The substation fence is a risk point because the public can touch it. IEEE 80 describes four practices: fence inside the grid and bonded to it, outside the grid and bonded to it, outside and bonded to its own electrode, or outside and without an electrode. The touch voltage at the fence must stay within the tolerable limit, and the safest solution in the cases analyzed is to extend the grid beyond the fence.
Short answer
The substation fence is very important for safety because it is usually accessible to the public. IEEE Std 80-2013 requires the touch voltage at the fence to be within the calculated tolerable limit, and the step voltage to be checked too, which is almost never a problem if the touch voltage complies. The fence can be bonded to the grid or kept separate from it; in the cases analyzed by the standard, the safest combination is to bond it to the grid and extend the grid beyond the fence. The fence is included in the substation grid design.
Why it is a risk point
During a fault, the grid rises with respect to remote earth, and the potential drops rapidly outside the grid area. A person touching the fence from the outside is standing on soil at a potential different from the fence’s. If the fence is bonded to the grid, it takes the grid potential (100 % of the substation ground potential rise), and the difference with the outside soil can exceed the tolerable limit.
If the fence has no bond to the grid, its potential depends on the contact of the posts with the soil, and the difference with the nearby soil changes along its length.
Fence grounding practices
If the fence is metallic, the standard describes four practices:
| Practice | Fence relative to the grid | Bond |
|---|---|---|
| 1 | Inside the grid area | Bonded to the grid |
| 2 | Outside the grid area | Bonded to the grid |
| 3 | Outside the grid area | Not bonded, with its own electrode |
| 4 | Outside the grid area | Not bonded and no electrode; relies on the contact of the posts with the soil through the concrete |
Practices 3 and 4 couple the fence to the grid only through the soil. To adopt them, three questions must be answered:
- Is an energized line falling on the fence a hazard? Lines that cross a fence usually have short spans and terminate at one or both ends, so the risk is normally not a concern. Designing against it would require coupling the fence very closely to the soil along its whole length, with acceptable touch and step voltages on both sides for a fault current almost equal to the substation maximum. The standard considers this impractical, because the fence is not bonded to the grid and the neighboring soil would have to dissipate all that current; in addition, the fault would damage the fence and it would be impossible to predict the clearing time and the voltages.
- Can hazardous potentials appear in other faults? Yes: the fence does not follow the equipotentials of the soil.
- Can total isolation between fence and grid always be ensured? It is unrealistic. Metal conduits or pipes can transfer the grid potential to the fence, and if the fence is not well coupled to earth by its own system, the hazard appears along its entire length. Insulating joints in the fence only reduce the risk in part and are not considered a practical solution.
Execution details
- Some utilities ground only the posts, with connectors, and rely on the fabric ties for continuity. Others ground posts, fabric and barbed wire.
- The grid must extend to cover the swing of all gates.
- Gate posts are bonded to the adjacent post with a flexible connection.
What the analyzed cases showed
The standard studied, by computer calculation, five ways of grounding the fence of a square substation 43.9 m on a side, with soil resistivity of 60 Ω·m and a grid current of 5,000 A. The touch voltage was measured for a person standing 0.91 m from the fence, at arm’s length.
| Case | Layout | Result |
|---|---|---|
| 1 | Outer grid conductor 0.91 m outside the fence; fence bonded | Touch voltage below the tolerable value: safe |
| 2 | Outer conductor right next to the fence; fence bonded | Very close to the limit when crossing the grid, and unsafe in some zones along the fence |
| 3 | Outer conductor 0.91 m inside the fence; fence bonded | Similar to case 2: in several zones the fence is not safe to touch |
| 4 | Outer conductor 6.7 m inside the fence; fence bonded | The touch voltage at the fence is not safe |
| 5 | Same as 4, but the fence with its own local ground, not bonded | Fence potential of 43.7 % of the ground potential rise; the largest difference, at the corner, only just exceeds the limit |
There are three conclusions:
- The touch voltage at the fence increases as the outer grid conductor moves toward the interior of the substation (cases 1 to 4).
- The step voltage at the fence is almost never the problem: in case 1 it did not exceed 3 % to 4 % of the ground potential rise, far below its limit.
- In case 5, if the fence were to be bonded to the grid by accident, its potential could rise to 100 % of the ground potential rise, with results similar to case 4. The cases with an insulated, ungrounded fence gave results very similar to those of case 5.
When the fence falls outside the grid
If the fence encloses an area much larger than the one the substation uses and the grid covers only that part and the fence, the areas without grid inside the fence are often used for storage or as a work area. Step and touch voltages must be checked there to decide whether more grounds are needed. A service area can also be enclosed with a separate fence, not bonded to the grid, with the transferred voltage risks explained in the standard. Service areas, switch handles and arresters are covered in grounding of switch handles, arresters and service areas. A metal fence can carry a transferred voltage outside the substation.
How we do it at Tiza Group
The solution depends on the grid geometry, the soil and the fault current. At Tiza Group we evaluate the touch voltage at the fence and propose the solution within the grounding design of medium, high and extra-high voltage substations. Learn about our engineering design and studies service or write to us. We deliver it remotely, worldwide.
Related guides
- IEEE 80: Safety in Substation Grounding
- Transferred Voltage: Cables, Pipes and Fences
- Grounding of Switch Handles, Arresters and Service Areas
- Step and Touch Voltage: Difference and IEEE 80
- Grounding of Distribution Substations: Design and Tests
References
- IEEE Std 80-2013, IEEE Guide for Safety in AC Substation Grounding, clauses 17.1, 17.3 and 17.4.
- NESC (IEEE C2), National Electrical Safety Code, cited by IEEE Std 80-2013.
Frequently asked questions
Must a substation fence be grounded?
If it is metallic, yes. The standard explains that, for example, the NESC requires grounding metal fences that enclose substations with energized equipment or conductors. This can be achieved by bonding it to the grid or to a separate electrode. If the fence is masonry or another nonconductive material, it is not grounded, except for exposed metal hardware or sections, such as gates.
Why is a fence grounded separately from the grid dangerous?
Because the fence does not follow the soil potential contours during a fault. It can reach soil potential where coupling is good and have a large difference from the soil at other points. In addition, an inadvertent connection to the grid through pipes or conduits can carry the grid potential to the whole fence.
What about the substation gates?
The grid must extend to cover the swing of all gates, and the gate posts must be bonded to the neighboring fence post with a flexible connection.