Low Voltage (LV) Switchgears in Electrical Construction Works



Low Voltage Switchgear are familiar to every people since they are the controlling mediators between low voltage utilities and equipment with human beings. Be it a common home DB or an isolator or control centres, they all are being manufactured to use untrained people ie., common people. The scope of works shall cover the supply installation, testing and commissioning of main and sub-distribution boards, panel boards, fusible safety switches, circuit breakers etc. Associated minor building works require for the erection of the distribution boards and panels shall be included in the scope of work.



LV Main Switch Board (MSB) Installation in Electrical Construction Contract Work

The contractor shall furnish and install complete all main switch boards as shown on the drawings and as described in these specifications. The main switch board shall be connected to a supply voltage system of 380/220V, 3 phase, 4 wire and 60 Hz.

The switch board shall be designed, built and tested in accordance with applicable portion of the latest editions of National Electrical Manufacturers Association NEMA PB-2 and Underwriters Laboratories No. UL-891 and the latest requirements of the National Electrical code. All sections and devices shall be UL listed and labelled.

The switch board shall be furnished with individually mounted main breaker and group mounted branches breakers, front and rear accessible. The door shall be formed steel hinged and shall be retained in the closed position by two captive, slotted, knuckled head screws.

The switch shall be completely self-supporting structure of the required number of vertical sections bolted together to form one metal enclosed switch board 90” high. Sides, top and rear covers shall be code gage steel bolted to the switch board, structure. The frame structure members shall be die-formed, 12 gage steel bolted together and reinforced at corners with rugged gaskets internal and external to the structure members. The switch board frame is to be suitable for use as floor sills in indoor installations.

The switchboard and devices shall have a fully rated short circuit rating of 65 KA, RMS symmetrical amperes. The switch board shall include protective devices listed on drawings and as specified in these specifications with necessary interconnection, instrumentation and control wiring.

The vertical bus in each section shall be tin-plated copper. This bus shall be rested in individual extruded channels in fibre glass base, to provide a barrier between vertical sections shall be tin-plated copper. Bus joints shall be bolted with high tensile steel bolts with spring loaded Belleville type washers. All bus joints shall be accessible from the rear of the switch board for maintenance.

The current density of the bus shall not be less than 1.6 A/mm2 (1000 A/sq. inch) cross section. The continuous current rating of the bus shall be equivalent to the frame size of the main circuit protective device. All feeder device line and load connection straps shall be rated to carry the full continuous current rating of the device frame (not trip rating). Load connection straps shall be insulated and extended beyond the main bus.

A ground bus shall be furnished secured to each vertical section structure, and shall be extend the entire length of the switch board. The size of neutral bus bar shall be 100% of phase bus bars. The size of ground bus bar shall be 50% of phase bus bars. Front to rear bus arrangement shall be used throughout to assure convenient and safe testing and maintenance.

The main device shall be completely isolated from the feeder sections of the switch board both in the device sections and the cable section of the cubicle. The cable section shall also be isolated from the main horizontal bus. The main device cubicle shall have a UL service equipment label.

When specified, space for future device shall include all necessary bus, device support and mounting and connections. All vertical sections comprising the switch board shall be aligned front and rear. Control wiring, necessary fuse blocks and terminal blocks within the switch board are to be furnished when required. All control wires leaving the switch board are to be provided with terminal blocks with suitable numbering strips.

Switch board is to be provided with adequate lifting means, and be capable of being rolled or moved into installation position and bolted directly to the floor without the use of floor sills.

Cable connectors shall be tin-plated copper and UL listed for copper cables. The connectors shall be mechanical type lugs, for short circuit rating at 100 KA, rms or less short circuit rating above 100 KA, shall have compression type lugs.

Switch boards shall be designed for bottom entry main feeders and top entry outgoing feeders. Switch board current rating including devices shall be based upon operation in 45 deg. C room ambient.

Installation of LV Sub-Main Distribution (SMDB) Panel Boards in Electrical Construction Work

The sub distribution panel board shall be connected to a supply voltage of 380/220, 3 phase 4 wires and 60 Hz. It shall be designed and constructed to withstand the thermal and mechanical stresses set up by short circuit faults levels of 43 KA at 380 V for clearance time of protective devices. The sub distribution panel board shall be fabricated to conform with: 1. National Electric Code, 2. NEMA standard.

The numbers, sizes and rating of units incorporated within the distribution board shall be as indicated on the contract drawings. Distribution board shall be floor mounted, metal clad, with good quality folded steel construction. 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 labelled (typewritten or stenciled). Doors shall be hinged and gasketted to give a damp and dust proof enclosure. The door fastening shall be capable of being locked.

The distribution shall be connected to a supply voltage of 380V/220V, 3 phase, 4 wire, 60 Hz. The distribution board shall be of group mounted main and sub-main breakers and shall be fabricated from galvanized sheet steel. A turned edge shall be provided around the front box for rigidity and attachment of the front.

The switch board shall be indoor type. The minimum of 45 cm clearance shall be provided from the bottom of the switch board for cable termination at the circuit breakers.

LV Bus bar System

The bus bar in the distribution board shall be of copper, silver or electrode tin plated contact surfaces. The rating of the bus bars shall not be less than that of the main breaker that services the switch board. The continuous current rating of the bus bars shall be based on an ambient temperature of 45 deg. C with a temperature rise of 50 deg. C.

Neutral and ground buses shall be 50% of phase buses. The short circuit rating of the main bus bars and sub-main bus bars structure are braced for a fault rating of 50 KA RMS symmetrical at 415 V, 3 phase, 60 Hz.

Metering and indication shall be provided as follows:

  1. One ammeter per phase with full scale deflection equal to rating of bus bars.
  2. One voltmeter 0 to 400V with phase selector switch - phase to phase and phase to neutral.
  3. Phase indicator lights - one per phase.

Branch Panel Boards

The panel boards shall be suitable to connect 380/220V, 3 phase, 4 wire 60 Hz system voltage.

The boards shall be constructed from minimum 16 AWG sheet steel folded and braced as necessary to form rigid enclosures and be complete with top and bottom gland plates. From panels shall also be removable for ease of wiring and lockable hinged doors fitted to each panel.

The board will be wall mounting type and fixed on to walls and suitable provision shall be made for the method of fixing. Each board shall be equipped with the number of moulded case circuit breakers (MCCBs) indicated on the drawings and shall be of the ratings specified.

Bus bars shall be consist of three high conductivity copper conductors of equal cross section, mounted on insulated pillars and rated in accordance with the details given on the drawings. Two independent connector links shall be provided inside the panel for the neutral and ground connections. The number of termination shall be equal to the number of outgoing circuit breakers.

MCCB banks shall have a corresponding number on the circuit list. Each distribution panel shall have an approved type of circuit directory fixed to the inside of the door. Each MCCB shall have a corresponding number on the circuit list. Each distribution panel shall have an approved type of circuit directory fixed to the inside of the door.

LV Power Break Circuit Breaker in Electrical Construction Work

The power break circuit breaker shall be made of insulated case power break and shall be UL listed in accordance with the UL standard 489 and meet the requirements of NEMA standard AB1-1975.

The power break circuit breaker shall be electrically and manually operated and shall have stored energy charging mechanism and independent closing function. The closing function shall be local only via a closing button on the face of the breaker. The local closing function shall employ a “hidden” ON button to preclude unintentional closing of a charged breaker.

The power break circuit breaker shall be equipped with the following internal control accessories as specified on the drawings.

LV Disconnect Switches in Electrical Construction Work

Disconnect switches shall be of the general duty, single throw, fusible type as shown on the drawings. Switches shall have continuous rating of the horse power rated, quick make, quick break, rated at 600 volts AC. Sizes and number of poles shall be as shown on the drawings of as required for the load served.

Enclosures shall be of the suitable NEMA/BA type for specific locations. Fusible type switches shall be provided with the proper size, class K5 dual or single element current HRC fuses which shall be coordinated to prevent current from exceeding the withstand capacity of the switch.

The operation mechanism shall be quick make, quick break with external operating handle mechanically interlocked with the enclosure cover. The interlock shall make it necessary for the disconnect to be in the ‘OFF’ position for normal access to the inside of the enclosure. Means for bypassing the mechanically interlocked door and handle shall be provided.

Enclosure shall have provisions for locking the operating handle in the off position. Position of disconnect shall be clearly indicated on the enclosure. The disconnect switches used in the outdoor areas shall be water tight type. Disconnect switches shall be as manufactured by General Electric company or approved equivalent.

Instrumentation in LV Electrical Panel Boards

Ammeters and voltmeters shall have moving iron spring controlled dead-beat elements in square bezel flush type cases 144 mm in size and suitable for switch board mounting. Meters shall conform to BS-89 and have Grade ‘A’ accuracy. Scale range shall meet with the requirements and shall be as indicated on the drawing or in the schedule of quantities.

Energy meters shall be two element switch board mounting type suitable for unbalanced loads. Meters should incorporate a KW/KVA demand meter with an integration time of 30 minutes. In case of two incoming feeders, a summation CT shall be provided with the meter. Meters shall conform to BS-37.

Metering in LV Electrical Panel Boards

The contractor shall liaise with the Electricity Supply Authority to establish the space and housing requirements for service termination and KWH metering equipment. The contractor shall be responsible for providing a suitable enclosure for such equipment to the Electricity Supply Authority specifications. Each KWH meters shall be clearly identified with proper numbering for area identification.

Testing And Commissioning of LV Electrical Panel Boards

All switch panels and panel boards shall be tested with 1000V insulation tester and the insulation valve between the live conductors and frame (ground) should not be less than 2.0 Meg. Ohms.