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What Is Step and Touch Potential and Reducing Resistance To Ground?

Step Potential


Touch Potential


Step Potential

When a fault occurs at a tower or substation, the current will enter the earth. Based on the distribution of varying resistivity in the soil (typically, a horizontally layered soil is assumed) a corresponding voltage distribution will occur. The voltage drop in the soil surrounding the grounding system can present hazards for personnel standing in the vicinity of the grounding system. Personnel “stepping” in the direction of the voltage gradient could be subjected to hazardous voltages.

Step Potential


Hazardous Step Potentials or step voltage can occur a significant distance away from any given site. The more current that is pumped into the ground, the greater the hazard. Soil resistivity and layering plays a major role in how hazardous a fault occurring on a specific site may be. High soil resistivities tend to increase Step Potentials. A high resistivity top layer and low resistivity bottom layer tends to result in the highest step voltages close to the ground electrode: the low resistivity bottom layer draws more current out of the electrode through the high resistivity layer, resulting in large voltage drops near the electrode. Further from the ground electrode, the worst case scenario occurs when the soil has conductive top layers and resistive bottom layers: in this case, the fault current remains in the conductive top layer for much greater distances away from the electrode.



Touch Potential


Touch Potential





  1. Reduction in the Resistance to Ground of the grounding system
  2. Proper placement of ground conductors
  3. 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 3-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 Resistance to Ground

Reducing resistance to ground (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 resistance to ground of the grounding system. Thus, reducing the Ground Potential Rise 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 Ground Potential Rise. For example, if the fault current for a high-voltage tower is 5,000 amps and the resistance to ground of the grounding system is 10-ohms, the Ground Potential Rise will be 50,000 volts. If we reduce the resistance to ground of the grounding system down to 5-ohms and the fault current increases to 7,000 amps as a result, then the Ground Potential Rise will become 35,000 volts.

As seen in the example above, the reducing resistance to ground can have the effect of allowing more current to flow into the earth at the site of the fault, but will always result in lower Ground Potential Rise values and step and touch voltages at the fault location. 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. Reducing the resistance to ground can be achieved by any number of means as discussed earlier in this chapter.

Proper Placement of Ground Conductors

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 inches 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: i.e., to minimize Touch Potentials.

It is important that all metallic objects in a GPR 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 grounding rods or deep wells extending into this layer will be ineffectual.

It is sometimes believed that placing horizontal grounding loop conductors very close to the surface results in the greatest reduction in Touch Potentials. This is not necessarily so, as conductors close to the surface are likely to be in drier soil, with a higher resistivity, thus reducing the effectiveness of these conductors. 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.

Reducing Step and Touch Potential Hazards



Telecommunications in High-Voltage Environments


Industry standards regarding these precautions and protective requirements are covered in IEEE Standard 387, IEEE Standard 487, and IEEE Standard 1590.These standards require that a Ground Potential Rise study be conducted so that the 300-volt peak line can be properly calculated.

