Electrical Sub-Main Distribution Boards (SMDBs) are the integral part of Electrical LV Power distribution network in the end user premises of a medium and large scale projects. SMDB acts as a mediator between Main Distribution Board (MDB) and Final Distribution Boards (FDBs) that serves approximately 100A to 400A load range.
The drawing given below explains how to install a floor mounted type SMDB. It requires a cable trench to enter the cables to panel board. These cables to be tightened with cable glands through gland plates fitted under SMDB. Adequate support system and fixtures to be given for mechanical strength when installing power cables. Power cables shall be entered to the cable trench through PVC ducts or GI ducts which connected to incomer as well as outgoing cables. The drawings given here shows how to install:
1. Floor mounted type SMDB
2. Wall mounted type SMDB
3. Electrical Schematic Diagram Drawing also known as Single line Diagram (SLD) of the SMDB.
This drawing shows how to install a wall mounted SMDBs. In this drawing as we see, Sub main distribution panel board installed on wall and cables connected directly through PVC/GI duct pipes and cable trench can be avoided but in some cases we provide cable trench too for the convenience particularly system has number of large size cables.
Below drawing shows the schematic diagram of a SMBD that has to be powered by MDB through a 400A TP MCCB and serves to various FDBs using 125A TP MCCBs.
Typical Specification For SMDB (Sub-Main Distribution Boards)
- The Contractor shall supply, install, test and commission sub-main distribution boards located as indicated on the drawings. They shall be rated for continuous duty at 50°C ambient. The ratings of circuit breakers shown on the schematic diagrams are nominal fixed ratings.
- The fault rating of the bus bars shall be 35 kA, ASTA certified. MCCBs shall have a service short circuit breaking capacity (Ics) of 25kA for MCCB up to 160 A and 35 kA, for MCCB of 200 A and above. It shall not be possible to close the circuit breakers on to a fault. MCCBs shall be of the fixed type.
- Operation of all outgoing MCCBs shall be possible at all times without the need to isolate the board or open main doors. Wardrobe type boards will not be acceptable.
- A sub-feed neutral assembly with a removable link for isolation shall be available as an integral part of the distribution board. Earthing bar assembly shall also be available to facilitate the correction of earth continuity conductors.
- Sub main distribution Boards are to be compartmented form 4, type 6 to IEC 60439-1, front accessible, with fully withdrawable feeder modules. The construction shall be made with total segregation between different compartments in order to avoid gases arising from faults in one section from spreading to an adjacent section. Selection of components to be certified by manufacturer that electrical protective devices are selectively coordinated up stream and down stream.
- Type of SMBD: General purpose type or industrial suitable for relevant ambient conditions. flush self standing and/or surface mounted as required, comprising box, trim, or trim and door to approved manufacturer's standards and sizes. Enclosure shall be certified for 1000-hour salt spread test.
- Construction of SMDB: Box, trim and doors, where required, are to be electro-galvanized sheet steel of gauges not less than 2mm and in accordance with the standards. Welded joints are to be galvanized after manufacture. Gutter spaces are to conform to the standards, adequate for the utilized cables/wires subject to the Engineers' approval and in no case less than 100 mm on all sides. Enclosure is to have pre-designed angles or threaded end studs to support and adjust mounting of interior panelboard assembly.
- The bus bars shall be mounted on non-hygroscopic insulators and shall withstand the rated short circuit level. The neutral bar shall be provided with a separate terminal for each incoming and outgoing feeders. A separate earth terminal block shall be provided which shall have a separate terminal for each feeders. All phases shall be identified and separate circuits shall be labeled (typewritten or stenciled).
- Trims are to cover and overlap front shield, covering all terminals and bus compartments, to form a dead front panel. Trims are to be fixed to cabinet/box by quarter-turn clamps engaging flange of box (use of screws engaging holes in flange of box is not acceptable). Screws where used are to be oval-head, countersunk and flush. Trims for flush mounted panelboards are to overlap box and front shields by at least 20 mm. Trims for surface mounted panel boards are to be exactly sized to form flush fit to box.
- Doors are to have concealed hinges integral with trim, and flush combination cylinder lock and catch. Doors over 1000 mm high are to have vault-type handle and multiple point latch mechanism. Locks are to be keyed alike.
- Hinged Front Panels: Allow access to breaker, metering, accessory, and blank compartments, maximum demand indicators (MDl) on incomer & power quality meters (PQM) with backup memory during power failure.
- Finish: Inner and outer surfaces of cabinet/boxes, trims, doors etc. are to be cleaned, phosphatized, chrome passivated and treated with final thermosetting epoxy powder modified by polyester resins providing high resistance to mechanical injury, heat, acid and alkali solvents, grease, aging and corrosion and of standard gray RAL 7032 outside and white inside colour to the approval of the Engineer.
- Directories under glassine, or an approved alternative durable arrangement, are to be provided on inside face of doors, or in metal label holders when trim without doors are specified. Directories are to be typed to identify panel boards and clearly indicate circuit number and description of load.
- Outdoor enclosures are to be heavy duty double walled aluminium cabinets, minimum 1.5 mm thick, fully weatherproofed (IP 55), without knockouts, but with removable sealed / gasketed bottom gland plates and gasketed doors.
- Glands: Cable glands used for armoured cables are to be of brass compression type, complying with BS 6121 and BS EN 50262: Part I, with earth tag and PVC shroud.
- Operating dollies of all circuit breakers/isolators to be door interlocked and it shall be possible to defeat the interlock by means of a tool.