MV switchgear and MV cables are to have separate main earthing bar connected to the framework or earth bar of each item by insulated conductor and the earth-electrode by 2 insulated earthing conductors, one at each end of bar, via testing joints. Earthing conductor is to be minimum 150 mm2. It is important to have an earthing or grounding method for transformer substation.

The earthing method for transformer substation interconnects equipment neutrals, equipment housings, lightning masts, surge arresters, overhead ground wires, and metallic structures so that they are in contact with the earth.

LV switchgear is to have separate main earthing bar connected to framework or earth bar of each item by insulated conductor.

Main LV switchgear panel is connected to main LV Earth bar in the electrical room which in tum connected at two ends by 2 insulated earthing conductors through testing joints to two earth pits. Earthing conductor size is to be minimum 150 mm2.

Transformer body earthing terminal is to be connected to MV main earthing bar by insulated copper earthing conductor not less than 20 mm2 per 100 kVA of transformer rating, with a minimum of 35 mm2.

Transformer neutral (star point) is to be connected by an insulated earthing conductor (color black) directly to an independent earthing electrode. The neutral earthing conductor is to be sized for maximum earth fault current for 5 seconds with final conductor temperature not exceeding 160 deg. C or size not less than 30 mm2 per 100 kVA of transformer rating, and with a minimum of 50 mm2.

Lightning arresters are to be directly connected to the earth electrode, following the shortest path. Each lightning arrester is to be connected to a dedicated earth rod.


The grounding system in substations is extremely important. Grounding systems are essential because:-

  • It provide a low impedance path to the ground for currents flowing from faults, lightning rods, surge arresters, gaps, and related devices.
  • Provide the earth’s neutrals with a connection to generators, transformers, capacitors, and reactors.
  • Enhance the relay system’s operation to eliminate ground faults.
  • Enhance the availability and reliability of the electrical system.
  • During maintenance, allow de-energized equipment to be grounded.