current split factor in ground grid design

[JLuc]

Hi,

according to IEEE 80-2000, the current split factor will decrease when the earth grid resistance increase -> more current will go in the transmission line neutral and less in the earth grid.(current split factor = Igrid/3Io)

which give us a lower touch potential.

what I understand from that is that a low grid resistance will give higher grid current and thus higher touch potential.

so there is no point of looking for a grid resistance as low as possible.

is my understanding ok?

thanks

JL

 

[jghrist]

Usually, the things that reduce the grid resistance (like smaller meshes, more wire, more ground rods, lower resistivity, etc.) also reduce the touch potential for a given grid current. So, even if the current split factor is increased, the touch potential will be reduced, just not as much as it would have without the increased split factor.

Lower grid resistance can also be important by itself if there are communication lines that are sensitive to GPR. Lightning protection can also be better with lower grid resistance, but this is not easy to quantify.

 

[cuky2000]

This is a compromise with two conflicting factors:

a) Low resistance: Most utilities in the US require a minimum grid resistance of 1 Ohm or less. As low is the grid resistance more current is injected to the ground.

http://cuky2000.250free.com/Gnd_Current_Division_Factor.jpg

b) Touch & step potentials: Actual ground voltage rise should be less than allowable voltage. The first factor is a function of the size of the grid, number of ground rods and other grounding path resistances. The second factor do not varies with the conductor instead it is a function mainly of the soil resistivity, the surfacing layer resistance and the fault clearing time.