Recommendations for earthing are given in BS 7430: 1991. This document covers the earthing of equipment and the principles of earthing for protection against lightning. All equipment metalwork must be bonded together and connected to the electricity supply earth point, the main earth terminal. In addition a connection must be made to the earthing system provided for protection against the effects of lightning.
Antenna systems by their nature are vulnerable to lightning strikes. Nearby taller structures may reduce the risk but precautions must still be taken. The zone of protection – a cone with its apex at the tip of the protecting structure and its base on the ground forming an angle of 45æ to the perpendicular – does not necessarily protect structures above 20 m high. BS 6651: 1985 is the Code of Practice for the Protection of Structures against Lightning. Lightning protection begins at the top of the mast or tower where, ideally, the highest item should be a finial bonded to the tower. However, a finial mounted alongside an omnidirectional antenna will affect is radiation pattern. Grounded antennas are preferred and the outer conductor of each feeder cable must be bonded to the tower at the top and bottom of its vertical run. Grounding kits for the purpose are obtainable. The feeder cables must also be bonded to an earthing bar at their point of entry to the equipment building. The codes of practice should be consulted regarding the routes to be taken by earthing tape and the methods of jointing. An important point is that neither very sharp bends nor ‘U’ bends are created.
The legs of the tower must be bonded together with earthing tape and each leg connected to an earthing system which may consist of a buried plate or rods. Several rods may be necessary for each subsystem to attain the specified earth resistance of not more than 10 ohms. The rods should, except in rock, be driven into ground which is not made up or backfilled or likely to dry out. Where several rods are necessary to achieve the specified earth resistance these should be spaced well apart, the reduction in earth resistance being small with parallel rods closely spaced. Joints to facilitate testing of each subsystem separately must be provided either above ground or in a purpose-built pit or chamber. Figure 19.1 shows a typical earthing system.
Lightning finial
bonded to tower
Feeder cable outer conductor
bonded to tower at lop and
bottom of vertical run via
feeder grounding kit
Feeder Feeder cable outer conductor
bonded to earth bar via feeder
grounding kit Earth bar below
feeder entry
Independent earthing functions, e.g. DC power, AC power, etc.
Each tower leg bonded via sacrificial earth lug to tower earth ring Tower and building
earth rings interconnected (driven electrode system shown)
The system should be tested regularly, at least annually, and the results recorded in a lightning protection system log book. The test comprises:
1. Measurement of the resistance to earth of each termination network
and each earth electrode.
2. Visual inspection of all conductors, bonds and joints or their mea
sured electrical continuity.
BS 7430 recommends the method of testing.
Other vulnerable systems are overhead cables for either the elec
tricity supply or telephone and control purposes.