Resistivity Test

[tin2779]

A contractor submitted a test proposal and added a small note:

"Please see that if GPR as well as the step and touch potentials cannot be achieved as required, additional on-site testing will be required and the possibility of having
a remote ground grid may have to be considered,".

I am just wondering that remoted grid will only help in reducing GPR but not in step and touch potential. Is this correct ?

Thanks

 

[jghrist]

Step- and touch-potential are proportional to GPR for a given grid configuration. Some of the fault current will return through the remote ground grid, lowering all the voltages. It is also possible that there may be step- and touch-potential problems at the remote ground grid.

 

[tin2779]

Thanks. So, technially it is a parallel combination of two resistances leading to a reduced resistance in total.

So, Earlier if one grid had Rg1= 5 ohm and now Rg2 the remote grid is also 5 ohms, the net resistance will be 2.5 ohms. So if on a conservative basis, if my fault current is 100 A, the GPR will be 250 V instead of 500 V.

So touch and step potential will be effected in the same lines as GPR.
I hope I got it right ?

Regards,

 

[jghrist]

It's not that simple. There is a mutual resistance between the grids so it is not a straight parallel combination. If the grids are far enough away, they would act as in parallel, but you would have to consider the impedance of the connecting wires, and the current that would go into the earth from the connecting wires (assuming they are bare).

Also see thread238-189113