A well designed earthing system is essential to ensure protection of personnel and equipment from danger associated with fault currents or lightning within buildings or areas falling within their 'zone of protection'.
Earthing system design considerations
To achieve an efficient earthing system it is essential that a low electrical resistance to earth is achieved, using good quality conductors with sufficient cross sectional area to carry the expected current flow. The conductors must also have a high corrosion resistance and must meet internationally recognised European and British
earthing standards.
The information contained in this section primarily refers to Lightning Protection Earthing Systems (LPS) and does not detail all earthing requirements for electrical wiring, although there are some similarities. Please refer to the lEE 16th edition wiring regulations for more details.
Other factors to consider when designing an earthing system are:
Earth terminations & networks
An earth electrode should be connected to each down conductor of a lightning
protection system (LPS). Earth rods need an earth inspection housing for periodic testing of earth resistance.
Earth rods are used in most applications and are driven into the ground as close as is practicable to the structure and the down conductor. They are normally spaced at specified intervals corresponding to the spacing on the down conductors.
Resistance to earth
For an LPS the earth termination network as a whole should have a combined resistance of not more than ten ohms, before bonding to building metalwork.
However, the maximum resistance value for earthing systems is application specific.
A single earth rod may not achieve the required resistance figure and several may be need to be fitted to achieve this; their combined resistance is proportional to the reciprocal of the individual rod resistances to earth. This rule holds true as long as each rod is situated outside the resistance area of any other. To ensure this is the case, it is generally accepted that the minimum spacing between rods should not be less than their driven length.
The expected number of rods required to obtain a particular resistance value, e.g. ten ohms, can be roughly calculated. To do this the soil resistivity needs to be taken into consideration. A soil resistivity test will need to be performed.
There are several methods used to obtain a lower resistance value:
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