A power substation requires the earthing system for three main purposes.
- The neutral point earthing for each network which has to be earthed at the power station or substation. In other words, it is mainly the earthing of neutral point of power transformers.
- Earthing of non-current carrying metalwork of different apparatus. For example, transformer tanks, power cable sheaths etc.
- Earthing the other metalwork not associated with the power systems. For example boundary fences, metal gates etc.
The purpose of earthing is to keep any voltage on metal frameworks below dangerous levels during normal or fault conditions. In directly earthed neutral system, it is not always possible to keep metal parts exactly at true earth potential when fault current flows. Therefore, the main goal is to provide low-impedance path to the earth of all the parts that a person may touch at the same time. So, the large fault currents do not flow through the body of the person.
The surge protection devices are connected to earth as directly as possible. High surge currents contain high-frequency components. These currents require earth connections with low resistance and low reactance. Therefore, we use short earth connections with minimum bends and direction changes for lightning arrestor.