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Substation Grounding

Posted By Leslie Stoch, Sunday, July 01, 2007
Updated: Sunday, February 10, 2013

Section 10 of the Canadian Electrical Code, Grounding and Bonding, is probably the least well understood section of the electrical code, but it is one of the most important. Opinions abound on the absolutely correct interpretations. But almost everyone will agree that correctly installed grounding and bonding is most critical to a safe electrical installation.

Rule 10-002 lists the objectives of correctly installed grounding and bonding as:

  • Protecting life from the danger of electrical shock and property from damage
  • Limiting the voltage on a circuit when exposed to higher voltages than designed
  • Limiting voltages-to-ground to 150 volts or less for interior wiring
  • Limiting circuit voltages when exposed to lightning strikes

High voltage substations (over 750 volts) are a special case. Here Section 36 of the CEC takes us several steps further and the requirements become more complex with the increased potential for harm. Section 36 deals with some special electrical hazards in the form of ground potential rise (GPR), step and touch potentials that can harm us when we are in the vicinity of a substation during the occurrence of a ground fault.

Ground potential rise is the voltage rise in the grounding system of an outdoor substation during a ground fault. The calculated GPR is simply the maximum available ground-fault current (usually obtained from the electrical utility) multiplied by the measured grounding resistance of the substation grounding electrode (Ohm’s Law) under all soil conditions.

Step voltage is defined as "the potential difference between two points on the earth’s surface separated by a distance of one pace, assumed to be 1 m in the direction of the voltage gradient.” Voltage gradients are the voltages produced when electrical current flows through the earth. Touch voltage is defined as "the potential difference between a grounded metal structure and a point on the earth’s surface separated by a distance equal to normal maximum horizontal reach.” GPR, step and touch voltages must be maintained within the limits prescribed in Section 36 of the code in outdoor substations operating at 7500 volts or higher.

During a ground fault, a person walking inside or in the vicinity of an outdoor substation runs the risk of an electrical shock due to step voltages in or around the substation. Similarly, a person standing inside or outside a substation and in contact with a grounded metallic object may receive an electrical shock due to a touch voltage between the person’s hands and feet. For this reason, Rule 36-304 of the CEC specifies that the grounding systems of outdoor substations above 7500 volts must be designed to limit the possibility of step and touch potentials to the requirements of Table 52.

Rule 36-304 requires that outdoor substations above 7500 volts phase-to-phase must have a GPR no higher than 5000 volts during a ground fault. Table 52 limits step and touch potentials within, in, and around the perimeter of an outdoor station to the following maximum limits:

As you can see, the maximum permissible step and touch voltages are increased as the duration of a ground fault is reduced and reduced for a longer duration ground fault.

An outdoor substation ground electrode is normally a combination of driven ground rods and buried conductors interconnected as a mesh, covering the entire substation area and at least one metre outside the fence. Formulae and guidelines for substation grounding design are found in IEEE Standard No. 80 and CSA Standard No. C22.2 No. 41. Computer software is used to calculate the ground potential rise, step and touch voltages.

To minimize voltage differences, the electrical code also requires that all metal objects, including structures, electrical equipment, metal piping, railway tracks, cable armour and metal sheaths, buildings and fences to be interconnected with the station grounding electrode. Concrete reinforcing steel may also be connected to help lower the station grounding resistance, but some conditions apply. Sometimes unfavourable soil conditions require supplemental grounding (interconnecting the substation grounding electrode with a remote grounding electrode so as to reduce the grounding resistance to an acceptable level).

A layer of crushed stone within and at least one metre outside the station fence provides further protection against step and touch potentials by isolating a person from the grounding system.

As with previous articles, you should also consult with your local electrical inspection authority for a more precise interpretation of any of the above.

Read more by Leslie Stoch

Tags:  Canadian Code  July-August 2007 

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