Electrical DB pictures

I have uploaded below a few pictures of electrical DB being installed at a high-rise office-building project. I only have enough time for these few pictures today, but I will be sending a more detailed post on the DB installation soon. Plus a few more of the DB pictures maybe.

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Picture 1 – Mounting position and location


These electrical boards are permanent boards, not a temporary electrical installation. I have been sending so many pictures of temporary electrical installations that I feel the need to make that clear. Otherwise, some readers may get confused because in these pictures you can see that the boards and the cable trunking are solidly mounted on the concrete wall.

Actually there are three DB’s in this picture. The two big ones on the left (your left side) are the electrical distribution boards (DB) and the one at the far right is the telephone distribution panel (some people call it “DP panel”).

These DB’s are located along one of the corridors on the office floor. Therefore, they will need to be hidden from view and protected from unauthorized access. Therefore, a cabinet will be built around these boards, and a lockable door will be provided.

The installation work is in progress and the building is still under construction. However, if you see closely to the doors of the distribution boards, you may notice that the DB doors are not provided with lockable door handles. They seem to be provided only with door screws.

With this type of doors, then they are not protected from access to the live parts of the boards. So, if there is no provision to build such electrical cabinet or rooms as I have mentioned and secure it with lockable keys, then the construction supervisor in charge of the electrical installation must insist for it.

There is no problem in installing the distribution boards exposed like that. However, they must be protected from unauthorized access. In this case, you have three choices:

Provide lockable doors and raise the mounting height of the DB’s above reach by hands.
Raise the mounting heights to unreachable heights without providing the lockable keys to the doors. This a little bit of a compromise already, but some may say that is already okay. In many scenarios this sort of issues are quite subjective.
Build a cabinet around the boards, enclosing all three electrical and telephone boards. A lockable door should be installed.
Build wall around them so it will become an electrical rooms, plus a lockable door.

Picture 2 – Dropper trunking from above ceiling


This second picture shows the metal trunking coming down from above the ceiling and into the distribution boards.

Some of you may be wondering why these trunking are like that… with the big trunking branching into the smaller ones above the ceiling. The reason is to make the space bigger for the wiring cables to crisscross each other while coming out of the MCB’s (miniature circuit breakers) inside the DB’s and into its intended trunking route (either one or the three branch trunking above the ceiling.

Actually, the three branches of trunking above the ceiling is for the following:

One for lighting wiring final circuits. All wiring cables supplying all lights and maybe ceiling fans and toilet exhaust fans. This is an air-conditioned office building, and ceiling fans are not used at all. However, wiring for the air-conditioning equipment is not allowed to run in trunking for lighting wires. Emergency light and the lighted exit signs also have their wiring installed inside this trunking, but the one from the ESSENTIAL SUPPLY board.

I should have mentioned this earlier. We have two electrical DB’s there because one is supplied from the standby electrical generator. This is the smaller DB. The bigger DB is supplied from the authority’s mains supply. The one supplied form the standby electrical generator is usually called “essential DB” or “essential distribution board”, while the one supplied from the mains is normally called “normal DB” or normal distribution boards.

The Essential DB is usually smaller because the number of electrical loads it need to supply is smaller. In ordinary office buildings, the number of light fittings that need to be turned on during a power supply failure is usually only one out of three lights or about thirty percent of the general lighting. So the Essential DB will have only half the number of final circuit wiring as the Normal DB. That is why they are generally half the size, which is what you can see in this picture.

The second branch trunking is for the small power final circuit wiring, the wiring supplying all the switched socket outlets and other small power points like window air conditioners, etc. Cables supplying power to other mechanical equipment such as water pumps, etc are not allowed to share this trunking. The need to be installed in a separate trunking, which is the third branch trunking.
This carries the cables and wiring for the mechanical equipment like the building’s water pumps, etc.

So the above describes what those three branches of the metal trunking are for. The same applies for both DB.

Picture 3 – Bottom cable entry


This picture shows the bottom entry for cables. Both DB’s has bottom cable entry. Some of the readers may be wondering why cables need to enter the DB cabinet from both to and bottom. The reason being this is an office building with an underfloor trunking system.

The electrical socket outlets, the telephone socket and the computer data sockets for all office tables are provided from outlet boxes of this underfloor trunking system. So there are three separate high impact PVC trunking running inside the concrete floor throughout all office areas of this multi-storey office building. At each worktable, an outlet service of approximately 12 inch x 12-inch box is provided and all the power, telephone and data sockets are provided there.

You can see that the orange electrical trunking under the DB’s going downward to the floor. The green telephone DB also runs downward to the same location. This is where they are connected to the 3-way underfloor trunking system. The piece near the floor is called the “vertical access box” of the trunking system.

That is all on the bottom trunking for now. In future, I will talk more on the underfloor trunking system and maybe show you some real construction photos.

The top entry trunking are for all other cables and wiring. The wiring to the window air-conditioning units run above ceiling, never inside the underfloor trunking. The nature of underfloor trunking installation make it very difficult and messy to extend. You need to hack the concrete floor to install additional runs and that can disrupts the operation of the whole floor. So only the wiring for the final circuits to the office work desks are run inside them. They are dedicated only for this purpose most of the time.

Usually some general-purpose power sockets are still provided on the walls even where underfloor trunking system is used. These sockets are usually run in the above ceiling trunking and a dropper conduit is used to make the connection to the sockets on the walls.

The lighting wiring are always run inside the above ceiling trunking. A system of trunking and conduit is the dominant method for this wiring purpose in almost all office buildings.

From the above it is quite clear that a much larger number of wiring runs above tha ceiling than under the floor. So that is the reason the bottom entry trunking is much smaller than the top entry ones. In this case the bottom trunking from both DB’s can be joined and still use the same size to connect to the vertical access box.

Telephone DB
As for the telephone DB, the trunking does not need to run above ceiling to go to the sockets at the worktables. The top entry trunking is for the incoming multi-core telephone cable from the telephone riser shaft. All telephone final wiring cables run inside the underfloor trunking. Since this blog is about electrical matters only I will not talk too much on telephone works. That may be a topic for one of my new blogs.

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Electrical DB installation

The electrical DB (distribution boards) for high-rise office installations shall be non-ventilated, naturally cooled, metal enclosed type, flush fronted and suitable for front access and shall be built to comply with IEC 60439-1. All louvers shall be covered with wire mesh.

The distribution boards shall contain MCCB (moulded case circuit breakers), MCB (miniature circuit breakers), RCD (residual current devices) and busbars as indicated in the contract documents. The DB shall be suitable for wall mounting or floor mounting as appropriate.

All necessary fixing materials, mounting bolts, etc shall be provided for the type of mounting mentioned.

All operating devices shall be mounted for installation, cable termination and maintenance operation at the front of the panel.

All distribution panel located outside the riser and public area shall have key lock handle.

Enclosure and Degree of protection
The DB enclosures shall be fabricated from rolled steel angle sections and shall be self supporting when assembled, uniform in height and depth from front to back. Sheet steel used shall not be less than 2 mm thick, anti-rust coated steel plate.

The distribution board shall be rigidly constructed to be stand alone units without any possibility of sagging, deformation or warping.

The front covers of the DB shall be of hinged-door type. The front doors shall be provided with lockable push button type. The doors shall be arranged to seal onto the board frame by means of a non-perishable dust-proof material. The sealing material shall be synthetic rubber and not foam. Doors shall be effectively earthed to the fixed enclosure by braided straps. Cover bolts or nuts shall be retained in place when undone.

For all internal DB installation, the degree of protection for the DB enclosures as per IEC 60529 shall be at least IP 41. When the doors are opened, the degree of protection to all live parts shall be at least IP 20.

All the distribution panels shall have Form 1 construction in compliance with IEC 60439-1.

The incoming and outgoing cable entry on the distribution boards shall be protected with rubber material to prevent damage to the cables.

Selection of components
All components shall be standardized as far as practical and shall comply with relevant IEC publications.

Busbars
Busbars for the DB shall be of plain hard-drawn, high-conductivity, electrolytic copper bars in accordance with BS EN 13601: 2002 and of adequate rectangular cross-section to carry continuously the specified current without overheating and shall be rigidly mounted on insulators so as to withstand any mechanical force to which they may be subjected under maximum fault condition.

The DB busbars shall be colored red, yellow, blue and black at appropriate points to distinguish the phases and neutral. No tapes shall be used.

Moulded case circuit breakers (MCCB)
Refer to the MCCB section for the details on MCCBs.

Type test certificates shall be produced for the MCCB selected from internationally recognized testing laboratories.

Miniature circuit breakers (MCB)
Refer to MCB section or details on MCBs.

Wiring
Refer to Electrical DB Wiring section for details on the DB wiring.

Residual current devices
Residual current devices shall provided as indicated in the design drawings and construction drawings.

Refer to Protection, Metering and Control section for details.

Type test certificates shall be produced for the RCCB/ ELCB selected from an independent internationally recognized independent testing laboratories.

Finishing and painting
Refer Electrical DB paintwork section for details on finishing and painting of electrical DB.


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Electrical busbars

The electrical busbars inside the switchboards shall be of plain hard-drawn, high conductivity, electrolytic copper bars in accordance with BS EN 13601: 2002, and of adequate rectangular cross-section to carry continuously the specified current without overheating and shall be rigidly mounted on non-hygroscopic insulators so as to withstand any mechanical forces to which they may be subjected under maximum fault condition.

Busbars shall be colored red, yellow, blue and black at appropriate points to distinguish the phases and neutral. No tapes shall be used. The main busbars shall be arranged in a horizontal plane and in the order of red, yellow, blue and neutral phases from back to front. On each panel connections shall be red, yellow, blue and neutral phases from left to right viewed from the front of the panel.

The phase and neutral busbars should be located in the top compartments of the switchboard.

The busbar system shall be readily accessible for inspection, construction and maintenance duties without the requirement of additional equipment. In case of a busbar short circuit, it shall be possible to clean or to replace the busbars and the support system without stripping the assembly.

In the busbar compartment the phase busbars may not be fully insulated. However, each phase busbar shall be able to withstand at least an applied AC test voltage of 2.5 kV for a period of 60 seconds.

The droppers shall have full segregation by insulated materials. The insulation of the busbar jointing and branching points shall be of equal quality to that of the main bars and shall be removable and easily replaceable for inspection.

The neutral bar may be not insulated in the busbar compartment but shall be insulated at all other compartments with the same insulation level as the phase busbars.

The earth bar shall be located in the top or bottom compartment and in all cable riser compartment of the switchboard and shall be easily accessible. Sufficient connection points with adequate terminating facilities shall be provided for terminating the cable earth leads. The earth bar shall be sized complying with IEC 60439-1.



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Electrical DB wiring

Following is a sample specifications for the wiring of an electrical DB. Refer the section on Electrical DB installation for specification details on the construction of distribution boards.

Photo 1 - Internal wiring of an electrical DB

Terminals for external conductors All conductor terminals shall be suitable for copper conductors.

All cable termination assemblies shall have facilities for entry of cables from both the top and the bottom of the distribution boards. Cable entries, cable clamping, earthing facilities and supporting devices that are provided shall be suitable for the type, size and number of the incoming cables.

The identification ferrules Both ends of each wiring conductor shall be provided with identification ferrules. The ferrules shall be made of an insulating material of a type not affected by oil or damp. The characters shall be suitably marked in black. (See photo here: Wiring identification ferrules)

The ferrules shall be of the continuous ring type. A slide-on type will not be acceptable. The markings shall be in accordance with the relevant manufacturer’s drawings.

The wiring cables The minimum allowable cross-sectional area of control cables shall be 1.0 mm.sq. Wiring cables with a cross-sectional area of 1.5 mm.sq or larger shall always be stranded. The secondary circuits of current transformers with a 5 A rating shall not be less than 2.5 mm.sq. Color-coding of all wiring shall be in accordance with IEC 60446. The color of earth wiring cables shall be green with yellow stripes.

Wiring between two terminals shall be continuous. Joints or interconnections at locations other than at terminals are not acceptable. All wiring cables shall be terminated at both ends at a connection terminal. As a minimum requirement, use shall be made of rail-mounted terminals (TS 32 rail assembly) of high-grade Melamine.

When different voltage levels are employed in a single DB enclosure, partitions shall be installed between the terminals of different voltages.

The wiring The wiring ends of stranded conductors which have to be connected into bus type of terminal contacts shall be provided with compression-type pre-insulated wire pins with insulation support.

When cable lugs are used, they shall be of a compression type. To accommodate and support the wiring cables, covered plastic trunking, channels, insulated tubes or plastic strips shall be used. Wiring cables shall never be mounted directly to a metal part of a distribution boards.

The space factor for channels shall never be less than 30 percent. Where supporting of a wiring cable is not feasible, the cable shall be as short as possible.

Note: This anchor post, Free electric installation pictures , may contain a summary of the materials you are looking for. It can be faster than clicking through each post title at the Blog Archive. I started it long ago but never actually got around to finishit.

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Switchboard electrical earthing

Following is a picture showing the earthing busbar of a main switchboard. Below the picture is a simple performance specifications for the grounding of the switchboard.
The are also two more posts on switchboard installation check and switchboard electrical tests. The latter is a general list of tests and inspections that are usually required of a major switchboard such as main switchboards and large subswitchboards.
Photo 1 - Earthing busbar of a main switchboard (MSB)
The earthing of an electrical switchboard shall be in accordance with BS 7430, and the color coding of protective conductors shall be in accordance with IEC 60646.
Switchboard earthing busbar
The switchboard earthing busbar shall be installed internally along the full length of the switchboard. The material of the busbar shall be the same as the material of the phase busbars. (See more photos: Switchboard earthing busbar.)
Equipment earth bar
Where the substation consists of 11 kV switchgears, transformers and LV switchboards, earthing copper tape shall be installed continuously along the full length of the substation.
It shall be connected to the main system earth bar at both ends using appropriate size earthing bolts with nuts and spring washers.
The earthing copper tape shall be made of hard-drawn high conductivity copper.
The cross-section of the earth bar shall be sufficient to carry the rated short-time withstand current of the switchgear for the allowable temperature rise and the time specified.
Internal branch earth connections
The branch earth connections made from the switchboard earthing busbar to the individual switchboard components shall consist of adequately sized copper strips, or green/ yellow striped PVC sheathed stranded copper conductor.
The termination lugs shall be of the compression type.
Earthing of metal parts
All metal parts of a switchboard where live components (such as relays, instruments, indicating lights etc) are mounted shall be earthed through branch connections to the switchboard earthing busbar.
Doors of the switchboard shall also be similarly earthed.
Frames of the draw-out circuit breakers shall be connected to the earthing busbar through a substantial plug type earthing contact.
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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.

Scope
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
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
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.

Voltmeters
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.

Ammeter
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
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.

Fuses
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
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
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.

Isolators
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.

Packing
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.

Maintenance
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.

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