Grounding of Distribution Substations: Design and Tests

What the grounding of a substation must achieve

When a ground fault occurs in a substation, the current disperses into the soil and raises the potential of the grid with respect to remote earth. A person who touches a metal structure or walks nearby can be subjected to a potential difference. The two critical voltages are:

• Touch voltage: the voltage between a grounded structure and the feet of a person touching it.
• Step voltage: the voltage between the two feet of a person walking on the ground during the fault.

These voltages, together with the voltages transferred outside, must not exceed the tolerable limits of the human body (IEEE 80). In a distribution substation, the work comes down to three decisions: how much fault current there is, how long it takes to clear and what the soil is like. For more detail see step and touch voltage and grounding grid. The types of distribution substation and their components are described in Distribution substations: types and components.

What the design must show

The design must verify, with a calculation procedure recognized by engineering practice, that step and touch voltages do not exceed the tolerable limits. This applies to substations, distribution networks and small-scale generation plants. The calculation takes a body resistance of 1,000 ohms and each foot as a 200 cm² plate with a force of 250 N. The maximum permissible values are calculated with the IEEE 80 methodology.

The basic design procedure has several steps. The main ones are:

The design must also investigate the voltages that can be transferred outside through pipes, fences, neutrals, cable shields and signaling circuits, and how to mitigate them.

Data provided by the utility. The maximum ground fault current at medium and high voltage is provided by the utility for each case. Without that value in writing, the design has no basis. The clearing time depends on the substation protections: a fault that clears faster exposes people for less time.

Pole-mounted substation. In end-user installations with a pole-mounted substation, the design can be simplified, but it must consider soil resistivity, fault currents and load type, and must always control step and touch voltages. The transformer neutral and its tank must also be solidly grounded.

Materials, joints and equipotential bonding

Construction requirements are checked at inspection:

• Electrodes: rods, tubes, plates, strips, bare wires or cables. Buried aluminum is not accepted.
• Underground joints: made with exothermic welding or with a connector certified for direct burial.
• Inspection boxes: accessible points to measure, with a removable cover. They do not apply to distribution networks or transmission lines.
• Interconnection: when a building has several grounding systems, they must be bonded together. Using a single ground for all needs, or separate and independent grounds, in the same building is not good practice.
• Structure: the main metal reinforcing elements of a building must have a permanent electrical connection to the grounding system, and this is essential on floors that support transformers or switchgear.
• Medium- and high-voltage ground conductor: sized with the fault current and the clearing time, using the equation of IEEE 80.
• Continuous conductors: without switches or disconnecting means, and joined by welding or certified connectors.

In indoor substations, grids and enclosures must be solidly grounded. In prefabricated ones, the enclosure must have ground terminals for the shell and for all its metal parts. The placement of equipment and of the grid on the substation site is covered in Physical layout of a substation: yard and clearances.

What is measured and when

For resistivity, the four-electrode Wenner method is accepted. For resistance, the methods of IEEE 81 apply, among them the fall-of-potential method, with the auxiliary current electrode at a distance of at least five times the largest dimension of the grounding system. In substations, step and touch voltages are measured up to one meter outside the enclosure. The thresholds that trigger these measurements depend on the local code and the utility, and they do not apply to pole-mounted medium-voltage substations.

If resistivity is high, fault currents are high or clearing is slow, there are measures to avoid exposing people: make critical areas inaccessible, install high-insulation floors or pavements, insulate devices a person might hold and establish equipotential connections. Also, insulate the electrode conductor where it enters the ground and post warning signs. Reference resistance values can be used as an indicator, but they do not replace compliance with the permissible voltages.

For the resistance test see the guide on ground resistance testing and our free ground resistance tool.

Maintenance

The components of the system lose effectiveness over the years through corrosion, electrical faults, mechanical damage and lightning strikes. Inspection is done by a competent person, who delivers records with the condition of the conductors, the level of corrosion, the condition of the joints, the resistance values, the deviations from the requirements and the changes since the last inspection. Tests include the equipotential bonding test and the resistance measurement. If the inspection shows that repairs are needed, they must be done without delay and not wait for the next maintenance cycle.

Common mistakes

• Using a resistance value as the only criterion. It is a reference; what must be met are the permissible voltages.
• Designing without the fault current from the utility. The calculation has no basis.
• Leaving the structure that supports the transformer or switchgear unconnected.
• Buried joints without welding or a certified connector. They are the first point of failure through corrosion.
• Not leaving inspection boxes accessible. They prevent measuring later.
• Measuring only once. Grounding is inspected periodically and with records.

Grounding design and measurement with Tiza Group

We calculate and measure the grounding of your substation, and deliver the records needed for inspection. Measurements are done in Colombia, or elsewhere on request; the design calculation can be done remotely. See our testing and commissioning and primary engineering, or write to us through contact.

Related guides

• Grounding grid
• Step and touch voltage
• Ground resistance test
• Industrial grounding
• Pole-mounted substation
• Pad-mounted substation
• Neutral grounding resistor
• Distribution substations: types and components
• Physical layout of a substation: yard and clearances

Frequently asked questions