Ground Enhancement Materials (GEM) Information
Ground Enhancement Materials (GEM) are used in areas with poor ground conductivity to reduce electrical resistivity. They are also called conductive backfills, ground soil additives, ground enhancement backfills, earth bonding materials, earth bond enhancement compound, soil conductivity amendments, earthing compounds, conductive concrete, conductive cement, grounding fills, and chemical earthing.
Electrical grounding is required to supply a ground for an electrical distribution system or to dissipate electrical discharges from lightning strikes or electrical faults. The resistivity or conductivity of the soil or earth surrounding the electrical ground rods must be sufficient to complete the circuit and provide a low resistance ground. In applications with high resistivity or low conductivity soil, additional ground conductors and ground enhancement materials are used to complete the circuit between ground rods, ground wires, ground grids, ground plates, ground mesh, lightning rods, air terminals, or corrosion protection anodes the surrounding soil or earth. Ground enhancement material (GEM) could be used directly around a metal housing or electrical enclosure to create many paths to ground.
Ground enhancement materials should be compatible with the grounding electrode alloy or materials (ground rod, mesh, ring, etc.) and environmentally friendly. Electrode compatibility means the GEM and the ground electrode alloy should not have potential favoring galvanic corrosion. The ground enhancement material should maintain environmentally friendliness by not containing any toxic agents and by not leaching out into the surrounding soil.
Some soil and earth compositions do not have the conductivity to complete the circuit and act like insulators. For instance, sandy soil or overburden in a very dry or low moisture dessert environments, mountain tops, and rocky ground can have good dielectric properties or low conductivity and insulate the rod from the ground. In addition to the type of soil, the temperature and water or moisture content also impacts soil resistivity because the electrolytes or ionic content also impacts soil resistivity because the electrolytes or ionic conductors carry the current in soils.
Location of 1D Earth Resistivity Models with respect to Physiographic Regions of the USA. Image: USGS
The USGS has created a Regional Conductivity Map, which shows the resistivity across the surface of the United States. Soil consisting of silt, clay or till have lower resistivity and higher conductivity. For instance, soil or overburden resistivity in the St. Lawrence Lowlands region was 4 ohm-m (see http://geomag.usgs.gov/conductivity/SL-1/ ). Shale, sandstone, limestone, magmatic rock and crystalline rock have higher resistivity or lower conductivity. The Piedmont formation has resistivity near the surface approaching 1000 ohm-m (see http://geomag.usgs.gov/conductivity/PT-1/ ). USGS maps shows the variations in soil resistivity with depth. The depth of placement of grounding electrodes can impact ground resistance of the grounding system.