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Construction & Specification of SMDB, MDB, FDB LV Switchgear Panels & Distribution Board

The switchboards, distribution boards including smbd and control panels are built in accordance with IEC 439 “Factory Built Assemblies for Low Voltage” or BS 5486 “Factory-built Assemblies of Switchgear and Control Gear for Voltage up to and including 1000 AC and 1200V DC.

Switchboards SMBD, MDB, FDB are always type tested conforming to tests conducted by ASTA or other recognized testing authority. Type test certificates obtained from the testing authority together with the construction details of the switchboard are submitted at the time of technical submission.


All factory built assemblies should be capable of withstanding the electrical, mechanical and thermal stresses of the prospective fault level experience. The prospective fault levels of the various factory built assemblies are indicated in the specific drawings for smdb, mdb and other LV panels and distribution board.

All factory built assemblies, as a complete unit shall have a rating equal to or greater than the integrated equipment rating as indicated in the project drawings. All factory built assemblies subject to rain or wet conditions or located outside electrical switch room should be weatherproof constructed to IP 65, able to withstand high impact strength of 60 KN/m² (min), temperature resistant with consideration of Ambient temper 50°C and average switch room temperature 35oC. Flame retardant and corrosion resistant.

Standards for Construction of SMDB, MDB, FDB

The LV switchboards and distribution boards should be constructed in accordance with the latest revision of the following standards:

  • BS EN 61439-1:2009: Low-voltage switchgear and controlgear assemblies. General rules
  • BS EN 61439-2:2009: Low-voltage switchgear and controlgear assemblies. Power switchgear and controlgear assemblies.
  • BS EN 60947-3:2009: Low-voltage switchgear and controlgear . Switches, disconnectors, switch disconnectors and fuse-combination units.
  • IEC 61439-1: Low Voltage Switchgear and Control Gear Assemblies, General Rules.
  • IEC 61439-2: Power Switchgear and Control Gear Assemblies
  • IEC 61439-3: Distribution Boards
  • IEC 61439-6: Busbar Trunking Systems
  • BS EN 60529: Specification of Degrees of Protection Provided by Enclosures.
  • IEC 529: Specification of Degrees of Protection
  • BS/IEC or other National standards not mentioned above but are applicable to the installation.

Construction of SMDB, MDB

The factory built assemblies shall be compartmented and shall utilize sheet steel plates of thickness as detailed below. The panels shall be vermin proof and constructed to a minimum degree of protection of IP ratings to BS 5420. The IP ratings indicated below shall be applicable unless otherwise indicated in the drawing for main and sub main distribution board.

  • 2mm thick sheet steel
  • Form 4b, Type 6 for Main Board and Form 2b, Type 2 for Sub Main Board unless otherwise specified to BS 5486 with separation of busbars from the functional units and separation of all functional units from one another
  • Minimum IP 42 and IP 2X after opening of doors & panels (of external covers) unless otherwise specified.
  • IP65 for location subject to rain wet conditions or located outside electrical switch room.
  • Switchboard frame shall be of modular detail and extensible.
  • The interior of each cubicle shall be dust inert and vermin resistant.

Switchboard smdb frames shall be fabrication of 2.0mm thick electro-galvanised steel. All joints should be neatly formed and finished flush with the adjacent surfaces by grinding and/or machining. No joints shall be located on a corner and all bare edges shall be lipped. Structural members and bracing, where necessary, should be welded or bolted to the frame.

SMDB MDB FDB Specifications

Doors, Frames & Mounting of Distribution board 

Full access shall be provided to service and maintain all smdb and mdb equipment inside each cubicle by means of a suitable hinged door that shall open a minimum of 120 degrees. All hinged doors shall have an earth braid connected to the cubicle.

Doors and panels shall be constructed of 2 mm electro-galvanised sheet steel, pressed or rolled so that edges are given a neat round finish and shall reinforced with a suitable frame welded to the inside folded edge of the door. An approved stiffener shall be welded to the inside of each door and/or panel.

The smdb and db switchboards shall be mounted on 50mm x 50mm C-channel. Wherever insulating panels are required to mount special equipment, they shall be of high quality black bakelite, polished on the front. All edges must be cut straight and square and shall be chamfered on the front edge with a 3mm chamfer.

Panel work of the switchboards shall be finished with electrostatic epoxy power coating of minimum 50 microns all treated for 180°C 12 minute oven baking. All metal work should be rust inhibited and sprayed with two coats of primer. The painting should be of best quality oven-bake epoxy power coated of specified color.

Approval Procedure for SMDB, MDB & FDB

Detailed construction drawings of all factory built smdb assemblies are submitted for approval before construction commences. Such drawings show the proposed method of construction of the cubicles, method of supporting equipment and busbars, full details of busbar layout, method of support, electrical control wiring diagrams, equipment weight, colours, surface treatment. The drawings for smdb, mdb  and fdb should also incorporate a full list of proposed materials. The construction should not be allowed to commence until the drawings are approved for construction. Factory and site testing procedures and report formats are also finalized and included in the technical submittal for LV switchgear i.e mdb, smdb, fdb and capacitor bank etc.

Quality Assurance Requirements for SMDB, MDB & FDB

Source Limitations: Switchgear or smdb panels should be obtained through one source from a single manufacturer. This will ensure reliability and easy integration of all components.

All the electrical components, devices, and accessories like ACB, MCCB, MCB and other type of circuit breakers should be listed and labeled as defined in local Electricity Wiring Regulations, by a testing agency acceptable to authorities having jurisdiction, and marked for intended use.

Delivery, Storage and Handling of SMDB, MDB & FDB
  1. Deliver switchgear including smdb’s in sections of lengths that can be moved past obstructions in delivery path.
  2. Store switchgear indoors in clean dry space with uniform temperature to prevent condensation. Protect switchgear from exposure to dirt, fumes, water, corrosive substances, and physical damage.
  3. If stored in areas subjected to weather, cover switchgear to provide protection from weather, dirt, dust, corrosive substances, and physical damage. Remove loose packing and flammable materials from inside switchgear smdb panels; install electric heating to prevent condensation.

Site Coordination for Installation of SMDB, MDB, FDB

Coordinate layout and installation of switchgear and components with other construction that penetrates ceilings or is supported by them, including conduit, piping, equipment, and adjacent surfaces. Maintain required clearances for workspace and equipment access doors and smdb panels. Coordinate size and location of powerboard and electrical panel for concrete bases. Concrete, reinforcement, and formwork requirements with local authorities.


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