Friday, November 13, 2009

Electrical switchboard installation

The following is a sample perfomance specifications for the assembly and installation of switchboards in an electrical contract for a new multi-storey office building.

The scope of this section is to set down briefly the requirements, methods, materials, workmanship, standards and regulations in connection with the complete assembly of switches, starters, and/ or control devices as used in an electrical installation.

Standard of manufacture
The equipment shall comply in every respect with the requirements of the latest edition of B.S. and M.S. specifications and be approved by the Director General, Office of Energy Commission. Switchboards, distribution boards and feeder pillars fabricated by manufacturers unauthorized or unregistered with the Director General, Electricity Supply Board, will not be accepted.

Copies of Type Tested Certificate issued by A.S.T.A. or other approved testing authorities shall be submitted together with the tender.

Electrical system
All switchgear under this section is for use on 415/240 volts, 3 phase 4 wire, 50 Hz alternating current system with solidly earth neutral.

Rupturing capacity
The switchboard shall be capable as a whole of withstanding without damage the electrical, mechanical and thermal stress produced under short circuit conditions, equivalent to 31 MVA at 415 volts for 1 second as defined in BS 5486.

Construction of switchboards
The switchboard shall be modular, cubicle metalclad, self contained, extensible, flush mounted, floor or wall mounted as specified, easily accessible and suitable for housing the individual switchgear and other necessary electrical accessories as specified hereinafter or on the drawings.

The switchboard is to be adequately ventilated and be designed to prevent ingress of all vermin, accidental contact with live parts and to minimize the ingress of dust and dirt. Precaution shall be taken to prevent overheating through hysteresis and eddy current loss.

Extensive labeling and marking shall be used in internally and externally in each switchboard in accordance with the drawings indicating the rating function, designation, number and/ or code letter of the equipment, switches, circuit breakers, fuses, indicating lights etc.

All switchboards must be rested on 100 mm high plinths.

Framework of switchboards
The framework of the switchboard cubicle shall be fabricated from rolled steel angle sections and shall be self-supporting when assembled. The switchboard shall be of rigid construction designed to withstand without any sag, deformation or warping, the loads likely to be experienced during normal operation, maintenance, transportation or maximum fault conditions. All structural members shall be folded on 12 SWG sheet steel. The front door and instrument panels shall be constructed of 12 SWG sheet steel pressed or rolled so that the edges are given a neat round finish. The doors and panels seated on a rubber gasket cemented to the frame. The sides, rear and top panels shall be constructed of 14 SWG sheet steel.

Paint finish and anti corrosion protection
The cubicle framework and sheet steel panels shall be chemically cleaned, degreased and lithoform etched prior electrostatically power sprayed with epoxy paint and oven baked to a touch and smooth finish coating.

All screws, bolts, nuts, washers and similar materials used shall be chromium plated.

Distribution boards
All distribution boards shall be completely enclosed in dust, splash, insect and vermin proof metalclad cubicles and their lids secured with knurled thumb screw. Fuse switches, isolators, contactors, busbar, meters, fuses, or miniature circuit breakers shall be provided as specified on the drawings. Earth terminal blocks, phase barriers, labels and charts for outgoing circuit identification shall be provided.

The distribution boards shall be designed for operation on a 415/ 240 volts, 50 Hz, A.C. system and generally comply with B.S. 214 and suitable for wall, floor or flush mounting as shown on the drawings.

The general construction of frame and panel shall be as specified under clause 5 and 6 but the panels shall be constructed of 16 SWG sheet steel.

The finishing of the metalwork shall be as specified under Clause 7.

The lids shall be designed and installed to ensure that they can be turned through 90 degrees for easy maintenance. All insulating boards provided shall be of hig quality and adequately supported.

Unless otherwise stated, all distribution boards shall be mounted such that their isolating switches are at least 1500 mm above the floor level.

Distribution feeder pillar
Distribution feeder pillar shall be suitable for outdoor service and weather proof type and be equipped with the number of incoming and outgoing circuits as specified in the drawings. It shall be designed with two lockable, hinged doors and removable back panels and provides easy access to cables and terminations.

The general construction of frame and panel shall be as specified under Clause 5 and 6.

The finishing metal work shall be as specified under Clause 7.

All feeder pillars shall be mounted on plinths, which should be at least 460 mm above the ground. They shall be separately earthed and labeled.

Adequate ventilation, insulating phase barriers for connections, cable boxes, end glands for all the incoming and outgoing ways stated in the drawings (including spare ways) shall be provided for each feeder pillar.

The feeder pillars shall be designed to prevent the entry of water during heavy torrential rain. Unless otherwise stated, barriers of railings shall be provided around each feeder pillar to provide some mechanical protection as necessary.

Busbars shall be of hard drawn high conductivity electro-tinned copper conductor with rectangular cross section capable to carry continuously the specified current without overheating. They shall be rigidly mounted on non-hygroscopic insulators to withstand any mechanical forces to which they may be subjected under the maximum fault conditions of 31 MVA. The busbars shall be arranged in a horizontal plane and be painted with red, yellow, blue and black colors at appropriate points to distinguish the phases and neutral. Size of neutral busbar shall be of the same as that of the phases.

An earthing busbar of appropriate cross section shall be run at the base of the switchboard.

Air circuit breakers (ACB)
Air circuit breakers shall be of the air break, metalclad, cubicle, horizontal withdrawable and dead front construction of the specified rating and supplied complete with all the necessary instruments, current transformers, trip fuses, trip devices and protective instruments as called for on the accompanying drawings. The breakers shall be suitable for operating on 415 volts 3 phase, 50 Hz, A.C. system and be certified by A.S.T.A. or KEMA for short circuit breaking and making capacity for 31 MVA or lower in accordance with B.S. 4752. The closing mechanism shall be of toggle action trip free type incorporated with mechanical “ON/OFF” and “ISOLATED/ PLUGGED” indicators shall be fitted with manual solenoid operated closing tripping coils allowing remote closing and tripping as specified.

A mechanical trip pushbutton shall be provided and electrical tripping of the circuit shall be accomplished by A.C. trip coils or if shunt D.C. trip coils are offered, a suitable trickle battery charger and nickel cadmium batteries shall be supplied and installed by the contractor.

All trip units are to be current transformer operated and current transformers shall be of suitable ratio, output type and class of accuracy for their function and shall comply in all respects with BS 3938.

Locking facilities shall be provided so that circuit breakers can be prevented from being opened or closed accidentally by means of padlocks, mechanical and electrical lockouts shall be also be provided to prevent closing of the circuit breaker after an over-current trip.

All circuit breakers shall be tested and set to the requirements of the S.O. by approved and licensed testing contractor. The expenses involved in testing the circuit breakers shall be borne by the contractor.

Busbar coupler
Where ACB is used as a coupler, electrical and mechanical interlocks shall be provided for the two incoming ACBs and the coupler cannot be “close” if both the two incoming ACBs are “close”.

Meters for the external panel mounting shall be of the flush pattern, with square escutcheon plates finished matt black and pressed steel cases. Indicating instruments shall be to B.S. 89 1-st grade, moving iron spring controlled (M.I.S.C) with 100 mm diameter dials (240 deg. Scale) with external zero adjustment, integrating meters shall be to B.S. 37 Parts 1 and 2 and Part 4 with cyclometric registers and protective relays to B.S. 142.

Each voltmeters shall be of the M.I.S.C. type, be of suitable voltage range to suit the circuit concerned and of a high degree of accuracy. The voltmeter shall be connected in circuit with a 6-position selector switch and protective cutouts fitted with HRC fuse links.

Every ammeter shall be of the M.I.S.C. type, be of suitable range to suit the current rating of the circuit. It is meant to operate through current transformers. It shall have a high degree of accuracy and be connected to three (3) busbar connected current transformers of the correct ratio, burden and class as stated on the relevant schematic diagram. Unless otherwise specified, ammeter shall also be connected in circuit with a 4 position selector switch.

Power factor indicator
Each power factor indicator shall be of the M.I.S.C. type and be suitable for measurement of 3 phase, 4 wire, unbalanced loads through current transformers (for the voltage coil circuit protection, HRC fuse-links shall be used). The indicator shall be scaled to read from 0.5 capacitive to 1 and from 1 to 0.5 inductive circuits.

Frequency meters
Each frequency meter shall be of moving coil type of range 40 cycles to 60 cycles operating in a frequency sensitive rectifier operated bridge network.

Protection relays
Relays shall confirm to BS 142 and shall be heavy-duty pattern, unaffected by vibration or external magnetic fields and be fully tropicalized.

Relay contacts shall be adequately rated and separate contacts shall be provided for alarms and tripping functions.

Relays shall be provided with phase colored flat indicators, which shall be of the hand-reset pattern and shall be capable of being reset without opening the case. Where two or more elements are included in one case, separate indicators shall be provided for each element.

Relays shall be provided with clearly inscribed labels describing their application and rating. Means shall be provided on the relay panels for testing of protective relays and associated circuits.

Overcurrent and earth fault inverse time protection
Relays shall be of the induction disc/ static, inverse time, overcurrent type. Overcurrent elements shall be supplied with adjustable settings for both operating current and time, the adjustment being possible on load. The range of current setting for phase fault elements shall be 50 to 200 % of rated full load with tapping at 25 % intervals and the time setting adjustment shall be 0 to 3 seconds at 10 times the normal operating current. Inverse time earth fault elements shall comply with the foregoing but shall have a range of settings from 10 to 40 % with tapping at 5 % intervals.

Overcurrent relays shall be of time delay induction disc/ electromagnetic type with adjustable setting of 50 % to 200 % of rated full load and adjustable time device shall open the contacts when overcurrent occurs.

Each leakage relays shall be instantaneous/ induction disc type with adjustable setting of 10 to 40 % and connected to the residual circuit of the current transformers accompanied to each phase which shall open the contacts of the circuit breakers when earth leakage occurs up to the allowable limit as set.

Earth leakage circuit breakers
Earth leakage circuit breakers of direct acting type (ELCB) with a tripping sensitivity of 100/ 300 mA to comply with the requirements of Electricity Supply Board and the National Electric Company shall be provided and installed as shown in the Drawings. Every ELCB shall operate successfully for every earth fault in the installation or circuit it controls even with an open circuit in the incoming neutral conductor.

Current transformers
The current transformer for protection, measuring and metering shall comply with B.S. 3938 and shall be of suitable ratio, output, type and class of accuracy for their function as specified in the Drawings. They shall be able to withstand short circuit rupturing capacity of 35 MVA for 1 second without damage.

The current transformers of the ring type shall have appropriate burdens, classes and ratios to suit the installation. Under no circumstances shall the ratio error at zero burden exceed +1 %.

All current transformers shall be mounted to the fixed position of equipment in such a manner that no pressure shall be exerted on their windings and easy access shall be provided for replacement and maintenance of current transformers.

All current transformers shall carry nameplates, identifying types, ratio, class, output and serial number.

Miniature and moulded case circuit breakers (MCCB and MCB)
The miniature and moulded case circuit breakers shall have the continuous ratings as indicated in the drawings at an ambient temperature of 40 degree C. They shall be suitable for use on a 415/240 volts, 50 Hz, A.C. system and the fault current withstand value shall at least be 5 kA for single phase and 10 kA for three phase units installed ot the distribution boards. For installation at switchboards, it shall withstand a fault current of 43 kA.

The circuit breakers shall be quick-make and quick-break type having center toggle mechanism and shall comply with B.S. 3871 part 1 and 2. All breakers shall have inverse time tripping with thermal magnetic trip elements and adjustments for the MCCBs.

All miniature and moulded case circuit breakers supplied shall be of approved manufacture.

At switchboards where MCCBs are used, there shall be facilities for locking out the toggle switch for maintenance.

Fuse switches
All fuse switches shall be of the indoor, metalclad, cubicle, flush or surface mounted, withdrawable and dead front construction type of the specified not less than those shown in the drawings.

They shall be suitable for operation on 415 volts, 3 phase, 4 wire, 50 Hz, A.C. system and be certified for short circuit breaking and making capacity of 31 MVA in accordance with B.S. 3185 and 861.

The switch contacts shall be of the heavy duty, double air break, slow make and quick break pattern with HRC fuse cartridges bolted onto the moving contacts complete with phase barriers, fully shrouded contacts, hard silver plate switch contacts.

The handles shall be fully interlocked and there shall be positive ON/OFF indication on the bodies of the switches. Interlocks shall be provided to prevent the cover fro being open when the switch is on.

Switchfuses shall be of the number of poles and current ratings as shown on the drawings and shall be of the totally enclosed pattern, metalclad with positive quick make and quick break action.

Switches shall be capable of passing and also interrupting their full rated current safely and without damage. Switch handles shall be interlocked to prevent opening the cover with the switch “ON”.

Fuse shall be of the HRC cartridge type.

All fuses supplied shall be HRC type with rating as indicated on the drawings to B.S. 88 or 1382.

All fuses for switches and feeder pillars shall be ‘Class P’ and ‘Class Q” respectively supplied complete with its appropriate carriers as necessary.

Spare fuses amounting to 20 % of the total requirements shall be provided.

Contactors provided shall be constructed and rated in accordance with B.S. 775 and the duty classification shall match the load controlled.

All contactors shall be provided with backup protection either in the form of fuses or circuit breakers for interrupting the fault currents.

All contactors provided shall have easily obtainable parts for repair or replacement.

Starters to be provided under this contract shall meet with the requirements for Electricity Supply Board’s approval and shall be direct-on-line, star-delta, auto transformer or rotor resistance types as to suit the requirements of motor.

They shall be enclosed in a metalclad cover, be provided with the correctly rated contactors, arc chutes and fault current protection such as trip coils or overcurrent coils.

The starters shall be suitable rated to continuously carry the full load current of the motor and also accept the starting current surges without tripping. In this respect, starters shall be fitted with time delays as necessary to prevent tripping when the motors are started.

Unless otherwise specified, all contactors used shall in the starters shall be constructed for heavy-duty operation.

Starters shall have facilities for manual and remote operation and parts shall be easily obtainable for repair, maintenance or replacement.

The isolators supplied shall be of the on-load type with single-pole and neutral, triple pole and neutral and double pole and with the rating as indicated on the drawings. Each unit shall be designed and constructed for heavy duty switching and shall carry the rated current continuously without overheating or damage. “ON” and “OFF” indications shall be provided on the body of the switches. Switch handles shall be interlocked to prevent opening the cover when the switch is “ON”.

Metering panel
The metering panel shall be manufactured from anti-rust zinc coated steel plate of not less than 16 SWG with removable hinged front cover to the electricity supply authority’s requirements. Facilities shall be provided on the panel for mounting of the authority’s meters, cutout fuse units and for wiring connection. Finishing shall be in grey enamel.

Working drawings
Prior to manufacturing of any switchboards, distribution boards and feeder pillars, the contractor shall submit complete working drawings to the S.O.’s reprentative for approval.

The switchboards, distribution boards and feeder pillars shall be factory assembled and tested before delivery to site and of suitable sizes for installation in the situation as shown on the drawings.

The contractor shall also supply:

a. One set of any special tools, gauges and handling appliances at each switchroom required for the normal maintenance of the standard equipment.
b. Sets, as required, of any special tools, gauge or other test equipment required for the assembly, checking or adjustment (but not for normal maintenance) of the standard equipment.
c. A 915 mm wide and 13 mm thick insulation rubber mat running full length in front of the switchboard
d. One set of “as built” wiring diagram in the main switchroom.

Copyright -Electrical switchboard installation


Anonymous said...

public announcement....
voltage for single phase now is 220vAC 50hz and for three phase is 400vAC 50hz....

Anonymous said...

Quite a useful information about Electrical Switchboard. Thanks for the post.

row anco said...

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Boon Electrical said...

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