Indoor MV Distribution Panel Boards In Electrical Construction Works



Building projects requires MV Distribution Panel Boards for distribution of power to each utilities. A good electrical distribution network will have proper design, right components, and future load provisions. Space utilization will also be an important factor for maintenance and handling of the equipment. Here we look into what are the features required for design, manufacture, install a MV Distribution panel board including its BIS (IS) standards that serving quality building projects.



1. Scope of Work

The scope of this section covers manufacture, supply, installation, testing and commissioning of indoor Medium Voltage Distribution Panels Boards in Electrical Construction Contract Works.

2. Applicable Standards For Indoor Type Distribution Panel Boards

The following Indian Standards (IS/BIS) shall be applicable. The list shown here may be outdated hence the manufacturer to make sure they use the latest or superseded BIS standards.

IS 4237 of 1982: General requirements for switch gear and control-gear for voltage not exceeding 1000 Volts.
IS 5578-1985: Guide for marking of insulated conductors.
IS 11353-85: Guide for uniform system of marking and identification of conductors and apparatus terminals.
IS 2516: AC Circuit Breakers. (Part I) Sec. I 1989.
IS 4064-1978: Heavy Duty Air Break Switches for voltage not exceeding 1000 Volts.
IS 2147-1962: Degree of protection provided by enclosure for low voltage switch gear and control gear.
IS 2675-1983: Enclosed distribution fuse boards and cutouts for voltages not exceeding 1000V.
IS 2551-1982: Danger notice plates.
IS 1818-1972: Alternating current isolators (dis-connectors) and earthing switches.
IS 8828-78: Miniature air break circuit breakers for voltages not exceeding 1000V.
IS 3202-1965: Code of practice for climate proofing of electrical equipment.
IS 722-1977: AC Electric Meters.
IS 2705-1964: Current Transformers (Part 1 to 4).
IS 9224-1979: HRC fuse upto 650 V.
IS 649-19: PVC insulated cables upto 1100V.
IS 8623-77: Factory built assemblies of switch gear and control gear.
IS 8544-1979: Motor Starters for voltage not exceeding1000 Volt AC or 1200 Volt DC.
IS 2959-1985: Contactors for voltages not exceeding 1000 volt AC or 1200 volts DC
IS 12640-1989: Residual current operated circuit breakers.
IS 1248-1983: Direct acting indicating (all parts) analogue electrical measuring instruments & and their accessories.
IS 4201-1983: Application guide for voltage transformers.
IS 8197-1976: Terminal markings for electrical measuring instruments & their accessories.
IS 3072-1965: Code of practice for installation and maintenance of switch gear.
IS 3231-1965: Electrical relays for power system protection.
IS 2516-1967: Molded Case Circuit Breakers.

3. Construction of the Indoor MV Distribution Panel Boards

The salient feature of construction of panels shall be as follows:

  1. Sheet Steel 2 mm thick for frames, cable glands plates and equipment mounting plates and 1.6 mm thick for front and rear doors and covers.
  2. Welded construction, with shipping section bolted together. All such joints to be gasketted. Lifting lugs to be provided.
  3. The cubicles shall be totally dust and vermin proof conforming to IP 54 of IS 2147.
  4. All doors to be hinged type except busbar chamber covers which shall be bolted type. The panel shall be of flush front design, suitable for access from front and rear.
  5. The construction shall be such as to facilitate easy extension at both ends.
  6. The design shall be such as to have individual feeders in separate compartments, with proper barriers between adjacent feeders, busbar chamber and cable box chambers.

4. Painting of The Panel Boards

All sheet steel work shall be properly cleaned and degreased. Rust and scale shall be removed by pickling and phosphatising. After phosphatising, two coats of finishing synthetic enamel paint of approved shade as per IS-5. The painting shall be stove enameled or epoxy paint as called for.

5. Air Circuit Breakers (ACB) Used In Panel Boards

  1. The circuit breaker shall be air-break, conform to IS 2516 (I,II,III) & shall be air-break, horizontal drawout feature shall show 3 positions viz.: SERVICE, TEST and ISOLATED. These positions along with "OPEN" and "CLOSE" positions shall be visibly marked.
  2.  All positions shall have provisions for locking. The ACB shall have shutter assembly and arc-chutes and mechanical trip features.
  3. The ACB shall have 6 NO + 6 NC auxiliary contacts rated at 10 A, 240 V, AC.
  4. 'RED' and 'GREEN' indicating lamps shall be provided on the cubicle.
  5. The ACB door shall not have any lamps or instruments. All such accessories shall be mounted on a separate compartment.
  6. The ACB shall have proper interlocks such that it cannot be 'plugged in or out ' 'SERVICE' position, if the breaker is in 'ON' condition. It shall not be possible to operate as circuit breaker unless it is properly engaged in any of the three positions.
  7. The ACB shall have series CT operated over-current and short-circuit releases with facilities to mount the under voltage and shunt- trip releases.
  8. The operating mechanism shall be independent, manual spring charged stored energy type. The mechanism shall ensure quick-break, quick make action and the ACB shall be trip-free in operation.

6. Air Break Switches

  1. The air break switches shall be of AC23,( heavy) duty, quick make-quick break, fault-make type as per IS 4047.The contacts shall be silver plated.
  2. The switches shall be capable of withstanding the mechanical and thermal stresses produced by overloads and short-circuits.
  3. All switches of all ratings shall have interlocks with the compartment doors. Switches of 250 A and above shall be lockable in the 'OFF' position.  All live parts shall be shrouded. It shall be possible to intentionally defeat the interlocks if required.
  4. 'RED' indicating lamp shall be provided for 'ON' indication.

7. Fuses

All fuses shall be of HRC cartage fuse-link type having a certified rupturing capacity of not less than 46 KA at 415 volts AC. The HRC fuses shall conform to IS 9224 1979. All fuses shall have visible indication to indicate "Blown" condition. The rupturing capacity of fuses shall be as per IS 2000-1962.

8. HRC Fuse Carriers

  1. The HRC fuse carriers/bases shall be of high grade phenolic moldings. The contacts shall be silver plated and the contact blocks shall be suitable to receive the rated conductors of aluminum, through lugs.
  2. The fuse carriers shall have an aperture to view the conditions of HRC fuse mounted inside.

9. Contactors

  1. The motor starter contactors shall be of the electro magnetic, double-break, non-gravity type rated for uninterrupted duty suitable for operation under AC-3 utilization category as per IS 2959.  The contacts shall be silver plated.
  2. 2 NO and 2 NC auxiliary contacts shall be included.
  3. The operating coils shall have class 'E' insulation of wire  and shall be suitable for operation of any specified control supply system.

10. Thermal Overload Relays

  1. The thermal overload relays shall be provided where specified, having 3 element, positive action, ambient temperature compensated with a time lag and adjustable settings. The setting range shall be selected in accordance with the ratings of the motor.
  2. The relay shall be self-reset/hand reset as called for. In the case of hand-reset, the reset button shall be fixed on the compartment door.
  3. The relay shall have atleast one 'NC' and one 'NO' or one change-over contact.

11. Molded Case Circuit Breakers

  1. The molded case circuit breakers, MCCBs shall be provided where specified. The MCCBs shall conform to the latest applicable IS 2516-1977.
  2. For AC Circuits the MCCBs shall be triple pole  construction and shall have independent manual opening and closing mechanism. The mechanism shall be quick-make and quick-break type and the breakers shall be trip-free in operation. The 'ON', 'OFF' and 'TRIP' mechanism shall be clearly indicated.
  3. Bolted type neutral link to be provided with TP MCCB.
  4. It shall be possible to mount accessories on the MCCBs like shunt-trip and under voltage release, alarm contacts, etc.
  5. The MCCBs shall have thermal/static trip devices.
  6. The MCCBs shall have rupturing capacities as specified in BOQ/Single Line Diagram.

12. Miniature Circuit Breakers

The MCBs shall be of single pole, double pole, triple pole or four pole as required. The CBS shall be of magnetic type with a maximum rupturing capacity of 9KA at 415V. The CBS shall conform to BS 3871 (Part I) 1965 and IS 8825.

13. Current Transformers

  1. The CT's shall be of dry type and shall have short-time withstand rating equal to the short-time withstand rating of the associated switch gear for 1 second.
  2. The measuring instrument CT's shall be of 15 VA, minimum accuracy class 1.0 and an instrument safety factor of 5.
  3. The protection relay CT's shall be of 15 VA, minimum accuracy class 5P and an accuracy limit factor of 10.

14. Indicating Instruments and Meters

  1. Electrical indicating instruments shall be 72 mm/96mm/ 144 mm square size, suitable for flush mounting.
  2. The zero adjustment shall be done from outside the cover.
  3. The dials to be parallax free with black numerals on a white dial.

15. Indicating Lamps

  1. Indicating lamps shall be of the filament type and of low watt consumption, provided with series resistors and HRC fuse link for protection.
  2. The lens shall be easily replaceable from the front.

16. Control and Selector Switches

  1. The control and selector switches shall be of rotary type, adequately rated for the application but with a minimum rating of 10 Amps at 240 V AC and 1 Amp at 220 V DC. The plates shall have clear position markings.
  2. The control switches shall have pistol grip handles spring return to normal. The selector switches shall have oval knobs and shall be contact stay-put type.

17. Push Buttons

  1. The push buttons shall be of the momentary contact, push to actuate rated for 10 A at 240V AC and 1 A 220 V DC.  The 'START' push buttons shall be green and shrouded.  The 'STOP' push button shall be red and unshrouded. All other push buttons shall be black.
  2. The elements shall be enclosed with 1 'NO' and1 'NC' contacts. It should be possible to add on easily extra elements to increase the number of 'NO' and 'NC' contacts.

18. Main & Auxiliary Buses

  1. The busbars shall be of high conductivity aluminium alloy of E91E, grade. The busbar shall be of uniform cross-section throughout the length of the panels.
  2. All main and auxiliary busbars shall be insulated with sleeves.  The sleeves shall be of high dielectric strength, non-corrosive and of phase and neutral colours.
  3. The busbars shall be supported on cast epoxy/resin/DMC/SMC Fiberglass insulators and the spacing of the supports shall be such as to withstand the stresses of the short circuit currents. The busbar spacings shall be adequate for 3 phase voltage upto 600V.
  4. The busbar shall be as chosen for specific current ratings with a minimum density of 1 amp for sq. mm area.

19. Internal Wiring

  1. All internal wiring shall be carried out with 1100 V/650 V grades PVC  insulated, stranded conductor copper wires. The minimum size of wires shall be 2.5 sq. mm copper and for CTs also 2.5 sq. mm copper.
  2. All individual control and CTs wiring shall be labeled with engraved identifications ferrules, yellow  in colour with black letters.
  3. All wiring shall be terminated on stud type terminal blocks through crimping sockets. No more than 2 connection shall be made on any one terminal block.
  4. All spare auxiliary contacts of contactors and relays shall be wired to control terminal blocks.

20. Terminal Blocks

  1. Terminal blocks for power and control shall be of reputed make stud type, with washers, nuts and locknuts.  All adjacent terminals shall have insulating barriers.
  2. All power terminal blocks shall be appropriately rated for current with a minimum of 30 Amps.  The control terminal blocks shall be rated for a minimum of 10 A and suitable for atleast 2 conductors each of 2.5 sq. mm
  3. All sets of power and control power terminal blocks shall be identified with engraved plastic labels, black background and white letters.

21. Identifications Labels

All labels shall be black plastic with white engravings of letters of minimum 6 mm sizes.

22. Earthing

All switchgears shall have continuous run of earth busbar.  The size and materials of the earth busbar shall be specified.

23. Space Heater

Each vertical cubicle shall be having a Space Heater of 60 watts with thermostat wherever necessary.

24. Tariff Advisory Committee and CPRI Tested

The switch gear shall be approved by the Tariff Advisory Committee(for Fire Insurance), and CPRI tested for short circuit test and enclosure test.

25. Tests

  1. High Voltage test at 2.5 KV.
  2. Power and Control Circuits Continuity Tests.
  3. Insulation resistance test with 1000 V Meggar.
  4. Operational Test.
  5. Three Sets of test certificates to be submitted.

26. Drawings

Three sets of general arrangement drawings and wiring diagrams of all types of feeders shall be submitted.