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Technical Requirements of Busbars And Current Carrying Parts of LV Switchgear

The manufacturing of the busbar system shall comply to the latest edition of BS 158 and BS 159. All busbars and current carrying parts shall be manufactured to carry a current density of not more than 1.55 A/mm² and shall be capable of carrying normal current continuously without the temperature rise of any part exceeding 30°C.

The location of busbar parts should be such as to ensure adequate spacing between conductors and they shall be securely fastened and braced to withstand all stress set up during transportation, erection and normal operation, and under short circuit conditions.


The busbars should not be damaged in any way under a fault of short circuit experience with minimum 35 MVA for 3 seconds. Where two or more conductors are used to form a phase, it shall be separated with an air gap sufficient to allow for the ventilation of the individual conductors all round.

Busbar supports should be made of slotted first grade “Bakelite” or glass fibre reinforced polymer able to withstand minimum operating temperature of 110°C.

All connections in current carrying parts shall be made by means of bolts with lock nuts. The use of tapped holes and studs or any other alternative method of connection in current carrying parts shall not be employed without the express approval of the COMPANY prior to manufacture.

Busbars shall be provided with approved shrinkable tube of adhesive stickers of appropriate phase colour to indicate phases.All connections, tapping and clamping shall be made in an approved manner to ensure minimum contact resistance.

All connections in the busbars shall be firmly bolted and clamped with even tension. Before assembly, all busbar joint surfaces shall be filed or finished to remove burrs, dents and oxides and silvered to maintain good continuity at all joints. All intermediate conducting material shall be used when there is a possibility of electrolytic action when contact of the copper busbars with dissimilar metals are made.


An earth bar minimum size comply with BS7671 and BS7430 with minimum 50mm x 6mm to each vertical section of the cubicle units and shall run the full length at the bottom of each switchboard. All metal parts of the switchboards and associated equipment shall be bonded and connected to this earth bar. Minimum 2 connections to main earthing system shall be provided.

The neutral busbar shall be the same size of that of the phase bars and shall be provided with an adequate number of terminals including cable lugs, bolts, etc. to suit the installation.

For circuits not less than 200A, busbar drops to moulded case circuit breakers or other apparatus shall be made of circular or rectangular busbar covered with Nylex tubing of appropriate colour and terminated in adequately sized concentric lug, of lug type, cone grip cable sockets or approved equal. Cables may be used for circuits less than 200A to type test configuration.

All visible rectangular busbars shall be bare tinned type size 1.55A/mm² based on current density.

All screws, bolts, washers used for the busbars shall be cadmium plated. All contact parts of the busbars and connections shall be sanded and coated with a thin coat of chemical inert petroleum jelly. All bolts shall be tightened with an even tension. Approved spring washers shall be used at all joints complete with cadmium plated high tensile steel bolts.

All busbars and switchgear terminals to which outgoing or incoming cables are terminated shall be manufactured for a maximum temperature of 30°C above ambient.


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