Solution Summary: Grounding Mat
A grounding mat is an engineering control that reduces the risk of electrocution for operators who work on the ground. Consisting of a flexible sheet (vinyl/polyester) and a grid of conductors (e.g. copper wire) connected to a grounded object (i.e. a ground rod), this solution creates an equipotential zone that protects workers who are standing on it from “step and touch potentials.” The Hierarchy of Control (HOC), a well-known framework to evaluate the performance of safety solutions in Occupational Safety and Health (OSH) research (Wakefield et al. 2014), consists of control measures with five levels of effectiveness: (1) Elimination, (2) Substitution, (3) Engineering Controls, (4) Administrative Controls, (5) Personal Protective Equipment; with elimination being the most effective. At the third level, engineering controls “use safeguarding technology to place a barrier to keep a hazard from reaching workers” (Zhao et al. 2015). Grounding mats can be considered an engineering control solution due to their ability to keep electrical current away from workers’ bodies.
As illustrated in Figure 1, a grounding mat provides an equipotential zone against electrical shock through a flexible sheet with a continuous conductive wire attached to a ground point on electrical equipment (McKelvy 2002). Workers who operate on the ground near de-energized power lines, pad-mounted transformers and switches, lift trucks, and mechanical equipment face possible hazards from step and touch potentials from induced voltage, lightning, or unexpected re-energization. Touch potential is defined as the voltage difference between the energized grounded object and the feet of a person who touches that object. Step potential is the voltage difference between the energized grounded object and the foot of a person who is near an energized grounded object (Figure 2).
Figure 1. Grounding mat
Figure 2: Step and touch potential
While touch potential (which is almost equal to the full voltage of object) is likely to result in more serious injury, a worker could be injured simply by being near an energized object due to step potential. Creating an equipotential area near electrical sources in this situation is the best way to protect workers. According to OSHA, in an equipotential zone, the electrical potential of an energized object and that of the worker are approximately identical. As shown is Figure 3, the grounding mat is a high-ampacity area (12) consisting of aluminum or copper braid (10) in a crosshatched pattern within the vinyl/polyester fabric (11). To ensure that the mat is of one potential, all braids are connected to the same source of power. On the other end, braids are connected to a node at the edge of the mat that should be connected to a grounding conductor (i.e., grounding rod) at the construction site. While working with grounded equipment connected to electricity, workers should stand on a ground mat to protect themselves.
Figure 3: Grounding mat diagram
When working with grounded live electrical equipment, line workers and electricians run the risk of electrocution due to human error, a ground fault, or equipment failure. Even when the equipment or powerline is de-energized, the risk of electric shock remains due to induced voltage or lightning. Typically, this happens when electrical current flows directly through the body while the worker is standing on the ground. This situation represents a risk of electrical shock that can cause severe injuries to a construction worker.
How Risks are Reduced:
Grounding mats reduce the risk of electric shock from step and touch potentials by providing an equipotential zone for electrical workers. Figure 4 illustrates how a grounding mat directs the electrical current away from the worker. Without a grounding mat (Figure 4, top), if for any reason the power line or electrical equipment becomes energized, electrical current would flow directly through the worker’s body, causing serious injuries or fatality. On the other hand, when the worker is standing on a grounding mat (Figure 4, down), the electrical current would flow through the mat and protect the worker from potential injuries.
Figure 4: Grounding mat diagram
Effects on Productivity:
Some features of grounding mats could have effects on worker productivity. For example, there are different types of mats available for use during rain, snow and ice conditions which can improve productivity because they are also slip resistant.
- The grounding mat must be kept clean and protected, to avoid corrosion, wear and tear.
- It is possible to connect two or more ground mats in series to expand protection to a broader area.
- The grounding mat must be bonded to the equipment being worked on with a safe and reliable ground cable assembly.
- Grounding mats are easy to clean (a standard hose pressure can be used). But it is important to let them dry completely before storing them again.
- The grounding mat offers a temporary protection measure while performing work with equipment connected to electricity.
Behzad Esmaeili, Ph.D. - University of Nebraska- Lincoln
Pouya Gholizadeh - University of Nebraska- Lincoln
Sogand Hasanzadeh - University of Nebraska- Lincoln
Ana Clara Carvalho - University of Nebraska- Lincoln
Kolak J. (2010). The basics of personal protective grounding. http://ecmweb.com/ops-amp-maintenance/basics-personal-protective-grounding (05/05/2016).
McKelvy, M. D. (2002). Grounding mat. https://www.google.com/patents/US6477027.
Wakefield, R., Lingard, H., Blismas, N., Pirzadeh, P., Kleiner, B., Mills, T., McCoy, A., and Saunders, L. (2014). Construction hazard prevention: The need to integrate process knowledge into product design. Achieving Sustainable Construction Health and Safety.
- Cut, shape, place and install conduit and wire trays
- Install and maintain electrical control systems
- Install and maintain fixtures, lights, motors and pumps
- Install and maintain temporary wiring systems on construction sites
- Install interior wiring systems
- Pull wire through conduits
- Splice and connect wiring systems
- Work with or remove old wiring
- Residential Construction
- Install electrical systems
Hubbel Power Systems, Inc.
To obtain information, visit http://www.hubbellpowersystems.com/lineman/grounding/ground-grids/ground-grid.asp or contact 1-573-682-5521
Kri-Tech Products Limited
To obtain information, visit http://kri-tech.net or contact 1-403-788-3883
Hastings Fiber Glass Products
To obtain information, visit http://www.hfgp.com/fb-catalog/#419 or contact 1-269-945-9541