A good earth connection should possess the following characteristics:
- Low electrical resistance between the electrode and the earth. The lower the earth electrode resistance the more likely the lightning current will choose to flow down that path in preference to any other, allowing the current to be conducted safely to and dissipated in the earth
- Good corrosion resistance. The choice of material for the earth electrode and its connections is of vital importance. It will be buried in soil for many years so has to be totally dependable
The standard advocates a low earthing resistance requirement and points out that the earthing system should have an overall resistance to earth path of 10 ohms or less. Three basic earth electrode arrangements are used:
- Type A arrangement
- Type B arrangement
- Foundation earth electrodes
Type A arrangement
This consists of horizontal or vertical earth electrodes, connected to each down conductor fixed on the outside of the structure.
Type B arrangement
This arrangement is essentially a fully connected ring earth electrode that is sited around the periphery of the structure and is in contact with the surrounding soil for a minimum 80% of its total length (i.e. 20% of its overall length may be housed in say the basement of the structure and not in direct contact with the earth).
Foundation earth electrodes
This is essentially a type B earthing arrangement. It comprises conductors that are installed in the concrete foundation of the structure. If any additional lengths of electrodes are required they need to meet the same criteria as those for type B arrangement. Foundation earth electrodes can be used to augment the steel reinforcing foundation mesh.
Overview & Usage
The earth termination system is vital for the dispersion of lightning current safely and effectively into the ground. The standard recommends a single integrated earth termination system for a structure, combining lightning protection, power, and telecommunication systems. The agreement of the operating authority or owner of the relevant systems should be obtained prior to any bonding taking place.