What is Ground Potential Rise?
In electrical engineering, a Ground Potential Rise (GPR) is a hazardous event when a large amount of electrical current enters the earth. Under normal circumstances, the earth's potential is zero, but during a fault condition, any current flowing into the earth and disappearing into the ground elevates the earth and surrounding soil voltage.
Substations and HV towers are often placed in open areas and exposed to natural disasters such as earthquakes and lightning strikes, causing a Ground Potential Rise. While a natural disaster may cause the event, in most cases, it is most likely to happen due to an electrical network fault during normal working conditions. Any conducting object connected to the substation earth ground, such as overhead power lines, rails, fences, or metallic piping, may be energised at the ground potential in the substation. Broken trees and branches can sometimes make accidental contact with the energised conductors. The Ground Potential Rise can also happen in remote locations because of an existing circuit fault in the overhead connection to an underground.
Ground Potential Rise creates two specific electric shock hazards - a Step and Touch Potential.
What is Step and Touch Potential?
Step Potential is the voltage difference between two points on the ground - a person's footstep standing near an energised ground (the soil potential gradient). A person could be at risk of injury during a fault by standing near the grounding point.
On the other hand, a Touch Potential is a potential difference between a person's body and a faulted metal object that might be in contact and the ground from which they are standing away.
The risks of a Step and Touch Potential
Under normal circumstances, unbalanced currents can rise to the neutral to earth voltage. While this is rarely dangerous, it can cause electric shocks. During a short-circuit caused by electrical faults, Step and Touch Potential pose a far greater danger, as the high voltages produced in the vicinity of the ground are typically fatal and, if not, cause a severe injury and long-lasting health consequences to the personnel. Currents that contact the ground may also potentially cause hazards to the people and damage the equipment outside the substations.
Reducing the risks of Step and Touch Potential
Electrical shock incidents frequently occur in the workplace, according to a report issued by the Australian Institute of Health and Welfare in 2018. 73% of all reported injury cases involve exposure to electric current, and almost half (47%) of the cases at work require hospitalisation. A safe work environment begins with understanding control procedures when electrical faults occur. We address the Touch and Step Potential hazards with several approaches, including the use of Electrical Hazard (EH) rated footwear and the use of more resistive surface layers.
Electrical Hazard (EH) rated footwear
One of the most direct and efficient features of reducing or preventing electrical shock is wearing insulating footwear such as EH rated boots or any footwear manufactured to a suitable standard. These footwears typically come in non-conductive materials for the best possible secondary source of protection from the hazardous shock while standing on the ground. In the event of a fault condition, EH rated footwear offers a safe environment to protect personnel in close proximity to those grounded facilities.
Equipotential zones installation
Equipotential zones are work zones where the personnel are protected from Step and Touch Potential hazards. The personnel are protected because there is a near-identical state of electrical potential between any two points on the body. The zone can be created by connecting a metal mat or ground mesh to a grounded object. These metal mats or ground mesh are usually connected to buried ground rods to increase contact with the earth and minimise grid impedance. In some cases, a grounding grid may be used to increase grid voltage equality. However, it will not protect personnel entirely or partially outside the protected area. It is also possible to minimise the voltage between objects by bonding conductive objects in the immediate work area. However, bonding to an object outside the work area may increase the touch voltage.
Resistive surface layers
Another technique used to reduce electric shocks is to add more resistive surface layers by adding crushed rocks such as blue metal or asphalt to the bottom of a substation to provide an insulation layer. It limits the amount of current flowing through a person and into the earth while providing a designated walkway.
Purpose of Earthing Study
An earthing study revolves around electrical safety and is often carried out to assess a substation's performance so that the current fault does not affect the continuity of service. In other words, we want to ensure that the fault conditions do not exceed safe limits, preventing electric shock and equipment damage. Here are the essential assessment requirements for an earthing study:
Earth fault clearing time
To determine the acceptance criteria
Earth conductor size
To determine the earth grid resistance
Earth grid resistance
To calculate earth potential rise, step and touch potential
Table 1: Essential requirements for an earthing study
The following is a worked example of an earthing study conducted to examine the performance of Substation X's earth grid concerning the addition of a new conveyor substation. Please note that our dealings with clients are often private and sensitive so some elements of the studies have been anonymised or changed to protect privacy.
Assessment of the earth fault clearing time
This study refers to the AS2067 Standard and Argon safety assessment to determine the acceptance criteria for step and touch potential limits.
a) AS2067 Standard
As shown in Figure 7, the touch voltage limit is based on the AS2067 Standard. The upper line (TDMEN) represents earth faults occurring at one per year with a fault duration of less than 500ms. Based on our calculation, the overall earth fault clearance time for a normal earth fault is 300ms. Once we knew the fault clearance time, we can determine the permissible Step and Touch voltage based on the calculation. Therefore the touch voltage limit is determined to be 450V.
b) ARGON Safety Assessment
The figures below show the criteria required to calculate the step and touch potential limits. ENA EG-0 defines individual risk that results in exposure to a single individual, while the societal risk is the occurrence of a hazard which results in simultaneous exposure for multiple people. The societal voltage limits are used as the acceptance criteria for this study's step and touch potential calculations.
Next, we take the result of a soil resistivity measurement from ETAP to determine the soil resistivity at various depths. Understanding how the soil resistivity varies with depth is important to accurately design and analyse ground protection.
A summary of the testing results used for the earthing study is shown below.
Earthing grid design and modelling
We also refer to the earthing design and grid for Substation X and ensure that the material details are required to comply with the Australian Standards AS2067 and AS/NZS 3000.
Earthing study results
Based on the results below, it is apparent that the calculations for the Step and Touch voltages for faults do not exceed the permissible voltages, as per AS2067 Standard and Argon safety assessment.
Individual Rod Resistance
Every substation is unique, and therefore, it is essential to recognise suitable engineering earthing requirements, such as those in the IEEE 80 Guide for Safety in AC Substation Grounding. The essential assessments mentioned above comply with the ENA EG1, AS2067 and AS/NZ3000 Standards as the driving criterion for a safety limit for step and touch voltages when designing and assessing earthing systems.
At Divergent Engineering, we partner with our clients to deliver compliant engineering solutions focusing on electrical safety to provide a safe working environment. We can prepare an earthing system design following the preliminary earthing studies. We also provide power system studies, installation, testing and commissioning for a safe working environment. Click here for more information on earthing studies provided by Divergent Engineering or contact our team at Contact@diveng.com.au or 0861156370.