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Substation Grounding

Posted By Leslie Stoch, Wednesday, May 01, 2002
Updated: Thursday, February 14, 2013

In this article, we will review the measurements needed to ensure that substation grounding resistance and resistivity are low, so we can be sure that people are able to work safely in and around an outdoor station.

The Canadian Electrical Code, Rules 36-302, 36-304, and 36-306 require that the grounding design of outdoor substations over 7500 volts phase-to-phase, must limit during a phase-to-ground fault:

  • ground potential rise (GPR) to 5000 volts maximum; and
  • step and touch voltages in and around substations to the maximum values given in Table 52.

You will probably recall that touch voltage is the voltage between your hands and feet when touching an object energized during a ground fault. Step voltage is the voltage between your feet if you happen to be walking in the station during a ground fault. Assuming that the substation surface is normally a minimum 150 mm – 3/4 inch crushed stone, Table 52 would specify:

  • 3143 volts step voltage for a ½-second fault and 2216 volts for a 1-second fault; and
  • 885 volts touch voltage for a ½-second fault and 626 volts for a 1-second fault

How do we ensure substation grounding resistance low enough so as not to exceed the above values for GPR, step and touch voltages? IEEE Standard No. 80 contains station grounding design formulae for calculating these values, and when the job is completed, we still need to confirm the results.

To do the calculations we need to know:

  • The maximum available ground- fault current at the station
  • The measured earth resistivity at the site in ohm-meters
  • Conductor sizes and lengths and number of ground rods, conductor spacing and overall area that make up the initial substation ground electrode design

Resistivity is a property of all materials that have a resistance to electrical current flow. Although tabulated earth resistivities are available for different soils, these cannot be solely relied upon, since each site may be a combination of different materials. Resistivity also varies vertically with distance below the earth and horizontally along its surface. Gravel is at the high end of the resistivity range and clay at the low end. Resistivity also decreases with increased moisture and temperature, and varies with soil’s chemical content. Unless resistivity at a new substation site is well understood, it should be confirmed by measurement before attempting the substation grounding design.

Earth resistivity measurements are done using a 4-terminal megger type instrument, flexible cables, alligator clips and short ground probes. Four short ground probes are driven in a straight row, an identical distance apart, in an arrangement known as the Wenner Array. The distance between the probes determines the depth for which the instrument readings are valid. Short distances between probes provide shallow readings and longer distances, deeper ones. Resistance measurements are taken at a sufficient number of locations on the site to provide confidence that the complete picture is known and recorded.

A two-layer model is used to provide resistivity data for a shallow vertical distance (say 1-1/2 meters) below the surface, and then for the rest of the earth. A 1-1/2 meter distance between the ground probes determines resistivity of the predetermined top layer. The spacings are increased to measure the remainder of the earth. Resistance measurements are converted to resistivities by the formula: resistivity = 2 x pi x S x R where:

  • R is the resistance readings at each set up
  • S is the equal distance between the ground probes in relation to the two layers
  • And pi is 3.14

Computer software is used to combine the 2-level readings into a composite number that can be plugged into the IEEE No. 80 standard calculations for that site.

We also need to know the actual station grounding resistance once the ground electrode is installed, and from time to time during the life of the substation. These measurements can be done using the same megger type instrument.The resistance of a substation grounding electrode is made up of three components:

  • Resistance of grounding conductors, rods and connections
  • Contact resistance between the ground electrode and the earth
  • Resistance of the earth

When properly installed, the first two can be assumed to be almost negligible, and 85 to 90 percent of the grounding resistance can safely be assumed to be the resistance of the earth.

Although the instrument can be used in several different ways, the most accurate measurements are achieved using the "fall of potential” method. The instrument injects a known current through the earth from the ground electrode under test to a remotely located current probe. A potential probe is driven in a number of locations beginning near the ground electrode and moving toward the current probe. Resistance measurements are read from the instrument, recorded and plotted against distance from the ground electrode. The resistance curve is drawn and the "point of inflection” identified — at a distance along the curve where it becomes most horizontal. This is the optimum ground resistance accuracy range.

As usual, you should consult the local electrical inspection authority in each province or territory as applicable for a more precise interpretation of this subject.

Read more by Leslie Stoch

Tags:  Canadian Code  May-June 2002 

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