Danger of ground potential rise
Mitigating step and touch potential hazards is usually accomplished through one or more of the following three main techniques:
- Reduction in the resistance to ground (RTG) of the grounding system
- Proper placement of ground conductors
- The addition of resistive surface layers
Understanding the proper application of these techniques is the key to reducing and eliminating any ground potential rise hazards. Only through the use of highly sophisticated three-dimensional electrical simulation software that can model soil structures with multiple layers and finite volumes of different materials, can the engineer accurately model and design a grounding system that will safely handle high-voltage electrical faults.
Reducing the RTG of the site is often the best way to reduce the negative effects of any ground potential rise event, where practical. The ground potential rise is the product of the fault current flowing into the grounding system times the RTG of the grounding system.
Thus, reducing the RTG will reduce the ground potential rise to the degree that the fault current flowing into the grounding system does increase in response to the reduced RTG. For example, if the fault current for a high-voltage tower is 5000 A and the RTG of the grounding system is 10-, the ground potential rise will be 50,000 V.
If we reduce the RTG of the grounding system down to 5- and the fault current increases to 7000 A as a result, then the ground potential rise will become 35,000 V!
On the other hand, further away from the fault location, at adjacent facilities not connected to the faulted structure, the increase in current into the earth will result in greater current flow near these adjacent facilities and therefore an increase in the ground potential rise, touch voltages and step voltages at these facilities.
Of course, if these are low to begin with, an increase may not represent a problem, but there are cases in which a concern may exist.
A typical specification for ground conductors at high voltage towers or substations is to install a ground loop around all metallic objects, connected to the objects.
Keep in mind that it may be necessary to vary the depth and/or distance that ground loops are buried from the structure in order to provide the necessary protection.
Typically these ground loops require a minimum size of 2/0 AWG bare copper conductor buried in direct contact with the earth and 3-ft from the perimeter of the object, 18 in below grade. The purpose of the loop is to minimize the voltage between the object and the earth surface where a person might be standing while touching the object – that is, to minimize touch potentials.
It is important that all metallic objects in a ground potential rise environment be bonded to the ground system to eliminate any difference in potentials. It is also important that the resistivity of the soil as a function of depth be considered in computed touch and step voltages and in determining at what depth to place conductors.
For example, in a soil with a dry, high-resistivity surface layer, conductors in this layer will be ineffective. A low resistivity layer beneath that one would be the best location for grounding conductors. On the other hand, if another high resistivity layer exists further down, long ground rods or deep wells extending into this layer will be ineffectual.
Furthermore, while touch potentials immediately over the loop may be reduced, touch potentials a short distance away may actually increase due to the decreased zone of influence of these conductors. Finally, step potentials are likely to increase at these locations. Indeed, step potentials can be a concern near conductors that are close to the surface, particularly at the perimeter of a grounding system.
It is common to see perimeter conductors around small grounding systems buried to a depth of 3-ft below grade, in order to address this problem.
One of the simplest methods of reducing step and touch potential hazards is to wear electric hazard shoes. When dry, properly rated electric hazard shoes have millions of ohms of resistance in the soles and are an excellent tool for personnel safety. On the other hand, when these boots are wet and dirty, current may bypass the soles of the boots in the film of material that has accumulated on the sides of the boot.
A wet leather boot can have a resistance on the order of 100Ω. Furthermore, it cannot be assumed that the general public, who may have access to the outside perimeter of some sites, will wear such protective gear.
Another technique used in mitigating step and touch potential hazards is the addition of more resistive surface layers. Often a layer of crushed rock is added to a tower or substation to provide a layer of insulation between personnel and the earth. This layer reduces the amount of current that can flow through a given person and into the earth. Weed control is another important factor, as plants become energized during a fault and can conduct hazardous voltages into a person.
Asphalt is an excellent alternative, as it is far more resistive than crushed rock, and weed growth is not a problem. The addition of resistive surface layers always improves personnel safety during a ground potential rise event.
Telecommunications in high voltage environments
When telecommunications lines are needed at a high-voltage site, special precautions are required to protect switching stations from unwanted voltages. Installing wires in a substation or on a tower could present a hazardous situation and therefore certain precautions are required.
Industry standards regarding these precautions and protective requirements are covered in IEEE Standard’s 387, 487, and 1590. These standards require that a ground potential rise study be conducted so that the 300-V peak line can be properly calculated.
To protect the telephone switching stations, telecommunication standards require that fiber-optic cables be used instead of copper wires. A copper-to-fiber conversion box must be located outside the ground potential rise event area at a distance in excess of the 300-V peak or 212-V RMS line (Figure 4).
The current formulae for calculating the 300-volt line, as listed in the standards, has led to misinterpretation and divergences of opinion, resulting in order-of-magnitude variations in calculated distances for virtually identical design input data. Furthermore, operating experience has shown that a rigorous application of theory results in unnecessarily large distances. This has caused many compromises within the telecommunications industry.
The most noted one is a newer standard, IEEE Std. 1590-2003, that lists a 150-m (~500-ft) mark as a default distance, if a ground potential rise study has not been conducted at a given location.
Reference // Standard handbook for electrical engineers – Grounding systems by David R. Stockin and Michael A. Esparza