![substation basics substation basics](https://i.ytimg.com/vi/dBPuDz-4cOs/maxresdefault.jpg)
Typical values of surface material resistivity of washed granite is 1.5*10 6Ω when dry and 5000Ω when wet. Rock resistivity needs be analyzed prior to application. Resistivity value of crushed rock depends on many factors such as type of locally available material, size of stone, washed vs un-washed stone (to remove fine material), moisture content, atmospheric contamination, type of water that can make the rocks wet (rain water vs sea water spray) etc. Surface materials also retard evaporation of moisture from the top soil layer.Īdding Surface Material in Electric Substation A common surface material is the #57 crushed granite which has stones mostly in sizes from 3/4 inch to 1 inch is shown in the picture.Īnother advantage of having a layer of crushed rock as surface material is that spacing of ground grid conductors (buried beneath the soil) can possibly be increased (saving copper) to meet the same step and touch voltage criteria.
![substation basics substation basics](https://sites.google.com/site/macnetriotsa/_/rsrc/1344876537688/NetworkBasics/substation.jpg)
This is the basic idea behind using crushed rock in electric substations. Since there is very little current in the layer with crushed rock, the surface voltage (step, touch potential) will be negligible. Since the crushed stone has higher resistivity than surface soil, most of the ground current remains confined in the surface soil below the crushed stone. If we spread a thin layer of crushed stone (gravel) which typically has higher resistivity than most surface soil it will help increase contact resistance (R F) between surface soil and a person’s feet.
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When this person touches a perfectly grounded object (at 0V) a shock hazard exists. A person standing on the substation surface will also attain this elevated potential. Under a fault condition, surface potential of substation is raised relative to remote earth (0V). Touch Voltage: The potential difference between the ground potential rise (GPR) and the surface potential at the point where a person is standing while at the same time having a hand in contact with a grounded structure. IEEE Std 80 defines step voltage as “ The difference in surface potential experienced by a person bridging a distance of 1m with the feet without contacting any grounded object”. This voltage is known as step voltage or step potential. This causes a current to flow from one feet-through his body- to other feet. Step Voltage: During a fault condition, a person walking on the surface of substation can experience a voltage potential between his two feet (typically assumed to be 3.3ft or 1m apart). Application of crushed rock is a means of controlling step voltage and touch voltage. This is known as Ground Potential Rise (GPR) which is directly tied to two parameters: Step Voltage and Touch Voltage. Flow of ground current over substation ground elevates its electric potential relative to distant grounding point assumed to be potential of remote earth. When current is injected in to substation ground during a fault, it tends to divide and flow depending on the resistivity of various soil layers.