MISCELLANEOUS INSTALLATIONS
B7.6 LIGHTNING PROTECTION SYSTEM
B7.6.1 Installation of Lightning Protection System
Lightning protection systems shall be installed in accordance with the Particular Specification and Drawings, and as directed by the Architect. B7.6.2 Scope
This Sub-section covers lightning protection systems applicable to buildings and similar structures. It does not cover the protection of overhead lines and other specialized applications. The installation shall conform to the requirements and recommendations set out in the British Standard BS 6651 “Code of Practice for Protection of Structures against Lightning”.
B7.6.3 Type of Lightning Protection System
A lightning protection system shall comprise air termination connected by down conductors to earthing electrodes. In larger schemes more than one of any or all of the above items may be used. Air terminations may be interconnected by means of roof horizontal conductors.
B7.6.4 Air Termination
Air termination shall consist of vertical or horizontal conductor(s) or combination of both.
No part of the roof shall be more than 5m from the nearest horizontal conductor. For larger flat roofs, this shall be achieved typically by an air termination network mesh of approximately 10m x 20m.
Section B7
On a reinforced concrete structure, the air termination shall be connected to the designated reinforcing bars in the number of positions required for down conductors.
All metallic projections on or above the main surface of the roof which are connected, intentionally or fortuitously, to the general mass of the earth should be bonded to, and form part of, the air termination network.
B7.6.5 Down Conductor
The dimensions of down conductors to be used shall be either of the following :
(a) annealed tinned copper strip 25 x 3mm (b) annealed copper rod 12mm diameter (c) steel reinforcement bar 12mm diameter
Where the steel work of the structure is used as down conductor, co- ordination with the Building Contractor is required to ensure the continuity of the steel work. In this connection the ends of the steel work shall be connected to the lightning protection system by a jointing method which is suitable for bonding dissimilar metals and approved by the Architect. A test joint shall be provided at each connection.
When down conductors adhered on the external wall are specified, they shall be fixed by means of suitable spacer saddles at regular intervals of not exceeding 1m apart. The down conductor shall follow the most direct path possible between the air termination network and the earth termination network. When more than one down conductors are installed, they shall be arranged as evenly as practicable around the outside walls of the structure starting from the corners.
Each down conductor shall be connected to an earth termination. B7.6.6 Test Joint
Test joints shall be of phosphor bronze, gunmetal or copper. They shall comprise a heavy circular base, to which a heavy circular cover of the same material shall be bolted. The flat copper conductor shall be overlapped and placed so that they are clamped between the cover and the base by means of at least four screws or studs.
The test joints shall be positioned so that they shall not be tampered with by unauthorized persons. In cases where this is not possible, the earth tape/rod connector clamp shall be used as an alternative test point in place of the test joint.
B7.6.7 Earth Termination
Unless otherwise specified, earth termination shall be of rod electrodes complying with Sub-section B6.3.
Each earth termination shall have a resistance to earth not exceeding the product given by 10Ω times the number of earth terminations to be provided. The whole of the lightning protection system shall have a combined resistance to earth not exceeding 10Ω. Where, due to local conditions, it is difficult to obtain a value of less than 10Ω by the sole use of rod electrodes, alternative or additional methods, e.g. the use of deep bore holes, the installation of tape or plate electrodes complying with Section B6, may be used at the discretion of the Architect.
B7.6.8 Spacer Saddle
Spacer saddles shall be of high quality metal. For copper conductors, brass or gunmetal shall be used.
Non-metallic spacer saddles shall only be used when down conductors are provided with PVC sheathing.
B7.6.9 Connection to Earth Termination
Down conductors and bonding conductors shall be connected to the earth electrode in accordance with Sub-section B6.4. Each connection shall be separately clamped and shall be fitted with a warning notice in accordance with Sub-section A3.3
B7.6.10 Bonding to Other Services
Lightning protection systems shall normally be kept segregated from the metalwork of other services. Minimum isolation clearances as given in BS 6651 shall be maintained. However, in certain circumstances, e.g. where fortuitous contact with other systems cannot be avoided, earth electrodes for lightning protection may, at the discretion of the Architect, be bonded to other such services including the metal sheath and armouring of low voltage cables, the consumer's low voltage earth system, metal water pipes and steel reinforcement for the foundation and structure. Cables and earthing systems belonging to the Supply Authorities, the Telephone Company and other authorities shall not be bonded except with the written permission of such Authorities. Gas pipes shall not normally be connected to any earth electrode.
B7.6.11 Bond
Bonds for use in connecting the lightning protection system of other metal work shall be of soft annealed copper strip of 25 x 3mm. Bonds connecting movable items to the lightning protection system shall be of flexible copper strand 475/0.5mm diameter or equivalent. Connections to other metalwork shall be of negligible resistance, metal to metal and mechanically sound with non-ferrous nuts, bolts and washers using clamps where necessary.
Section B7
B7.6.12 Joint in Conductors
Joints in roof conductors and/or down conductors shall be made by double riveting or by suitable tape clamps. When clamps are used, the tape clamps shall each be provided with at least 4 screws or bolts. All such joints shall be tinned and shall have negligible resistance and good mechanical strength.
Alternatively, connectors may be jointed together by means of exothermic or thermal welding utilizing the high temperature reaction of powdered copper oxide provided that the proper material and equipment are used in accordance with the manufacturer's recommended process.
B7.6.13 Testing
On completion of the installation, the lightning protection system shall be tested for continuity between air terminations and earthing terminations and the resistance shall be recorded. The earthing resistance of each earthing electrode shall also be tested and recorded. The tests shall be carried out in accordance with Part D of this Specification and shall be submitted to the Architect for approval. All instruments used for testing shall be properly calibrated.