BS 6651:1990 pdf free.British Standard Code of practice for Protection of structures against lightning.
BS 6651 For the purposes of lightning protection, the thermal effectof a lightning discharge is confined to the temperature rise
of the conductor through which the current passes.Althouahthe current is high, its duration is short and the thermal effect on the protection system is usually negligibte.
NOTE.This ignores the fusing or welding effects on damaged conductors or those which were not adequate in the initial installation.ln general, the cross sectional area of a lightning conductoris chosen primarily to satisfy the requirements of
mechanical strength, which means that it is large enough tokeep the rise in temperature to 1°c.For example, with acopper conductor of 50 mm2 cross section, a severe strokeof 100 kA with a duration of 100 us dissipates less than 400 J per metre of conductor, resulting in a temperature riseof about 1°C.The substitution of steel for copper results in a rise of less than 10 °c.
Where a high current is discharged along parallel conductorsin close proximity or along a single conductor with sharp
bends, considerable mechanical forces are produced.Securemechanical fittings are therefore essential (see figure 2
and table 1).
Adifferent mechanical effect exerted by a lightning flash is due to the sudden rise of 30 000 K in the air temperatureand the resulting explosive expansion of the adjacent air in the channel along which the charge is propagated.This isbecause, when the conductivity of the metal is replaced by that of an arc path, the energy increases about one hundredfold. A peak power of about 100 MW/m can be attained in the return stroke and the shock wave close to this strokereadily dislodges tiles from a roof.
Similarly, with a side-flash inside the building, the shockwave can result in damage to the building fabric.Table 2 gives some recommendations for materials to beused in the manufacture of the component parts oflightning protection systems. Characteristics for some ofthese materials are given in table 3.PSA Standard Specification (M and E) No.1 includessections covering the components and installation ofearthing and lightning protection systems.
Copper and aluminium are recommended for installationsrequired to have a long life.If there is any difficulty in theuse of copper or aluminium, galvanized steel of the samecross section as that recommended for copper may be usedand this material may be preferred for some short life installations,e.g.exhibitions.BS 6651 pdf download.