During a ground fault, the current entering the soil creates potential gradients in and around the substation. A person suffers a shock when their body bridges two points at different potential, with enough current and time to do harm. IEEE 80 describes the typical situations: step, touch, mesh, transferred and metal-to-metal.
Short answer. In a ground fault, the current that the grid discharges into the soil raises the potential of the substation and creates potential differences between points of the ground and of the structures. A person suffers an electric shock when they bridge two of those points with their body, the current through it is sufficient and it lasts long enough to cause harm. Understanding those conditions is what makes it possible to design a safe grounding system.
The basic problem
IEEE Std 80-2013 (clause 4.1) states that a safe grounding system pursues two objectives. One is to carry currents into the earth, in normal operation and in a fault, without exceeding equipment limits or affecting service. The other is to reduce the risk that a person near the grounded facilities receives a critical electric shock.
Many people assume that any grounded object can be touched without risk. The guide warns of the opposite: a low grid resistance does not guarantee safety, because there is no simple relation between that resistance and the maximum current that can flow through a person. That is why the potentials at the ground surface are analyzed, not just the resistance. The path of the current through hand, foot and soil is detailed in body resistance and the accidental ground circuit.
What happens in the soil during the fault
When a ground fault drives current into the soil, the flow from the grid and the other buried electrodes produces potential gradients in and around the substation. If the design takes no precautions, the maximum surface gradients can be dangerous for anyone in the area. Dangerous voltages can also appear between grounded structures or enclosures and the nearby ground.
How much the potential rises also depends on how the fault current returns to the source. If the substation is fed by an overhead line without a shield wire or neutral, almost all the current enters the soil and the local potential rises sharply. If there is a shield wire, a neutral, a gas-insulated bus or underground feeders, part of the current returns to the source through that metallic path and the ground potential rise is lower.
The five factors that must coincide
Clause 4.2 lists the circumstances that make an accident possible:
| Factor | What it means |
|---|---|
| High fault current | High relative to the area of the grounding system and its resistance to remote earth |
| Soil and current distribution | Resistivity and currents that produce high surface gradients |
| Person on site | The body bridges two points with a large potential difference |
| Little series resistance | No contact resistance or other resistance to limit the body current to a safe value |
| Duration | The fault and the contact last long enough to cause harm at that intensity |
If any of the five is missing, the accident does not occur. That is why the design acts on those that can be controlled: reducing gradients, adding resistance between the soil and the feet, and clearing the fault quickly.
Typical situations in an outdoor substation
Clause 8.2 shows five basic situations. In each one, what changes is between which points the body is placed and which voltage drives the current:
| Situation | How it occurs | Driving voltage |
|---|---|---|
| Step | The person has one foot on each of two surface points, touching nothing grounded | Step voltage |
| Touch | Hand on a grounded structure and feet on the ground | Touch voltage |
| Mesh | Contact at the center of a grid mesh | Mesh voltage, the maximum touch inside a mesh |
| Transferred | A person inside touches a grounded conductor at a remote point, or a remote person touches one connected to the grid | Transferred voltage |
| Metal-to-metal | Hand-to-hand or hand-to-feet contact between metallic objects in the substation that are not at the same potential | Metal-to-metal voltage |
The worst touch in a conventional substation is usually between the hand and the feet at the point of greatest reach. The guide defines the step voltage as the potential difference at the surface between two points 1 m apart.
The metal-to-metal case
Between structures bonded to the grid, the metal-to-metal voltage is considered negligible in a conventional substation. It appears when a metallic object within the site is not connected to the grid, for example pipes, rails or fences inside or next to it. The guide recommends avoiding the hazard by connecting those points to the grid, because calculating the actual voltage is complex.
Potential transfer
Transferred voltage is the hardest to control. It can equal or exceed the total ground potential rise of the grid, not just the fraction found in a normal touch. With voltages induced in communication circuits, shield wires or pipes, it can even exceed the sum of the potential rises of two substations. IEEE 80 states that designing the grid for that contact is impractical, and often impossible. It is avoided by isolating or neutralizing the outgoing circuits and treating them, with clear labeling, as if they were energized lines. The case in which the hazard comes from outside the substation is explained in transferred voltage and transferred potentials.
Gas-insulated substations (GIS)
In a GIS the metallic enclosure of the switchgear is grounded, but the current flowing through the inner bus induces voltage in the enclosure. For that reason there can be potential differences between parts of the enclosure and the substation ground. Clause 8.3 considers two origins: an internal fault, such as an arc between the conductor and the enclosure wall, and an external fault whose current through the bus induces currents in the enclosures. Since the person may be standing on a grounded metal grating, hand-to-hand and hand-to-feet contact must be checked. For the rest of the installation see GIS substation.
What is done about it
The design seeks to keep the voltage reaching the body below the tolerable one. The grid distributes the current and controls the gradients. A high-resistivity surface layer adds resistance between the soil and the feet. Fast fault clearing limits the exposure time. The tolerable values and how to calculate them depend on the current the body withstands and are explained in chapter 6 of IEEE 80. For field diagnosis when the measured voltages are high, see high step and touch voltages in the grid. The current the body withstands as a function of shock duration is in the guide on tolerable body current.
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We analyze which of these situations can occur in your substation or plant, calculate the expected voltages and define the measures to control them, including transfer to the outside. See the engineering design and studies service or write to us through contact.
Related guides
- IEEE 80: Safety in Substation Grounding
- Tolerable body current according to IEEE 80
- Transferred voltage: cables, pipes and fences
- Step and touch voltage: difference and IEEE 80
- Substation ground grid: what it is, design and tests
References
- IEEE Std 80-2013, IEEE Guide for Safety in AC Substation Grounding: clauses 4.1 (basic problem), 4.2 (hazard conditions), 8.2 (typical situations in air-insulated substations) and 8.3 (typical situations in gas-insulated substations).
Frequently asked questions
Why do so few accidents happen if ground faults are frequent?
Because an accident requires several unfavorable conditions to coincide at the same time: a high fault current, a soil that produces high gradients, a person at the exact point and moment, little series resistance and a fault that lasts long enough. IEEE 80 attributes the low frequency of accidents to the low probability that all of them coincide.
Does a low-resistance grid prevent electric shock?
Not by itself. IEEE 80 explains that a substation with a low ground resistance can be dangerous and another with a high resistance can be less dangerous, or be made less dangerous by careful design. What decides is the potential that appears between the points a person can touch.
What changes in a gas-insulated substation (GIS)?
Besides step and touch, metal-to-metal contact must be considered: the metallic enclosure can be at different potentials at different points because of voltages induced by the bus current, in both an internal and an external fault.