LIGHTNING PROTECTION OF BURIED TOLL CABLE 281 



^ 



after the return stroke is initiated, however, is the potential large enough so 

 that arcing will occur for appreciable distances. 



When a stroke current / enters the ground, the electric force in the ground 

 at the distance r from the stroke point is given by 



E{r) = Jp/lirr (71) 



p being the earth resistivity. If the breakdown voltage gradient of the soil 

 is Cq , breakdown of the soil will take place until E{r) = ^o , or for a distance: 



ro = (/p/27reo)^ • (72) 



For a distance ro around the stroke point the soil may then be regarded as a 

 conductor of negligible resistivity. 



The resistance encountered by the lightning channel in the ground is then 



P 



\27rjJ 



27rro ^^^' 



Measurements of the surge characteristics of grounds of fairly small 

 dimensions'^' "^ indicate that the breakdown voltage of the soil may vary 

 from roughly 1000 volts per cm to some 5000 volts/cm. The data are, 

 however, quite limited and there is no assurance that the above values 

 represent the limits. In the first of the papers referred to, measurement 

 was made of the resistance encountered when a current of 10,000 amperes 

 crest value was discharged into the ground from an electrode suspended 

 above the ground. The measured resistance is in satisfactory agreement 

 with that obtained from (73) using the earth resistivity and breakdown 

 voltage determined from other measurements in the paper referred to. In 

 connection with the present study some small scale measurements were 

 made of the breakdown voltage of sand between plane electrodes, and of the 

 resistance encountered in discharges into the sand over point electrodes. 

 These measurements indicated a breakdown voltage of 5000 volts per cm 

 for dry sand having a resistivity of 3700 meter-ohms and 2400 volts/cm 

 when sufficient water was added to reduce the resistivity to 100 meter- 

 ohms. The measured resistance of point electrodes was a satisfactory 

 agreement with that calculated from (73) on the basis of the measured re- 

 sistivities and breakdown voltages. These experiments were made with 

 currents having crest values from 1 to 50 amperes. 



Measurements of the breakdown voltage of various types of soil between 

 completely buried spherical electrodes indicate substantially higher voltages 

 than those given above, from about 11 to 23 kv/cm.-- It is possible that 

 for surface electrodes, breakdown at the lower voltage gradients occurs 

 along the surface of the ground, so as to form a conducting plane of radius 



