Copper is the most used material in grounding grids for its conductivity and corrosion resistance; copper-clad steel is used where conductor theft occurs; aluminum is not recommended buried, and steel is kept for soils harmful to copper. A copper grid forms a galvanic cell with buried steel structures and lead-sheathed cables. Connections must meet the same requirements as the conductor.
Short answer
In a grounding grid, copper is the most used material for its high conductivity and its resistance to underground corrosion. Copper-clad steel is used mainly where conductor theft is a problem. Aluminum is hardly used and is not recommended buried. Steel or stainless steel are kept for soils harmful to copper, paying attention to their own corrosion. Connections, buried or not, must meet the same requirements as the conductor and are evaluated with IEEE Std 837. Field installation is described in grounding grid construction.
What the material must meet
A well-chosen conductor keeps the grid intact for years if it has the right cross-section and the soil is not corrosive to that material. The standard requires every element to have enough conductivity, resist fusing and mechanical deterioration, be robust, and keep its function even if it corrodes or is damaged.
The materials, one by one
| Material | Use in grids | Why |
|---|---|---|
| Copper | Very common | High conductivity; it is cathodic to almost all metals buried nearby, so it resists most underground corrosion |
| Copper-clad or copper-bonded steel | Common, especially where theft occurs | Steel core with a copper layer; the standard cites it where conductor theft is a problem |
| Aluminum | Rarely | It can corrode in certain soils, the corroded layer does not conduct, and it is anodic to steel |
| Steel or stainless steel | Soils harmful to copper | Its corrosion needs attention; galvanized or stainless steel with cathodic protection is typical |
Aluminum: why it is ruled out
At first sight, aluminum seems natural in a gas-insulated substation with aluminum enclosures. The standard lists its drawbacks: it can corrode in certain soils with a nonconductive layer, and gradual corrosion from alternating current can be a problem. It does not recommend using buried aluminum conductor, although, like steel, it would ease the problem of accelerating the corrosion of other buried objects.
Galvanic corrosion
A copper or copper-clad steel grid forms a galvanic cell with buried steel structures, pipes and the lead alloys of cable sheaths. That cell can accelerate the corrosion of those elements.
Some utilities have tried tinned copper: it reduces the cell potential against steel and zinc by about 50 % and almost eliminates it against lead. Its drawback is that it accelerates and concentrates the natural corrosion of copper at any small uncoated spot.
Other usual methods:
- Insulate with a coating the surfaces of the metal that is sacrificed, such as plastic tape, asphalt compound or both.
- Cross pipes at right angles with the copper conductor, and apply an insulating coating to one of the metals where they come close; normally the pipe is coated.
- Use cathodic protection, with sacrificial anodes or impressed current.
- Use nonmetallic pipes and conduits.
In a gas-insulated substation, cathodic protection may also be required for external installations, such as cables in pressurized pipe or lead-sheathed cables. Because of the complexity of those installations, the standard asks that all aspects of corrosion prevention be considered before designing the grounding. Underground corrosion and cathodic protection are broad subjects that the standard does not develop.
Metal pairs at a glance
| Grid metal | Against what it behaves | Effect |
|---|---|---|
| Copper | Buried steel, pipes and lead sheaths | It is the noble metal of the pair: the others corrode faster |
| Copper | Almost all metals buried nearby | Resists underground corrosion by being cathodic |
| Aluminum | Steel and many other metals | It is anodic: it sacrifices itself to protect the other metal |
| Galvanized or stainless steel | Soils harmful to copper | Accepted with cathodic protection and monitoring of its own corrosion |
That is why the material is not chosen by conductivity alone: what is buried around the grid and what soil surrounds it are also considered.
Connections
All connections of the grounding network, above and below ground, must meet the same general requirements as the conductor: conductivity, corrosion resistance, current capacity and mechanical strength. They must be massive enough that their temperature rises less than that of the conductor, withstand the electromagnetic forces of the maximum expected fault current and resist corrosion over the expected life.
IEEE Std 837 brings together the application and testing of permanent connections for substation grounding. Connections that comply satisfy the four criteria.
What this means for the design
- Material and size are chosen together: a more resistive material needs more cross-section.
- Soil corrosion and the proximity of pipes or metal-sheathed cables can change the choice.
- When the soil promotes corrosion, a larger size may be advisable even if the conductor and connection have passed the tests.
- Connections are chosen with standardized tests, not by habit.
Material and connection practices for distribution substations, under the local code, are in grounding of distribution substations.
How we do it at Tiza Group
Choosing material and connections requires knowing the soil, the nearby buried structures and the fault current. At Tiza Group we include it in the grid design of medium, high and extra-high voltage substations and industrial plants. Learn about our engineering design and studies service or write to us. We deliver it remotely, worldwide.
Related guides
- IEEE 80: Safety in Substation Grounding
- Grounding Grid Conductor Size: Thermal Capacity
- GIS Substation Grounding per IEEE 80
- Industrial Grounding: Design, Testing and Codes
- Grounding of Distribution Substations: Design and Tests
References
- IEEE Std 80-2013, IEEE Guide for Safety in AC Substation Grounding, clauses 11.1, 11.2 and 11.4.
- IEEE Std 837, Standard for Qualifying Permanent Connections Used in Substation Grounding.
Frequently asked questions
Why isn't buried aluminum recommended in a grounding grid?
Because it can corrode in certain soils, and the corroded aluminum layer does not conduct for practical grounding purposes. It can also suffer gradual corrosion from alternating currents. In addition it is anodic to many metals: if joined to steel in the presence of an electrolyte, it sacrifices itself to protect the other metal.
What happens when the copper grid touches buried steel pipes?
A galvanic cell forms that can accelerate the corrosion of the pipe, of buried steel and of cable sheaths made of lead alloys. The usual measures are to insulate with a coating the surface of the metal that is sacrificed, cross pipes at right angles, use cathodic protection or use nonmetallic pipes.
How do I know a grid connection is adequate?
Connections must meet the conductivity, corrosion resistance, current capacity and mechanical strength of the conductor. IEEE 837 defines their application and tests, and connections that comply satisfy all those criteria.