Updated: May 6, 2021
The metallic enclosure,frame of an electrical equipment is connected to earth. The earth has zero potential. In normal condition the metallic enclosure,frame and support remains at zero potential and no current flows through it. However in case of insulation failure the frame,enclosure or support may attain the live potential. To safeguard the equipment and people, the metallic enclosure,frame and support is connected to earth pit through a GI wire or GI strip.
The main purpose of earthing is to ensure protection of the equipment,people and to minimize the risk of fire.
The live part which is at zero potential in normal condition is connected to earth is known as as grounding or system grounding. During fault, the neutral point's potential may reach equal to the phase voltage. In this condition, the healthy phase voltage increase up to 1.73 times of the phase voltage and this situation may cause insulation failure of the equipment. This is condition similar to the floating ground. In order to protect the equipment neutral point of the power supply system system is connected to earth.
A current transformer is mounted in the neutral to earth circuit to trip the breaker to avoid damaging of the electrical equipment. Thus, the neutral grounding provides safeguarding to electrical equipment and the power system.
When three phase are connected in star, neutral point is formed where all the three phases are joined at one point. The neutral point voltage is zero when the three phase current is balanced. In case of unbalanced phase current,voltage is developed at the neutral point. The neutral point is earthed to enhance the reliability of the power supply system. The neutral point also serve the supply source of single phase load. If the phase to phase voltage is V volts, then phase to neutral voltage is 58 % of the phase to phase voltage. 220 Volts supply can be derived from 440 volts three phase system.