Earth Impedance 1

[keith2015]

When the ground subjected to the lightning, it will effect the value of earth impedance to be change. This will cause the ground potential rise to a dangerous level. What method can use to measure the earth impedance due to the lightning surge?

 

[busbar]

No location given. Suggest IEEE or NFPA listings.

 

[Graeme38]

The only way I can think is to measure the voltage gradient at different injected current frequencies into the ground.

I worked for some time on substation earthing systems design, and we used to model the effects on a software computer package called CDEGS. Generally what you find is a very sharp voltage gradient in the ground in the vicinity of the earth rod. The gradient is obviously dependent upon the soil resistivity, but is much sharper than say for an earth fault at power frequency.

In the UK we would not go to such elaborate methods of testing, generally we work to a total impedance of no greater than 10 ohms. The provision of lightning protection to buildings is really to ensure the safe discharge to earth in the event of a strike. You can never guarantee that it will protect life.

 

[Bung]

I'm not sure that the impedance actually changes for practical purposes - the potential rise is due to the voltage drop across the impedance due to the current flow.

Bung

 

[redtrumpet]

See "Westinghouse Electrical Transmission and Distribution Reference Book", Chapter 17, Section 10 "Tower-Footing Resistance" and Section 11 "Lowering Tower-Footing Impedance (Rods and Counterpoise)(1964 edition). Doesn't give a measurement method but has interesting comments regarding impulse values of resistance.

Note that for driven ground rods in soils of low to medium resistivity, the T & D book says the impulse resistance is usually less than the 60 Hertz value. For high ground resistances the impulse resistance is considerably less.