PLATTLINE Zinc Ribbon Anodes
Earthline™ Grounding Rods for Ground Grids
Earthline Grounding rod is selected over Copper when the application requires a conductor that will not only be able to withstand high electric fault currents for a short period but also provide the ability to not interfere with the existing cathodic protection system. When a dissimilar metal event arises between an existing copper ground grid and a new installation it necessitates an impractical upgrade of the cathodic protection system to protect the existing underground pipelines and other structures. Earthline’s zinc-coated steel cored product was developed to resolve this dissimilar metal conflict because the zinc layer produces less impact on the DC facility cathodic protection system than bare copper. The steel conductor core was selected to carry the local utility’s requirement of line-to-ground fault current and installation of the Earthline system is a great deal more cost effective than upgrading a complete existing ground grid.
Installation applications that have used Earthline with great success include a 138 kV electrical substation, power control buildings, isolation transformers, compressor buildings, suction header and scrubber areas and tank farms. On one of the first installations of the Earthline product the owner/utility’s 138 kV electric transmission system included an overhead ground wire system from the substation back to the sources of power to this facility. A majority of the fault current flows back to the sources via “remote earth” through the grounding system.
The initial design for the substation ground grid was bare copper wire and the initial design for the grounding system of the power control building, compressor building, suction header and scrubber area, tank farm was an insulated copper wire. The bare wire and insulated copper wire were substituted with the zinc coated steel conductor because the insulated conductors did not allow an adequate path for the fault current to return to the power system through remote earth which left a touch voltage problem in most parts of the facility.
The material selection used in the grounding system design depends primarily on the following factors:
- Fusing characteristics & current carrying capabilities
- Conductor resistance
- Mechanical Strength
- Cost of conductor material
Copper is by far the most common metal used as ground grind conductors. There are four reasons why copper has been used primarily as ground grid conductor:
- Familiarity of electrical characteristics of copper
- Conductivity making it suitable for installations with high fault currents
- Good mechanical strength
- Freedom from underground corrosion. Grid integrity will not be compromised, if conductors are adequately sized and not subject to any mechanical injury.
One disadvantages in using copper that may override the benefits in some situations is that it forms a “Galvanic Cell” with buried steel pipes, conduits and rebar in the vicinity and corrodes the steel. In the event of short circuits (faults) and transient phenomena (lighting and switching operations), a safe grounding practice has two major objectives, which are Personnel Safety and Equipment Protection.
That said, there are the three major considerations in the design of grounding grid systems:
- The grid must be able to withstand the maximum ground fault current without the danger of burn-off or melting.
- The grid must produce sufficiently low voltage between any two points on the ground to prevent all personnel hazard. This takes into account the acceptable limits of “Step, Touch and Mesh Potentials.
- The grid must minimize the “Ground Potential Rise with respect to remote ground (or zero potential point) by having a low contact resistance to ground (commonly referred as “Ground Resistance”).
In addition to these three starting points in the development of the ground grid design there are five other parameters to be considered:
- Soil Resistivity (most predominant factor)
- Tolerable Body Current (determines allowable “Step” and “Touch” potentials)
- Power System Network Configuration (determines the “Current Division Factor” and the actual amount of current flowing into the ground)
- Single-Line-to Ground Fault Current magnitude at the station and the X/R ratio
- Grid Geometry (determines the “Mesh, Step and Touch” voltage)
In assessing which conductor material and what conductor size or what maximum allowable temperature limit need to be applied in individual design situations; the final choice should always reflect the following considerations. Each element of a grounding system, including grid conductors, joints, connecting leads, and all primary grounding electrodes, should be so designed that for the expected design life of the installation, the element will:
- Have sufficient, conductivity, so that it will not contribute substantially to local voltage differences
- Resist fusing and mechanical deterioration under the most adverse combination of a fault current magnitude and duration
- Be mechanically reliable and rugged to a high degree, especially on locations exposed to corrosion or physical abuse.
The presence of a grid of copper or copper-clad steel forms a galvanic cell with buried steel structures, pipes, and any of the lead-based alloys that might be present in the cable sheaths, it is also likely to hasten the corrosion of the latter. Tinning of copper has been tried by some utilities: that reduces the cell potential with respect to steel and zinc by about 50% and practically eliminates this potential with respect to lead (tin being only slightly sacrificial to lead). The disadvantage of using tinned copper conductor is that it accelerated and concentrates the natural corrosion of the metal in a small area.
Earthline provides the answer when these copper grids and steel structures meet. Earthline is driven into the ground, allowing fusing characteristics & current carrying capabilities, conductor resistance, corrosion and mechanical strength at a cost advantage to upgrading the CP system originally installed.
To help with your projects development we suggest the following publication:
Standard IEEE Std 80-2000, the IEEE GUIDE FOR SAFETY IN AC SUBSTATION GROUNDING, IEEE Power Engineering Society.
Please visit http://standards.ieee.org to purchase your guide and access all other IEEE publications and information.