, demand factors, percent loss of equipment life under short time emergency overload conditions, voltage regulation, short circuit capacities, the ability to start large motors, load characteristics, client standards, and relevant codes and standards.
    • The design criteria will attempt to recommend the lowest cost sizing without lowering reliability, future expansion or safety to limit the installed cost and minimize future spare costs.
    • The electrical system will be economically designed for continuous and reliable service, safety to personnel and equipment, ease of maintenance and operation, minimum power losses, mechanical protection of equipment, interchange ability of equipment, and addition of future loads. System protective devices (relays, fuses, breaker trip units, etc.,) will be selected and co-coordinated to ensure that the interrupter nearest the point of short circuit (or high overload) will open first and minimize disturbances on the rest of the system.
    • The electrical distribution system will be designed and installed to meet the power and grounding requirements of the electronic load equipment.
    • The electrical distribution system will also be arranged to minimize service interruptions, provide flexibility for growth and maintenance, and provide continuous and reliable power under all desired conditions.

     Power Supply
    • Power supply will be provided by two independent sources (different incoming distribution lines). The minimum capacity of each supply feeder will be sufficient to supply 100% of the sum of the full load maximum site ratings of the connected transformers.
    • General building loads (such as lighting, heating, ventilation, air conditioning and process cooling equipment) and electronic load equipment (klystrons, power supplies, beam lines, etc.) will be supplied from separate switchgears respectively. The switchgear for general building loads will be designated “dirty power switchgear” and switchgear for electronic load equipment will be called “clean power switchgear”.

     Reliability
    • The design of the power system will be based on the need to provide a stable source of electrical power and to minimize any down time associated with the system as a whole or the individual components Thereof.
    • The reliability of the system will be enhanced by:
    o a reliance on accepted national and international standards,
    o a careful screening of suppliers,
    o application of redundancy principles in system design if required.
     Provision for Future Expansion
    • Sufficient power capacity will be installed to service the expected peak loads for the ensuing five years. As the forecast for future energy increases, additional equipment may be required to install.
    • Any increase in capacity will be achieved through the installation of additional equipment as opposed to replacement with larger sizes.
    • All switchgear (low, medium and high voltage) and operator control panels will be manufactured and installed to permit future additional cubicles to be easily added to the lineup.
    • 6.15.1 Critical loads or loads requiring a high degree of availability will be supplied by a UPS system and/or a standby generator capable of automatically supplying the required power within 10 seconds after a power failure.
     Design Factors
    • Electrical power and associated control equipment will be designed to withstand the effects of voltage depression resulting from a three phase short circuit on the distribution network.
    • The network will be designed such that any piece of electrical equipment can safely be taken out of service for maintenance purposes.
    • Available fault levels within the electrical system will be sufficient to start and operate any electrical load without disrupting operation of other equipment.
    • Rating of protective equipment will be adequate to detect and isolate electrical faults anywhere within the system.
    • Voltage drops at normal operating conditions are not to exceed 3%.
    • Voltage drop at motor terminals during starting is not to exceed 20%.
    • Voltage drop on a feeder bus during starting is not to exceed 5% (10% for large motors with infrequent starts). Appropriate measures like capacitor assisted starting, reduced voltage starting, soft start and transformer onload tap changers will be selected so as not to exceed voltage drops
    • Motors greater than 20 kW will be provided with reduced voltage closed transition, autotransformer starters, or load controlled solid-state soft-start starters.
    • Motors in excess of 40 kW will be provided with local power factor correction.
    • Where motor anti-condensation heaters are utilized, the control circuit will be designed for automatic operation of heaters whenever the motor is off and, in the case of medium voltage motors, when the switchgear is in the racked-out position.
    • Transformer impedance will be selected to limit short-circuit currents to values within the ratings of the connected equipment and to optimize voltage regulation.
    • The power circuit breakers will be manually operated for non-motor loads. Static trip devices will be furnished on all load center power circuit breakers.
    • Breaker-protected combination starters will control motors fed from MCCs.
    • Office workstation areas will be designed to accommodate one separate dedicated branch circuit wiring and receptacle for electronic load equipment and another separate wiring and receptacle circuit for convenience loads or high impact loads.

     Critical AC System
    • An uninterruptible power supply (UPS) will be provided for critical loads such as critical field instrumentation necessary for monitoring and safe shutdown of operations.
    • UPS will include an inverter, static transfer switch, and manual bypass switch. A failure or fault within the inverter will result in an automatic transfer of the UPS loads to a nonregulated back-up power supply.
    • The manual bypass switch will be used to transfer the UPS load to the back-up source for maintenance on the inverter.
    • To compensate for harmonics created by the connected equipment, the continuous rating of the UPS systems will support 100% unbalanced and 100% non-linear loads, with a crest factor of three.

     Lighting
    • The lighting fixtures, transformers, panels, receptacles, switches, wire, and raceways, and their design will comply with the requirements of IEC. Illumination levels will be in accordance the recommendations of the CIE
    Area lux
    Offices 500
    corridors 150
    Bathrooms 150
    Lobbies & entrance 150
    Stairs 100-150

    • Lighting for control rooms, instrument boards and other similar installations will be designed to illuminate vertical board-mounted equipment and details without glare.
    • Interior lighting will be switched with local switches throughout.
    • Emergency lighting required for egress from buildings will be provided by an emergency generator.
    • Locally switched and pilot lighted lighting will be provided in mechanical duct systems, at filter locations and near mechanical units where frequent maintenance is required. In storage areas the lighting will be designed to illuminate the lower shelves as much as possible. Fluorescent lighting will be provided in crawl spaces and/or chases.
    • Lighting panels will include individual labeled circuit breakers. The panels will be designed so that, initially, approximately 20 percent spare breakers and load capacity will be available for future use.
    • Where practical, the lighting panels will be located in corridors so that service and inspection can be done without interfering with the occupants.
    • In main areas, the circuits will be on a staggered basis so that if a single branch circuit breaker will trip any given area will not be in total darkness.
    • Security lighting will be provided for the fenced areas, building entrances, outside storage areas, parking areas and other specified areas.
    • Lighting will enable personnel to safely exit enclosed areas following the loss of electric power and lighting circuits.
    • Electromagnetic contractors to enable the switching of all outdoor lighting fixtures from a central location will control power supplied to all new outdoor lighting.

     Lighting Fixtures
    • For selection of lighting fixtures (metal halide or fluorescent), economic factors will be considered. HPS lamp fixtures will be considered in areas where flood lighting is required.
    • In general, suitable rapid start fluorescent fixtures will be used in low ceiling indoor areas requiring high illumination levels such as offices, control rooms etc.
    • Fixtures for general room or area lighting requirements will be symmetrical lens and fluorescent types. For control rooms, a ceiling metallic grid parabolic system will be provided.
    • Metal Halide and high-pressure sodium fixtures when used will have constant wattage high power factor ballasts and colour-corrected lamps.
    • Fluorescent fixtures will utilize T8 lamps with 4100K temperature and CRI of 80 or better. Quiet ballasts (sound rated class A) will be used in offices, conference rooms and similar low noise level areas.

     Receptacles
    • All offices will be provided at least with two duplex receptacles adjacent to the desk location. The receptacles will be placed in separate boxes at least 150mm centre-to centre and not installed in one box.
    • Lobbies and corridors will be provided with sufficient number of outlets to require no more than a 15 meter cord for power-driven housekeeping machines. One of these outlets will be provided near each caretaker’s office and these outlets will be on separate circuits than outlets in user spaces.
    • Duplex receptacles will be provided in the mechanical duct systems at filter locations and near mechanical units in the ceiling spaces and crawl spaces where frequent maintenance will occur.
    • Receptacles will be provided to serve portable lights and tools for maintenance of outdoor installations of equipment and facilities as follows:
    a. Outlets will be located within 5 m of the equipment to be serviced and about 1 m above grade or platform.
    b. Outside areas where the equipment or facility is served with permanent lighting.
    c. The Owner will review the final number and location of outlets. These will be protected by ground fault circuit interrupters.
    • Receptacles in buildings will be provided, as required, to supply electrical equipment not supplied by permanent wiring and to serve portable electrical devices.
    • Receptacles will be single-phase AC and will have a separate contact for connection to the grounding pole in the plug. Ground contacts in plugs and receptacles will be arranged so that the grounding circuit is made first and broken last.
    • Outdoor outlets will meet the following:
    a. Plug will have shrouded contacts so that contacts remain enclosed until circuit is broken.
    b. Plugs will be held in the plugged-in position by locking rings, twist lugs or equivalent.
    c. Arcs resulting from breaking loads will be contained. Plug and receptacle will incorporate arc-quenching design of the main contacts, with means of delaying full withdrawal until extinction is complete.
    • Branch circuits supplying outlets for general use will have an ampacity not less than the ampere rating of the largest receptacle supplied by the circuit. One circuit will supply not more than six outlets.
    • To ensure a reliable, low resistance connection, all wiring terminations to receptacles will be by screw-compression wiring contacts. Push-in wiring contacts will not be accepted.




     Raceway System
    • Cable Tray
    • The main selection criteria for designing and installing a proper cable tray system will be based upon the following:
    o CSA load class
    o Width and height
    o Type of tray bottom
    o Material
    o Span
    o Deflection
    o Fittings
    o Bonding
    o Support structures
    • For power and distribution, generally ladder, ventilated or solid tray will be specified.
    • For instrumentation, data and communications generally channel or centre hung tray will be specified, although solid and ventilated tray may occasionally be used as well.
    • Cable tray and accessories will be rigid steel, hot-dipped galvanized, CSA Standard load classification E.
    • If covers are used, the weight of the cover will be taken into account and added to cable tray loading. For outdoor applications, wind and snow loading will be added to the weight of the cables, thereby reducing cable tray load capacities.
    • Cable tray supports will be field located by the installation contractor and placed at intervals not exceeding 6 metres measured along the tray centerlines and also in accordance with standard details.
    • Cable trays must be supported either from overhead or adjacent structural members. Closer supporting may be required for outdoor installations, vertical installations, and installations where more than one level of tray share the same supports.
    • Where possible, cable entries to electrical power sources (i.e., switchgear, MCC) will be from below to simplify tray systems.
    • Trays will be located so that the lowest part of the cable tray support assembly is at least 2.1 metres above floors to maintain minimum headroom requirements. Trays in cable spreading rooms may need to be less than 2 metres due to the high concentration of cables in the area.
    • Cable trays will not be routed through areas where there is potential for accumulation of oil or other combustible materials on the cables. If cable trays must be routed through these areas, the cable trays must be provided with tray covers designed to minimize the amount of such material reaching the cables.
    • Trays will not be located near heat sources (burner fronts, steam piping, heat exchangers, etc.) unless cables are adequately derated and suitable for the higher ambient temperatures. If this is not practical or possible, a protective heat barrier will be installed.
    • Circuits in cable spreading areas will be limited to those performing control and instrument functions and those power supply circuits and facilities serving the control room and instrument systems.
    • Where routed through cable spreading areas, power supply circuits to instrument and control room distribution panels will be installed in conduits.
    • 6.30.1.17 Extra consideration must be given to the strength of the support elements (beam clamps, anchor bolts, hanger rods, etc.) used to support vertical stacks and long vertical runs of cable tray.
    • Each section of cable tray will be connected to adjacent sections using splice plates or approved coupling device and located within ¼ of the span from the supports.
    • Where cable trays are located over any electrical equipment, the minimum vertical separation of approximately 0.90m from the top of the equipment to the bottom of the tray will be maintained.
    • The cable tray system will be mounted so that sufficient space above the tray is provided to permit installation of any approved cable-pulling equipment.
    • A minimum variety of tray sizes and fittings will be chosen to simplify design and inventory.
    • Fittings will be limited to 45 and 90 degrees. Special, 30° and 60° fittings will be used only when required to satisfy special requirements.
    • The choice of radius for tray fittings will be a minimum of 8 times the diameter of the largest nonshielded cable or 12 times the diameter of the largest shielded cable to be installed, whichever is larger. A minimum variety of radii will be used.
    • Except as indicated otherwise herein, all indoor vertical trough and ladder type trays will be furnished with louvered ventilated covers. All indoor horizontal trays located under grating floors or insulated pipe be furnished with solid covers which extend at least 610mm beyond that part of the trays directly exposed beneath the grating floor or insulated pipe. Indoors, covers may be omitted on those lower trays of stacked trough and ladder type trays where a covered tray at a higher elevation in the stack provides complete vertical shielding to the lower tray. All outdoor trays will be furnished with solid covers. Trays that are specified to have solid bottoms will also have solid covers throughout, including all horizontal runs, all fittings, and all vertical runs.
    • The cable tray system will be electrically continuous. All trays containing power circuits will be provided with a continuous ground conductor installed in or on the entire length of the tray system. This ground must be connected to the station ground grid at locations indicated on the grounding drawings. For cable trays containing control or instrument circuits only, a ground conductor is not required; however, the tray will be connected to building steel at intervals not exceeding 45 meters, and will be mechanically connected to any enclosure or raceway to which the tray terminates. Where connection of control and instrument tray to building steel or at terminations as indicated is not possible, ground jumpers will be used as required to maintain electrical continuity. Cable trays will be grounded at intervals not exceeding 15 m.
    • Effective fire stops will be provided for cable entries into equipment. All penetrations through walls for cable trays especially into cable spreading rooms and all vertical penetrations through floors will also be provided with fire stops.
    • Where trays extend vertically through concrete floors and platforms, curbs or other suitable means will be provided to prevent water flow through the floor or platform opening.
    • The electrical conductors for redundant systems will be separated by arrangement of cable trays and/or protective barriers such that no single event will prevent operation of the required number of redundant systems. The degree of separation required varies with the potential hazards in a particular area.
    • Cable trays containing circuits for redundant systems will be arranged to minimize the possibility of a fire damaging more than one system or propagating from one system to another.
    • Conductors of systems sensitive to electrical noise will not occupy the same tray with conductors of power or control systems, and will be run in a separate instrument tray system. This tray system will be solid bottom with solid covers. Requirements of system manufacturers must be followed when routing cables for noise sensitive systems.
    • Trays for cables of different voltage levels will be stacked in descending order with the higher voltage above. Instrument cable trays will be lowest.

     Power Factor Correction
    • Improvements in power factor may be desired for financial reasons (to lower utility costs associated with power factor penalties) or operational reasons (to lower system losses, increase system reserve capacity, or improve voltage conditions).
    • Power factor capacitors, if specified, will be added as necessary to minimize the electrical kVA power demand. Larger, higher voltage capacitor banks are generally more economical than capacitor units installed with individual motors.
    • Extreme caution will be used when applying capacitors to ensure that they do not cause resonance conditions that can magnify harmonic levels and cause excessive voltage distortion.
    • Load harmonic profiles will be calculated or estimated (current harmonic profile estimated from typical individual pieces of electronic load equipment).
     Hazardous Locations
    • Classifications will be shown on the “Area Classification” drawings.
    • 6.35.3 Each equipment enclosure will be suitable for the respective area classification in which it is installed ( must be explosion proof).
     Lightning Protection
    • A lightning protection system will be provided to protect facilities from damage due to lightning stroke or discharge.
    • The lightning protection system will be an active attraction system designed to attract the lightning strike to a preferred point through an air terminal and to convey the energy safely to earth.
    • The lightning protection system will include the following components:
    o An enhanced active air terminal of the type designed to minimize corona emissions and optimize streamer inception at a predetermined time.
    o An insulated low impedance down-conductor to conduct the energy to earth safely and effectively.
    o A 6 meter copper-clad steel earth rod c/w access earth well and chemical electrodes filled with conductive electrolytes to provide better grounding conductivity (if required to reduce earthing resistance to acceptable level).
    o A lightning event counter.
    • The air terminal will be insulated from the protected structure under all conditions. The mast will be adequately rated for wind shear loading and guy wires will be provided as appropriate to local environmental conditions.
    • The down-conductor will consist a plastic filler (to increase effective diameter of core conductor), main copper conductor, semiconducting stress control layer, polyethylene high voltage insulation, semiconducting stress control layer, copper tape screen and electrically conductive plastic sheath. Insulation breakdown ratings between main conductor and copper tape will be no less than 200 kV based on 1/50 s wave shape . The lightning event counter will have an electronic register that activates one count for every discharge where the peak current exceeds 1500 A. The test wave shape will be the 8/20 s standard. The lightning event counter will be suitable for outdoor installation in –40C to + 40C.

     SYSTEM STUDIES
     General
    • System studies will be performed to verify proper design of electrical power systems and equipment for new facilities and major additions to existing facilities.
    • Short circuit calculations will indicate that all distribution equipment is suitable for continuous operation at full load and will be capable of withstanding thermal and electromagnetic forces due to short circuit and fault conditions.
     Short Circuit Analysis
    • A Short Circuit Study will be performed to cover three phase, single line to ground, line to line, and line to line to ground faults in time frames of first cycle, one to four cycles and 30 cycles. The Short Circuit Study will focus on:
    o Verifying switching equipment momentary and interrupting ratings based on worst case three-phase to ground fault levels.
    o Confirmations of short time withstand ratings of system components.
    o Providing maximum and minimum fault levels for relay coordination studies.
    • Short circuit studies will be performed assuming the maximum ultimate transient fault-current availability or minimum ultimate source impedance.
    • The maximum short-circuit current will be limited to a value no greater than 95 percent (90 percent during preliminary design) of the fault current rating at the point of common coupling assuming the following conditions:
    o Maximum system voltage (at contingency levels)
    o Maximum system fault MVA
    o Transformer impedance reduced by allowable tolerances
    o Motor fault contribution of motors that could possibly be running simultaneously
    o Facility loads at maximum expected value
    • Values obtained from worst-case analysis will be used to size and purchase electrical equipment.
    • Unless load flow analysis or actual system operating practices dictates differently, all transformer tap settings will be assumed to be at the midpoint.
     Voltage Regulation
    • Electrical equipment is designed for optimum operation at its nominal nameplate voltage. Any deviation from this rated voltage can result in decreased efficiency, damaged electronic equipment, and severely reduced life of electrical control and utilization equipment.
    • The allowable limits of voltage regulation will be as follows:
    o The voltage at the terminals of motors will be within ± 10 percent of the motor rated voltage, under steady-state operating conditions.
    o The voltage at the terminals of any single motor while it is starting will be at least 80 percent of the motor rated voltage. The largest motor connected to the bus under consideration will be assumed to start with all other motors running.
    o The transient voltage on running motors while another motor is starting will be at least 75 percent of motor rated voltage.
    • Voltage regulation studies will consider the following conditions:
    o Condition 1 - Establishing Minimum Bus Voltages
     Minimum system voltage (at contingency levels)
     Facility load at maximum expected value
     Transformer impedance’s increased by ANSI allowable tolerances
    o Condition 2 - Establishing Maximum Bus Voltages
     Maximum system voltage
     Facility load at minimum value (e.g., before initial startup)
     Transformer impedance’s decreased by ANSI allowable tolerances
    • Steady-state and transient voltage analysis will be performed to ensure that proper operating voltage is maintained (inadequate voltage may affect the performance of electronic load equipment such as operational problems, synchronization problem or risk equipment damage.
    • Design assumptions:
    o Actual system data and constraints will be used to calculate voltages.
    o Steady-state voltages will be evaluated at maximum, normal, and minimum.
    o The maximum voltage on each circuit will be calculated assuming that all motor loads are disconnected and in the case of a double ended substation that both transformer banks are operational and the bus-tie circuit breaker and/or switch is in its normal state.
    o The normal voltage of each circuit will be calculated based on the maximum operating loads.
    o The minimum voltage of each circuit will be calculated based on the normal operating load plus the operating load of the largest spare (standby) motor if the spare motor is not interlocked to prevent starting while the primary motor is running.
    o Voltage drop calculations will be based on the minimum short circuit level of the supply.
     Motor Starting Study
    • Motor Starting Study will be completed using both static and dynamic motor starting models to cover:
    o Assessment of motor starting capability.
    o Determinations of bus voltage drop.
    o Identification of assisted starting requirements (capacitor assisted starting, reduced voltage starting, soft start, etc.) if required.
    o Transformer tap adjustments so as not to exceed the limits of voltage drops.
     Load Flow study
    • A Load Flow Study will be performed to:
    o Identify equipment overload conditions (normal and contingency operation).
    o Identify steady state voltage problem areas (bus voltage profiles).
    o Select optimum transformer tap adjustments.
    o Identify poor power factor (branch power factors).
    o Identify system losses (current power flow & feeder capacity).
     Protective device Co-ordination
    • Relay and protective devices will be selected and co-coordinated to provide a system that permits the interrupting device nearest to a fault to operate first.
    • The design will be validated by a comprehensive coordination study.
    • Relay and fuse co-ordination studies will be performed to include all protection time/current characteristic devices from the largest single protective device connected to the low voltage main distribution bus, up to and including the incoming power supply protective devices.
     Harmonic Analysis
    • Calculation or estimation of load harmonic profiles is a necessary requirement for all power distribution systems intended to supply electronic load equipment, to comply with IEEE standard 519 and 399.
    • Based on project requirements, a harmonic load flow study will be executed to calculate the fundamental voltage and current and the load data from the harmonic source. The load flow report will be reviewed to ensure that the system is operating properly at 50 Hz.
    • Alternatively, harmonic profiles of load currents will be measured and recorded at required locations and appropriate mitigation measures recommended.


    _________________

    "لا اله الا الله العليم الحليم،لا اله الا الله رب العرش العظيم،لا اله الا الله رب السموات والأرض ورب العرش الكريم."



     


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