When a lightning wave hits a tower or lightning conductor, most of the lightning current will flow into the ground through the grounding device. If the grounding resistance is too high, the lightning current will generate a very high voltage on the top of the tower, causing the insulator to flash over and cause the line to trip. 

In recent years, power outages caused by lightning strikes have occurred successively in many areas of our country, most of which are caused by too high grounding resistance. Therefore, accurate assessment of grounding resistance of grounding devices has important guiding significance for rationally designing the type of line tower grounding body and reducing lightning damage. 

The evaluation index of grounding body usually adopts grounding resistance, which can be divided into power frequency grounding resistance and impulse grounding resistance. Now many engineering inspectors simply judge the power frequency grounding resistance as the impulse grounding resistance. This approach is wrong and will cause some big problems. The two grounding resistors correspond to different application scenarios and should not be confused. When an impulse current such as lightning flows to the earth through a grounding device, the resistance presented by the grounding device is called impulse grounding resistance, which is usually called impulse grounding resistance.

Impulse grounding resistance is the ratio between the amplitude of the impulse voltage waveform of the grounding body to the ground and the amplitude of the impulse current waveform. In order to make the impulse ground resistance have a clearer meaning, the ratio of the maximum value of the impulse response voltage of the grounding body to the maximum value of the impulse current is taken as the impulse ground resistance. 

The ratio between the impulse grounding resistance and the power frequency grounding resistance is called the impulse coefficient. However, it is found in actual measurement that when the impact coefficient and power frequency grounding resistance are used to obtain the value of the impact grounding resistance, the larger the grounding body, the greater the error. 

For the long and narrow distribution of large grounding bodies such as urban rail transit lines, the shape of the grounding body of the rail line is very different from the shape of the current common ground network or grounding body, and the existing relevant impact coefficient calculation curve cannot be applied to urban rail transit lines. The grounding body model should be used as far as possible to obtain the impulse grounding resistance by targeted actual measurement methods.