Thursday, December 17, 2009

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.

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