Ground grid testing

[collies99]

I helped performed testing on a small rectangular sub where the diagonal grid length was measured at 17 metres. The current probe was installed at 9x the diagonal dimension from one of the grid electrodes.

Voltage measurements were done at .05, .1, .2, .3, .4, .44, .48, .52, .56, .58, .60, .62, .64, .68, .70, .74, .80 & .90 of the current probe distance from the reference ground electrode. A second set of voltage measurements were performed at 105 degree compass direction from the first.

I was surprised that the current probe was not relocated to the direction where the second voltage measurements were made.

Is this the normal field practice to leave the current at 1 location ?2

 

[marks1080]

The current probe should be moved during testing. The more directions you test the better your results will be. If your current probes are always in the same place you are only getting information about that section of your ground grid. The more locations your pick for testing the better picture you get of the ground grid. I would suggest using compase points... N, NxNE, E, SSE, S, etc... all the way around the circle.

 

[eamonkerrigan]

Sounds like you are describing two different types of tests, the standard fall of potential and the "90 degree" fall of potential. If you take your voltage measurements at a 90 degree angle from your current probe, then you can avoid having induced currents in your voltage leads. It's important to realize that your plots for these two tests will look completely different. Even if it's not exactly 90 degrees apart, the idea is the same. As long as your lead wires aren't laying next to each other, you can reduce the cross-talk / interference between the two. I agree with marks1080 as well about changing your reference test location to at least a couple of different points in the station. You never know how good those connections actually are.

 

[collies99]

That is correct, the resistance plot along the path where the remote current probe is the standard curve, whereas the 90 degree plot start out similar but plateaus ftom the 50% distance area, the values are slightly less.

Is it customary to only perform the one set of test or should more than one corner ground electrode be tested ?