podobing
Electrical
- Jan 28, 2013
- 49
As is required by underground coal mining laws in the US, any three phase power that goes underground must be resistance grounded to a safety ground bed located at least 25 feet away from the substation ground bed. The typical value of the grounding resistor is either 15 or 25 amperes. The voltage can be 7200 vac or 12470 vac. I am not saying that I agree with the concept of separation or the distance of 25 feet, but that is the law. Some of the areas where these substations are located have relatively high resistivity soil - 1000 ohm-feet. Our objective is to get a ground bed resistance of 2 ohms or less.
Currently, when constructing ground beds, we use 8 foot copper ground rods (typically on 20 foot spacing) and tie these rods together with 4/0 stranded copper wire - 18 inches below the surface. All connections are exothermically welded. My concern is the amount of real estate that we will need to meet the 2 ohm objective in these soil conditions. My question is - Has anyone had any luck with soil treatments or chemically treated grounding rods to establish a low resistance bed when surface area is limited?
Dave
Currently, when constructing ground beds, we use 8 foot copper ground rods (typically on 20 foot spacing) and tie these rods together with 4/0 stranded copper wire - 18 inches below the surface. All connections are exothermically welded. My concern is the amount of real estate that we will need to meet the 2 ohm objective in these soil conditions. My question is - Has anyone had any luck with soil treatments or chemically treated grounding rods to establish a low resistance bed when surface area is limited?
Dave