Substation Grounding Crushed Rock

[RSChinn]

We have been using 3000 ohm-meters for our crushed rock values as a typical from examples in IEEE80. However, we did tests of our crushed rock and came up with about 9600 ohm-meters. Our tested crushed rock is 1-1/4" in size and we wetted it prior to taking measurements. Has anyone else did some empirical testing of their substation rock? Values vary with type of rock, of course, but is 9600 ohm-meters what others have used as their values... or are we way off base?

 

[dpc]

I would suspect your measurement techniques may need to be reviewed. The interface between the test leads and the material under test is probably critical to the measurement.

I'm sure the IEEE 80 values are conservative, so I could believe a higher resistivity measurement. But I would not want to take the responsibility for using something greater than recommended in IEEE 80.