Substation Grounding Resistance

[Nyceguy]

I am trying to see if anyone knows if vegetation in a substation, (i.e. weeds, grass, moss, any type of organic matter on top of a gravel layer), would decrease the resistivity of the gravel if someone was standing on top of the vegetation. I am trying to figure out if it is a safety hazard for a worker to be standing on vegetation in a substation in the event of a fault to ground. Is there a formula out there that says for so much square foot of area of vegetation, resistivity of the gravel goes down by so much? Also if there is vegetation growing on top of, or through, the gravel doesn't that mean that the vegetation is creating a kind of short around the gravel to the lower resistance soil below?

Thank you for any insight!

 

[jcormack]

Low is good

Low is good

In a substation you actually want the resistivity to be as LOW as possible. Ground grids are placed under the gravel to provide a lower step potential between the areas where your left & right foot are placed and between your body/feet and any station equipment you may come in contact with. Lower resistivity is GOOD. My Guess (IMHO) is that grassy vegatation would not present a major problem - woody growths would be higher imp. Gravel is used to keep the area clean, drain well, and be non-muddy. You should keep vegetation down to provide easy egress & not allow any plants to grow into the equipment and lines.

 

[jghrist]

The gravel provides an insulating layer and increases the allowable step- and touch-voltage. The low soil resistivity is needed below the gravel layer. I would not consider any insulating value for gravel that has any vegetation.

 

[Nyceguy]

So it sounds like having grassy vegetation in a substation, not climbing on anything, would not pose any problems and would not lower the insulation properties of the gravel, but having woody, small trees, could pose a problem if they were being touched. In other words, if a substation had an approved ground grid, someone inside the substation would not have any increased risk standing on a grassy area?

 

[jghrist]

So it sounds like having grassy vegetation in a substation, not climbing on anything, would not pose any problems and would not lower the insulation properties of the gravel, but having woody, small trees, could pose a problem if they were being touched. In other words, if a substation had an approved ground grid, someone inside the substation would not have any increased risk standing on a grassy area?

This is certainly not what I said. If the step- and touch-voltage analysis of the station assumed a crushed rock insulating layer, then grassy vegetation will make the analysis invalid and someone standing on a grassy area will have increased risk.

 

[richxtlc]

This is certainly not what I said. If the step- and touch-voltage analysis of the station assumed a crushed rock insulating layer, then grassy vegetation will make the analysis invalid and someone standing on a grassy area will have increased risk.

I agree that the gravel provides a reduction in the step/touch potential and any vegetation would to some degree reduce this if the person were standing on it.
Some people that I know think the gravel is just to keep the weeds down. It is a design factor in the safety to the station personnel.

 

[Nyceguy]

Is there a formula that realates how mush grassy area on top of gravel to percentage reduction in insulation level? Or is just a rule of thinking that it seems like the insulation level of gravel would be reduced (no direct calculation)?

 

[topgone]

Is there a formula that realates how mush grassy area on top of gravel to percentage reduction in insulation level? Or is just a rule of thinking that it seems like the insulation level of gravel would be reduced (no direct calculation)?

If the original specs called for "gravel" top layer, anything less or more than the specified material will have to be corrected, IMHO. From my experience, we keep our substations free of any kind of weed/growth. We have set schedules on when we scoop out the gravel and remove unwanted soil that may have been deposited over the gravel layer so no weed can grow on. My former mentor told us to KISS it (keep it stupidly simple), no math involved!

 

[rcwilson]

Most utilities and knowledgeable industrial facilites do not allow any vegetation on switchyard gravel. Vegetation can short out the gravel insulating layer that the grid designs depend on to keep the amount of current flowing through a worker's body during a fault below lethal values.

I am not aware of any rule of thumb, calculation, or other method to determine how much or little grass on the gravel is "safe".