7.4 Over Voltage Protection and Grounding Device
7.4.1 Over-voltage protection against direct lightning stroke shall be installed for outdoor switchgear installation and open-air oil tank，which may be lightning rods or overhead grounding wires.
7.4.2 Lightning rods may be installed on the frames of high-voltage switchgear of 110kV or above，but shall not be installed on the frames of high-voltage switchgear of 35kV and below，nor on the transformer gantry frames.
7.4.3 In the electrical distribution network of neutral point solidly earthed system below 1kV，grounding protection shall be applied to the metal casing of electric equipment.
7.4.4 In the design of grounding devices，artificial earthed networks shall be set up while the following natural earthed bodies shall be used:
1 The first layer of steel bars in the reinforced concrete hydraulic structures that are in contact with water all year round.
2 Embedded metal parts of penstocks，gates and trash racks.
3 Metal bodies left underground or submerged in water.(https://www.daowen.com)
7.4.5 The natural earthed bodies and the artificial earthed networks shall be connected with at least two connection points，and earth resistance measuring wells shall be installed at the connection points.
7.4.6 In a small earthing short-circuit current system，the earth resistance of electric equipment shall not be greater than 4Ω，which shall not be greater than 1Ωfor the monitoring system.In a big earthing shortcircuit current system，the earth resistance of electric equipment shall be in compliance with the formula below:
Where:R——the maximum earth resistance with consideration of seasonal changes（Ω），
I——the earthing short-circuit current that passes through grounding devices（A）.
Independent grounding devices should be installed for independent lightning rods（or overhead grounding wires），while its power-frequency earth resistance should not exceed 10Ω.In high earth resistivity regions，the lightning rods may be connected with the main earthed network，and the length of connecting wire underground shall not be less than 15m.
