Thursday, October 29, 2009

Lightning protection specifications

The following provides some example specifications that can be used in a contract for the installation of the lightning protection system in a multi-storey office building.

The installation of the Lightning Protection must comply with the specifications below, and the information given in the accompanying drawings, but in any case must always be in accordance with the British Standard Code of Practice BS 6651: 1985 and to the satisfaction of the S.O.’s representative. All materials should conform to the latest edition of B.S. Specifications.

Air Termination
Materials for the air termination must be medium or hard drawn copper or phosphor bronze rod of at least 15mm in diameter.

The terminations shall project at least 460mm above the highest point of the building.

They shall be firmly and permanently fixed with saddles or brackets and protected from mechanical damage.

Roof conductors
Materials for the roof conductors must be high conductivity annealed copper tape with the dimensions of 25mm x 3mm.

The roof conductors shall interconnect all air terminations to all down conductors and form a conducting air termination network covering the area to be protected with closed metal loops.

All exposed metallic structures shall be bonded in a solid and permanent manner to the roof conductor network.

Care should be exercised to avoid corrosion by the bonding of dissimilar metals.

Ferrous metals should not be used.

All joints and conductors shall be protected against mechanical damage.

Roof conductor runs shall be as direct as possible with spacer bars or tape clips at two meter intervals.

Bends shall be made as large as possible and in any event the bending radius shall not be less than 150mm.

Down conductors
Materials for the down conductors shall be of high conductivity annealed copper tape with the dimensions of 25mm x 3mm.

Down conductors shall interconnect the roof conductor network to the earth termination.

The route shall not interfere with the architecture and shall as direct as possible.

The down conductors shall be firmly and permanently fixed to the outside of the building or structure with spacer bars or tape clips at one meter intervals.

All independent metallic structures and other building structures within 600mm of any down conductor route shall be bonded to the nearest down conductor.

Lift shafts and duct shafts shall not be used as down conductor routes.

There shall not be any “up-turn” for the down conductor routes.

Joints and bonds
All joints shall be tinned and soldered and double riveted.

Clamped, bolted and screwed joints shall not be used, except at testing points and at rod connections.

Joints or bonds shall be as few as possible. All joints shall be coated with bituminous paint and protected against moisture and corrosion.

The maximum resistance of a joint shall not exceed 0.5 miliohms.

Tape to tape bonding clamps shall be “Furse” type having tinned contact surfaces complete with 8mm phosphor bronze fixing screw.

Test and junction clamps
Materials for the clamps should be phosphor bronze.

Test clamps shall be provided on each down conductor in an easily accessible position for testing purposes at a height of 2000mm from finished ground level.

Test clamps shall be protected from unauthorized interference.

Test clamps shall be provided on the roof conductor network in such a way that all parts of the network can be tested independently.

The test clamps shall be of approved type and shall not constitute an electrical resistance within the system.

After installation and testing they shall be painted with bituminous paint to prevent corrosion.

Earth termination
Materials for the earth terminations shall be of hard drawn copper or phosphor bronze rod. The minimum diameter of the rod shall not be less than 16mm.

The diameter of the rod shall be chosen according to the site condition but it shall always be such that it can be driven into the soil without bending or deforming the rod. Proper driving head and coupling similar to “Furse” type shall be employed.

The length of the rods shall be made up from the standard 1200mm lengths with internal screws and socket joints.

Minimum lengths of earth driven electrodes shall be 2400mm.

The length or the number of rods shall be increased if the maximum permissible earth resistance of five ohms cannot be achieved with the standard electrodes as specified.

The distance between two driven electrodes shall be equal to or greater than their driven length.

When more than one down conductors are used and terminated at earth terminations, all earth rods shall be bonded together to form one network.

The lightning protection earthing shall not be used in whole or in part as part of electrical earthing without the approval of the S.O.

Chemical treatment of soil at termination points to decrease the earth resistance shall not be carried out without the approval of the S.O.

Connections to the earth electrodes shall be by means of approved connector clamps adequately tightened. Earth connections shall be protected from damage by means of suitable electrode housing. The actual connection of the rods must be accessible and clearly visible when the housing cover is removed.

Testing of earth resistance and conductor continuity shall be carried as specified in the British Standard Code of Practice BS 6651: 1985.

The test shall be carried out in the presence and to the satisfaction of the S.O.

The resultant earth resistance of the electrodes shall not exceed five ohms and this resistance value shall be determined by an Earth Resistance Megger.

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