Distribution Switchboards


  1. Switchboards shall comply with BS 5486, IEC the Machinery and Occupational Safety Act 1983, and Local Authority’s requirements.  Switchboards shall be of the freestanding type with front and rear access. The degree of protection shall be IP31 minimum to IEC 144 with gasket front doors for internal locations, IP65 for external locations
  2. The complete installation shall comply with all relevant IEC Standards Codes of Practice and, where indicated, with other Standards and Specifications, and all amendments thereto. The relevant issues shall be those current three months before the date for the return of tender unless alternative dates are indicated
  3. Where practicable, each item of equipment shall be clearly and indelibly marked to indicate the standard with which it complies. Alternatively, a certificate of compliance shall be provided if required.
  4. Where equipment or services are indicated to be manufactured or provided under a particular certification, licensing, or quality assurance scheme, the manufacturer or supplier shall be a current participant in the relevant scheme. A certificate of compliance shall be provided
  5. Equipment not manufactured in the European Union shall be a standard that ensures its compliance with all appropriate IEC Standards.
  6. The completed switchboards shall be inspected by the Engineer at our Factory/works prior to despatch.  We shall inform the Engineer timeously when the switchboards will be ready for inspection.


All distribution boards shall be of ample size to accommodate the specified switch gear and not less than 30% additional space for future switch gear shall be provided, unless otherwise specified.

All specified external dimensions for distribution boards shall be adhered to.  If the clearances specified cannot be adhered to as a result of restricting external dimensions the we (Contractor) shall consult the Engineer before manufacturing of distribution boards.

Moisture and Vermin

All distribution boards shall be moisture, vermin proof, and adequately ventilated.  Distribution boards at all times shall be protected against damage, moisture, dust, and vermin.


  1. All distribution boards shall be equipped with doors unless otherwise specified in the Detailed Specification.
  2. The doors shall be of a smooth flat finish suitably braced to ensure stiffness and when in the closed position shall be flush with the architrave frame.
  3. The doors shall be of sheet steel and on all four sides be bent inwards to ensure stiffness.
  4. The corners shall be welded and smoothed.
  5. All doors shall be fitted with rubber or neoprene seals.
  6. Door Handles, Latches and Hinges
  7. All distribution board doors shall be fitted with lockable handles and catch, consisting of either a pushbutton handle combination with a spring-loaded catch or a rotary handle and catch combination shall be installed.
  8. Square key-operated catches may be fitted to the main board in substations and lockable shafts.
  9. Doors shall have adequate fastenings with provision for locking in the open and closed positions

Fault Levels

  1. The fault level rating of each board shall be as specified and indicated on the single line diagrams.
  2. Distribution boards with asymmetrical fault level short circuit rated to 50kA for 1 sec, 415v, with fully compartmented busbars ASTA certified to 50kA for 1-sec minimum
  3. In both cases, we shall submit copies of the test results or submit test certificates including the relevant drawings.
  4. Tests will be carried out for all bus bars, cabling, and wiring from the main bus bars to the input side of all switchgear.
  5. Small wiring direct from bus bars shall be protected by means of protection fuses mounted directly onto the bus bars.
  6. Bus bar-mounted fuses shall be provided with insulated backplates to afford complete safety from hand contact with live parts.


  • All circuit breakers, contactors, timers, switches, relays, pushbuttons, terminals, etc., must be clearly labelled as to their title or function.
  • Labels shall be of engraved Trifoliate or similar plastic material and shall be fixed by means of machine screws or aluminum channeling.
  • Lettering shall be black on white background.
  • All wire ends to contactors, relays, terminal strips, etc., will be clearly numbered by means of slip-on plastic ferrules similar or equal to Critchley or Parfex ferrules.
  • All terminal strips will be numbered on both incoming and outgoing sides by means of suitable clip-on numbers.
  • A suitable legend card covered by a perspex panel shall be provided and installed inside the door or on the face of the panel.  This legend card shall give the designation of the various outgoing circuits, the corresponding circuit breaker numbers in the board, and the area where the circuit is taken to e.g.: 3 Phase socket-outlet – SSO 3 – Workshop.  The legend card shall provide a description of where the board is fed from.
  • Final legend cards will be typed and inserted into the document holder.

Bus bars

  1. The bus bars shall comply with relevant IEC standards and shall be braced for the fault currents specified.
  2. Bus bars shall be of rectangular section manufactured from copper or aluminum, shall have joints of adequate contact area for the current rating as specified.
  3. All joints in the busbars shall be firmly bolted together with suitable high tensile galvanized or cadmium plated steel bolts and nuts.  Spring washers shall be used.  Busbars shall be easily accessible from the non-restricted side of the board and both ends shall be drilled and tinned as to be readily extendable.
  4. Bolted joints shall maintain contact pressure permitting rated current at rated temperature and thermal expansion without buckling of busbars and shall not loosen under vibration.  Joints on aluminum busbars shall be suitably prepared.
  5. All busbars shall be carried on insulated supports spaced at intervals of 600mm for main busbars and 300mm for busbar droppers and shall be adequately identified in phase colors.  Connections to busbars shall be bolted.  The insulators shall be tested in accordance with the relevant standards.
  6. Busbar droppers to and from circuit breakers shall have a suitable intermediate insulated support.  Heavy outgoing cables shall be supported so that there are no undue strain on the circuit breaker or circuit breaker cradle terminals.
  7. Where stub bars are used to accommodate large cable ends at the circuit breaker terminals, we (contractors) shall ensure that the clearance between phases is adequate.

Earth-bar / Neutral Bar

  • Earthing of the switchboards shall be carried out in accordance with the ( BS7671 16th Edition of the IEE Regulations) and BS5486. An earthed bar shall be provided along the length of the switchboard and all protective conductors and bonding conductors shall be connected to this bar.
  • The bar shall be bonded to the entire switchboard chassis, ancillary metalwork to each cable termination plate. The bar and all protective conductors shall be sized in accordance with BS5486. Earth bars shall be fully accessible with adequate space for making off of connections.
  • Removal of wiring or any other components to access the earth bar will not be accepted.
  • The neutral bar shall be mounted on stand-off insulators and shall be complete with high tensile, plated bolts/nuts/washers and spring washers.  The neutral bar shall have the same cross-section as the main busbar.  Only one termination per bolt/screw shall be allowed.


  1. All internal wiring shall either be laced or run in wiring channels and shall be clearly marked with numbered ferrules and color-coded.
  2. All connectors, including those of earth continuity conductors, shall be secured against accidental loosening.
  3. All cables shall be run continuously from terminal to terminal without intervening joints.
  4. Conductors of different circuits may be laid side by side, or may occupy the same wireway, or may be contained in the same cable. If they are operated by different voltages, they shall either be separated by suitable barriers or all be insulated for the highest voltage to which any conductor within the same cover may be subject. Wiring shall be fully accessible, installed neatly without cross-overs as far as practicable. Soldered lugs shall not be used.
  5. All terminals shall be placed in easily accessible and strongly constructed enclosures or compartments providing protection against accidental contact and against mechanical damage.
  6. Wiring used inside enclosures or compartments should permit the laying of additional cables at a later date if required. A minimum of 33% spare capacity shall be provided.
  7. For the apparatus mounted on a door, fully shrouded terminal strips shall be provided fixed on the enclosed or on the door. The connections shall be made with flexible conductors allowing for frequent and easy opening of the door without the risk of damage. The conductors are anchored to the door on the one hand and to the enclosure on the other, independently of the fixing of the ends on the terminals.
  8. Conductor material shall be copper unless otherwise stated and the cross-section shall be suitable for the required duty and protection, after taking into account any derating factors applicable to `loomed’ or `bunched-in-trunking’ circuits.
  9. In general, all wiring shall be PVC insulated to BS6231 and of the appropriate voltage grade. Wiring for electronic or other special devices or where excessive heat may be present shall be suitable for such temperature and for the proper functioning of the equipment.
  10. Sufficient space shall be left above the terminal block to allow the proper making off of outgoing and incoming cables.


  • Cable gland plates shall be at least 3mm thick and galvanized or plated.  The gland plate shall be bonded to the main earth bar with a copper earth strap.  When cables enter the top of the board a cable gland plate shall be incorporated in the top of the switchboard.  Spare entry holes in the top gland plates shall be provided with galvanized or plated blank covers.
  • Gland plates and cable support bars must be placed at suitable heights to allow for the bending radius of the cables concerned and making off of cables.  Gland plates for single-core cables shall be manufactured from nonferrous material of adequate strength.


  • Boards’ enclosures shall be fabricated from heavy quality sheet steel rust-proofed and powder spray or stove-enameled, with hinged, lockable doors. Type B three-phase boards shall be used.
  • Epoxy powder coating could be offered.
  • The color internally shall be white.  All drilling, cutting, and other metalwork must be completed before the painting process is commenced.


  1. Drawings in triplicate showing the general layout and arrangement of the boards, along with the details and arrangement of the equipment to be installed therein will be submitted to the Engineer for approval before manufacture is commenced (where applicable).
  2. The Engineer’s approval (Client’s rep) will be required for contractual responsibilities and obligations. We shall not be responsible for discrepancies, errors, or omissions on drawings or other documentation supplied by the Client, where they have been approved by the Engineer, where such discrepancies, errors, or omissions are due to incorrect information given in writing by the Engineer.
  3. Drawings for boards containing control gear will include detailed wiring diagrams on which the wire and terminal numbering code will be clearly shown.
  4. Construction of Free Standing/wall mounting Distribution Boards


Depending on the size of the distribution boards they will be of the floor standing type or wall mounting, be designed for installation over a cable duct or a manhole.  Equipment terminals, busbars, etc., shall be fully accessible from the front, sides, or back of the board as specified in the detailed specification.


  1. The boards shall be manufactured from not less than 1.60mm thick steel suitably folded to form the necessary angle and channel frames.  All corners and joints shall be butt-welded and smoothly finished as to be invisible on the finished board.
  2. Large boards shall be built up in a modular form, the sections of which shall be bolted together for ease of handling.
  3. The front of the board shall be provided with hinged 1,6mm thick cold-rolled sheet steel panels folded with returns on all sides to ensure adequate panel rigidity.  These panels must be provided with square key-operated quick-release fastenings.
  4. Side and back panels shall be manufactured from not less than 1.60mm thick sheet steel suitably folded to ensure adequate panel rigidity.  Removable side and back panels shall be manufactured from 1,6mm thick sheet steel and shall be fixed by means of 6mm plated bolts and captive nuts.
  5. The board shall be mounted on a rigid steel channel base which must fully support the board.
  6. Front doors shall be made from a 1.60mm steel plate suitably folded and stiffened to ensure rigidity.  The maximum size of a single door panel shall be 750mm wide by 1 800mm high.  Larger doors may be permitted but these must be given special consideration.  Boards shall be vermin and dustproof.

Busbar Couplers

  • The busbars couplers shall consist of an on-load isolator rated as shown, triple pole with a bolted neutral link.
  • The operating mechanism is to be manual, spring-assisted, and is to be trip-free.
  • Mechanical ON/OFF indication is to be provided and facilities for Castell interlocking shall be fitted when specified.



Metering equipment shall be in accordance with BS 89 and/or IEC 51.


  • Voltmeters shall be of the moving iron flush mounting type, with Class 1,5 accuracy as specified in IEC 51, locally manufactured with 96mm square flush panel mounting and fitted with unbreakable macro long glass fronts.  The voltmeters shall be suitable for operation on a 50 Hertz system.  The voltmeters shall be manufactured in accordance with BS 89 to industrial-grade accuracy as specified therein.
  • The voltmeters shall be protected by high rupturing capacity cartridge fuses to IEE standards housed in suitable insulated fuse carriers with a panel-mounting base.  Voltmeter selector switches shall be provided and shall have an off and 6 metering positions, between the phases and between each phase and neutral.


  • Unless otherwise specified, Ammeters shall be of the moving iron flush mounting type, with zero adjustment screw, and shall match with the voltmeters, unbreakable macro long glass fronts, and scale range as specified.  Ammeters for use in motor circuits shall have a suitably compressed overload range.
  • Combined Maximum Demand and Direct Indicating Ammeters
  • The ammeters shall comply with BS 89.
  • The instruments shall be flush panel mounting, locally manufactured with 96mm square face match the voltmeters, and fitted with unbreakable macro long glass fronts.
  • The ammeter shall be of a moving iron type showing the instantaneous current value, combined with a maximum ammeter employing a bimetallic spiral device which will indicate the mean current value over a 15 minute period, fitted with a residual pointer to indicate the maximum mean current reached during any period between manual resetting.
  • The bimetallic system shall incorporate ambient temperature compensation.

Frequency meters

  • Frequency meters may be of the vibrating reed type or the direct indicating type consisting of a moving coil ammeter and a current/frequency transducer.
  • The meters shall be locally manufactured, with a 96mm square face and with an unbreakable macro long glass front.
  • Unless specified to the contrary, the indicating range shall be 45 Hz – 55 Hz and the accuracy class shall be class 0,5 in accordance with IEC 51.

Power Factor Indicators

  • Instruments shall exactly match the ammeter or voltmeters with regard to size and appearance with shadow-less scale plates and in accordance with BS 89.  The current ratio shall be 0,5 to 5 A continuous at the rated voltage.  Power factor range shall be from 0,5 PF lead to 0,5 PF lag, and size shall be as specified.
  • Indicators shall be suitable for flush mounting.
  • Voltmeter Selector Switches
  • Unless otherwise specified each distribution board shall be provided with one voltmeter and selector switch.
  • The voltmeter selector switch shall have an “OFF” position and six measuring positions for all phases/neutral and phase to phase readings.
  • The switch shall be of the rotary type with CAM or WHIPE action air break contacts with two openings per pole.
  • The positions shall be clearly and accurately indicated on the faceplate in relation to the handle position.
  • The selector shall be suitable for flush mounting.

Voltmeter Fuses

  • The voltmeter fuses shall be of the cartridge type with fuse carrier and fuse base with terminals.  The fuse shall only be removable after drawing out of the fuse carrier and removing the cartridge from the carrier.  Rewireable fuses shall not be acceptable.  The fuse holder base shall be designed and manufactured for flush mounting and shall be installed on the same panel with the selector switch and the voltmeter.

Current Transformers

  • Current transformers shall be of the ring type and shall be neatly installed.  The area for the conductor shall be just big enough for installation to prevent inaccuracy.  The current transformers shall be designed for the specified primary current with a 5 Amp secondary current.
  • The current transformer rating shall be designed to take the instrument load to prevent any misreadings.
  • For indicating instruments the accuracy shall be Class 2.  For measuring purposes the accuracy up to 200A shall be Class 1, from 250 to 600A Class 0,5 and from 900A up shall be Class 0,2.  Safety Class shall be 5P or 10P as applicable.
  • All current transformers shall be provided with labels with non-removable lettering, indicating :
  1. • Manufacturer
  2. • Series number and type
  3. • Primary and secondary currents
  4. • Frequency
  5. • Rating and accuracy class
  6. • Highest Installation voltage
  7. • Insulation provision
  1. The current transformers shall be designed and supported to accept the dynamic powers of the specified fault level.

Indicator Lights

  • The indicator lights shall be in accordance with BS 1050 where applicable.
  • The indicator lights shall be incandescent, neon, or LED-type lamps.  The lamp voltages shall adapt to the control voltage, but the control voltage shall be of a lower value than the lamp voltage.
  • Indicator lights shall be suitable for mounting in distribution board panels and doors and shall consist of a mounting pedestal with a lamp holder, an interchangeable lens, terminals, and facilities for the installation.
  • Lamps shall be replaced from the front of the panels without the use of any tools.  All indicator lights shall be of the same standard range of one manufacturer, and shall have the same physical dimensions, and be interchangeable with key switches and push buttons.


A digital multimeter of the PM500 type with a LAN connection capable of recording all relevant electrical measurements shall also be accepted.

Routine Tests

The following tests will be carried out on all distribution boards in our works and if the Engineer so requires must be witnessed by him.

  • All tests shall be carried out in accordance with the procedure laid down in BS 159.
  • A high voltage test of 2 000 Volts, 50Hz, AC for one minute between each phase and earth in turn with the remaining phases earthed.
  • A 1000 Volt megger test on all insulated sections, from phase-to-phase and phase-to-earth Resistance readings shall exceed 1000 Megohms.
  • Full operational tests on any relay gear or which gear installed on the board.
  • A trip test on all earth leakage units by creating an earth fault of appropriate magnitude.
  • A current injection trip test on all main breakers to ensure that the trip at their set currents.
  • Such other tests as may as may be required by the regulations.