Section – Two Part – 6
Electrical Specification Distribution Boards
Part - 6
DISTRIBUTION BOARDS
1.0 GENERAL
The Contractor shall supply and install the distribution boards (DB, EDB) as indicated in the Schedules, where shown on the Drawings, and as herein specified. They shall include bus assembly, cabinet and front, circuit breakers and all necessary parts to install complete distribution boards.
Distribution boards shall be factory assembled.
Distribution boards shall be of a dead-front safety type, equipped with protective devices as shown on the Schedules.
Distribution boards shall have 415/380 V volt duty rating, with other ratings as shown on the Drawings.
Distribution boards circuit numbering shall be such that, starting at the top, odd number shall be used in sequence down the left-hand side and even numbers shall be used in sequence down the right-hand side.
Distribution boards shall conform to BSEN 60439-1:1994 and EN 60898.
Distribution boards shall incorporate plug-in type miniature circuit breakers of tripping characteristics B, C or D for lighting, miscellaneous power or motor loads as necessary complete as specified and as shown on the drawings and to the approval of Engineer.
Main breakers of all main breaker type distribution boards shall be plug in type miniature circuit breaker or bolt-on molded case type circuit breaker as shown on the drawings.
2.0 CABINETS AND FRONTS
The distribution board bus assembly shall be enclosed in a galvanized sheet steel cabinet. Cabinet shall be of sufficient size to provide a minimum gutter space of 10 cm on all sides. The thickness of the sheet steel shall be minimum 1.5 mm.
Fronts shall include doors and have flush, brushed stainless steel cylinder tumbler-type locks with catches and spring loaded door pulls. The flush lock shall not protrude beyond the front of the door. All distribution board locks shall be keyed alike. Fronts shall have adjustable indicating trim clamps which shall be completely concealed when the doors are closed. Fronts shall have approved directories with name of panel, number of phases, wires and voltage written on them.
Doors shall be mounted by completely concealed steel hinges. Fronts shall not be removable with door in the locked position. A circuit directory card shall provide a space at least 0.5 cm high x 7 cm long or equivalent for each circuit.
The directory shall be typed to identify the load fed by each circuit.
Fronts shall be of code gauge, full finished steel with rust-inhibiting primer and baked enamel finish. Color shall be gray to ANSI No. 61, BS 381 C or approved equal.
Joints shall be welded, galvanized and reinforced where necessary and galvanized after fabrication.
3.0 DISTRIBUTION BOARD BUS ASSEMBLY
Bus for connections to the branch circuit breakers shall be the "Distributed Phase" or "Phase Sequence" Type.
Three-phase, four-wire bussing shall be such that any three adjacent single-pole breakers are individually connected to each of the three different phases in such a manner that two or three pole breakers can be installed at any location. All current carrying parts of the bus assembly shall be plated.
Main and neutral buses shall be minimum 98 percent purity, rectangular copper bars, provided with bolted-type lugs as necessary.
Buses of miniature circuit breaker distribution boards shall be suitably braced for a minimum short circuit duty equal to 10,000 Amps unless otherwise indicated.
Buses shall be rigidly supported and insulated and be so designed that branch circuits can be removed without disturbing adjacent units or changed without additional machining, drilling or tapping.
Necessary bussing, drilling and blank plates shall be provided for installation of future circuits when so indicated in the Schedules on the Drawings.
All screws and bolts used for making copper connections shall be equipped with lock washers. Rivetted connections will not be acceptable.
Mains shall be equipped with solderless pressure indent type connectors and have means to prevent swiveling of connector.
Neutral terminal strip shall be full size and shall incorporate one neutral terminal for each single pole and neutral way.
Aluminum shall not be used for any interior parts of the distribution board.
Back pan or mounting on which buses and branches are mounted shall be rigid to properly support the component parts.
Reinforcing of back pan shall be by flanging or addition of angle iron.
Buses, connectors, and terminals shall be silver plated to a minimum thickness of 0.1 mm.
4.0 SPLIT BUS DISTRIBUTION BOARDS
Split bus distribution board shall incorporate a main switch or circuit breaker for the entire distribution board and two or more sub-main earth leakage circuit breakers that shall protect each section of the distribution board circuits against earth leakage.
For the wiring of circuits consisting of wire sizes 10 mm2 and larger shall be of the bolted pressure type, with a pre-insulated sleeve.
Connectors shall be manufactured from high conductivity copper, electro tin-plated.
5.0 EARTHING
An acceptable terminal bar for equipment earthing conductors shall be provided with a minimum number of cable terminations equal to the single pole number of ways of the distribution board.
A cable connection shall be made from the earth bar to the cable gland of the incoming feeder as applicable.
Cabinet shall be provided with an earth connector welded to it.
6.0 CIRCUIT BREAKERS
All plug-in type and bolt-on molded case circuit breakers shall have trip settings, and number of poles, as indicated on the Drawings. All circuit breakers shall have their ampere trip rating clearly marked and visible.
Breakers shall have quick-make, quick-break, toggle mechanisms; and shall provide positive trip-free operation on abnormal overloads. Stationary and movable contacts shall be adequately protected with effective and rapid arc interruption. Each pole of the breaker shall be equipped with an inverse time delay thermal overcurrent trip element and magnetic instantaneous overcurrent trip elements for common tripping of all poles for multiple breakers. Multiple pole breakers shall have a single handle mechanism. Automatic tripping shall be indicated by the breaker handle assuming a clearly distinctive position from the manual ON and OFF position.
Miniature circuit breakers shall provide tripping characteristics in accordance with EN 60898 in an ambient temperature of 40 deg. C. in accordance with the latest amendment.
Circuit breakers shall have minimum RMS symmetrical interrupting capacities equal to 10,000 Amps at 380 V for miniature circuit breakers and capacities equal to the values indicated in the drawings for molded case circuit breakers, and shall in no case be less than the busbar short circuit bracing of the distribution board:
Molded case circuit breakers shall have frame sizes equal to or greater than their trip ratings unless otherwise indicated and shall be suitable for operation at an ambient temperature of 50 degree C.
7.0 CURRENT LIMITING CIRCUIT BREAKERS
Current limiting circuit breakers shall have a maximum interrupting rating of 100 KA rms symmetrical amperes. All circuit breakers shall be UL listed and meet NEMA Standard No. AB1-1975, and Federal Specification W-C-375B/GEN where applicable.
Current limiting circuit breakers shall be supplied in unit molded case construction and shall consist of a common trip, thermal magnetic circuit breaker with an independently operating limiter section in series with each pole.
The conventional breaker section shall have an over center, trip-free toggle-type mechanism with quick make, quick break action and positive handle indication. A button shall be provided on the cover for mechanically tripping the circuit breaker. The current limiting breaker shall have permanent trip units containing individual thermal and magnetic trip elements in each pole. The thermal trip element shall be calibrated for 50oC ambient temperature.
The limiter section shall consist of three current limiting elements electrically coordinated with the conventional circuit breaker trip elements. The contacts of the limiter section shall be electromagnetically and electrodynamically opened and held open until interruption is complete. The unit shall not contain replaceable elements and the limiter shall automatically reset after circuit interruption.
On high level fault currents the limiter portion of the circuit breaker shall operate to limit the rise of fault current. Integral resistance shall be introduced into faulted circuit to dissipate and limit let-through energy and to provide a voltage transient-free interruption at rear unity power factor.
The current limiting circuit breaker shall have front removable lugs. Lugs shall be UL listed for copper conductors.
8.0 EARTH LEAKAGE CIRCUIT BREAKERS
Each leakage circuit breakers shall be current operated type providing protection against overloads, short circuit, and low level earth faults of 30 mA, 100 mA or 300 mA as applicable or as shown on the drawings.
It shall fit in standard distribution boards.
A push-to-test mechanism shall be provided to ensure proper operation.
Enclosure shall be glass reinforced.
Breaker shall have 6000A interrupting capacity unless otherwise indicated.
Breaker shall otherwise be similar to miniature circuit breakers.
9.0 DISTRIBUTION BOARD INSTALLATION
Distribution board shall be aligned, leveled and securely fastened to the building.
Connecting conduits shall not be used to support the distribution board.
All unused openings in distribution board cabinets shall be properly closed.
Distribution board interiors shall not be installed in cabinets until all conduit connections to the cabinet have been completed.
Trim for flush mounted cabinets shall be installed in plaster frame, flush with finished wall.
Trim shall be installed plumb and square to finish painting.
Concealed surfaces of cabinets shall be given on site a heavy application of emulsified asphalt prior to installation.
10.0 SHOP DRAWINGS
The Contractor shall submit shop drawings for the switchboards including schematic diagrams with all protective devices, control, instruments and instrument transformers details, dimensions of the assembly, etc. A copy of these shop drawings shall also be submitted to the local power authorities for approval. Any modification required by the Engineer or the local power authorities to allow the equipment to comply with the codes, standards and specifications called of hereinbefore shall be carried out without additional charges.
11.0 APPLICABLE STANDARDS
1.Requirements for type tested or-BSEN 60439-1 1994
partially type tested assemblies-EN 60898
2.Specification for color identification-BS 381 C: 1988.
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