Grounding Grid Construction: Trench and Plow

Short answer

A substation grounding grid is almost always built with one of two methods, trenching or plowing the conductor, and with a combination when convenient. Both use machines; when they do not fit or the job is small, it is dug by hand. IEEE Std 80-2013 describes the sequence, the connections and the safety precautions in its clause 18. What is built must match the design: depth, size and bonds define the step and touch voltages the grid will produce. The design that gets built comes from the IEEE 80 design procedure.

What decides the method

The standard lists the selection factors: grid size, soil type, conductor size, burial depth, available equipment, labor cost and physical or safety restrictions from existing structures or nearby energized equipment. There are two methods, both with machines; manual excavation is used where machines cannot move because of lack of room or the small size of the site.

Trench method

Two sides of the perimeter are marked with flags to set the spacing between parallel conductors; those markers guide the trenching machine. The sequence, in general:

1. The trenches are opened with the machine, normally along the side that has more parallel conductors, to the specified depth, which is usually about 0.5 m.
2. The conductors are installed, the rods are driven and connected to the conductors. The pigtails for equipment grounds can also be left at this point.
3. Those initial trenches are backfilled up to the location of the cross connections.
4. The trenches for the cross conductors are opened, often shallower, taking care not to snag the conductor already laid at the crossings.
5. The cross conductors, the remaining rods and the pending pigtails are installed, the cross connections between perpendicular conductors are made and the trenches are backfilled.

An alternative is to open all the trenches at the same depth at the same time, in small sections so as not to hinder other work, or across the whole substation.

Plow method

A special narrow plow, coupled to a tractor or a four-wheel-drive truck, or pulled by a winch at the edge of the yard, opens the furrow and lays the conductor to the bottom. The conductor can be laid on the ground ahead of the plow, carried on a reel mounted on the tractor or on a sled, or fixed to the bottom of the blade and dragged along the bottom of the furrow. In the latter case, the conductor must be kept from rising through the loose soil.

The cross conductors are plowed at a somewhat shallower depth so as not to damage those already installed. Then the crossings and rod points are uncovered, and the connections are made there.

With the right equipment and no heavy rock, the method works for all sizes and depths in normal use. The standard refers to IEEE Std 590 for more information.

Connections, pigtails and rods

With the conductors in the trench, the connections are made. There are many types and they depend on the joint, the material and each utility’s practice. During installation:

• Damage to the outer layer of any bimetallic material is avoided.
• Corrosion at exposed conductor ends is considered. Extending the end beyond the connection can mitigate it.
• Pigtails are left at the right places to connect structures and equipment; they can be the same size as the grid or a different one, depending on the number of grounds per equipment item, the magnitude of the fault current and the design practice. They remain accessible after backfilling.
• Rods are normally driven with a hydraulic or pneumatic hammer or another mechanical device. Two rods are joined by the exothermic method, by crimping or with a threaded or unthreaded coupling. The joint between the rod and the conductor admits several methods.

Construction sequence

The grid is normally installed after grading the yard, pouring the foundations and installing and backfilling the deepest pipes and conduits. If the latter are not in place beforehand, the trenching work must be coordinated in a logical way. The security fence can be installed before or after.

Later excavations

The layer of clean surface material or gravel is a safety aid in a fault. For that reason, when excavation is needed after the stone layer has been placed, mixing the lower-resistivity soil of the excavation with the stone around the area must be avoided.

When excavating later there is more chance of snagging the grid conductor. In that case:

• Check whether there is a break in the conductor or in the bonds.
• A break in the conductor or in the bonds is repaired immediately.
• Before repairing it, a temporary ground connection is installed around the break, because voltage may exist between the two ends of the conductor.
• The temporary connection must be adequate and placed following safe practices.

The same precautions apply when expanding an existing grid: temporary connections are installed between the old and new conductors before handling them to join them.

What to check during construction

• That the spacing and depth of the conductors match the design, because they determine the step and touch voltages.
• That the pigtails end up where each equipment item or structure will require them and accessible after backfilling.
• That no later trench cuts or loosens a conductor already laid.

What is verified afterward

A built grid must be measured: impedance, surface potentials and integrity. The tests are in the ground resistance test guide.

How we do it at Tiza Group

At Tiza Group we measure and verify newly built and existing grounding systems in substations, industry and solar plants. Learn about our testing and commissioning service or write to us. Field work is done in Colombia, or elsewhere on request.

Related guides

• IEEE 80: Safety in Substation Grounding
• Grounding Grid Materials and Connections
• Potential Map and Integrity of an Installed Grid
• Ground Resistance Test: Fall-of-Potential and IEEE 81
• Grounding of Distribution Substations: Design and Tests

References

• IEEE Std 80-2013, IEEE Guide for Safety in AC Substation Grounding, clauses 18.1 to 18.5.
• IEEE Std 590 (cable plowing), cited by IEEE Std 80-2013.

Frequently asked questions