Solution Summary: Temporary Protective Grounds
Grounding sets can be considered as engineering controls because they provide a short circuit and an overcurrent device to trip and de-energize the system.
According to OSHA, whenever lightning or other events energize a previously de-energized overhead power line, “ground sets” can protect workers by providing a faster path for electrical current to reach the ground (see Grounding for employee protection). The main part of ground sets are the cables of proper size and material that can carry large amounts of electricity to the ground. These cables could be connected to power lines by means of clamps. Finally, ferrules are used to properly connect cables to clamps. One should know that, although grounding sets are the main part of grounding procedure, there are other parts such as grounding brackets and rods that are required to complete the path from power lines to ground and safely direct electrical current to ground.
The Hierarchy of Control (HOC) is a well-known framework to evaluate the performance of safety solutions in Occupational Safety and Health (OSH) research. HOC consists of 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”. Grounding is the “process of removing excess of charge on an object by means of transfer of electrons between it and another object of substantial size. The ground is capable of transferring electrons to or receiving electrons from a charged object in order to neutralize that object”.
To protect linemen from electric shocks near powerlines, several methods and devices have been designed to discharge induced voltage to the ground through a system of conductors. Fig 1 is a sketch of the procedures to be followed while grounding an overhead powerline.
Fig 1. Power line grounding procedure
Personal grounds are defined as “portable devices that temporarily connect a de-energized conductor or pieces of equipment to electrical grounds prior to beginning the work”. Line workers usually carry sets of personal grounds on their trucks, based on the conductor sizes they are servicing. Medium voltage is normally used for distribution in urban and rural areas, limited to 69 kV. Conductor sizes range from 12 mm2 (#6 American Wire Gauge – AWG or 0.47”) to 750 mm2 (or 29.5”), which is normally used for high voltage overhead power transmission lines.
While personal protective grounds can enhance workers’ safety, their application requires a set of particular skills. They are furnished in grades, according to the ASTM standard, and these grades are calculated based on short circuit properties. An experienced person should confirm the compatibility of chosen sets with circuit size and standards based on the intended use and worksite environment.
Protective ground sets are composed mainly of 1) grounding cables, 2) clamps, and 3) ferrules. They can be found separately or assembled in the market.
Grounding cables have flexible elastomer or thermo-plastic jackets, according to ASTM F 855, primarily for mechanical protection of the conductor they cover. Either aluminum or copper ferrules may be used with copper cables.
IMPORTANT: Cables shall be of continuous length. Splices are not permitted.
Fig 2. Grounding Cable (Source: http://www.hubbellpowersystems.com)
Clamps are used to connect grounding cables to the conductor and the neutral. They are graded from 1 to 7 (according to ASTM F 855) and also classified into classes (A and B), according to the contact surface.
- Class A: smooth contact surfaces
- Class B: serrated surfaces
Fig 3. Types of Clamps (Source: http://www.hubbellpowersystems.com)
These components attach the stranded grounding cables to the clamps. They are classified in grades and according to the size of the cable to which they will be connected.
The products found in the market are sized in 4 wire types, according to the American Wire Gauge (2, 1/0, 2/0 and 4/0). The properties of each type of wire are discussed in the cables section.
Fig 4. Ferrules (Source: http://www.hubbellpowersystems.com)
Several companies also offer assembled grounding sets. They are composed of all the materials described: clamps, ferrules, and cables.
Fig 5. Grounding Set (Source: http://www.hubbellpowersystems.com)
Linemen are exposed to several risks while working. The following scenarios may be fatal for these workers:
- Induced voltage from adjacent energized lines
- Fault-current feed-over from adjacent lines
- Lightning strikes anywhere on the circuit
- Switching equipment malfunction or human error
- Accident initiated contact with adjacent lines
All these scenarios create one big risk for workers: the risk of electrical shock or electrocution. The effects of electric current on the human body are shown in Fig 6.
Fig 6. Effect of electricity on human body (Source: https://www.usbr.gov)
How Risks are Reduced:
Temporary protective grounding provides adequate protection against electrical shock that can cause death or injury to linemen working on de-energized lines. Whenever any of the aforementioned scenarios (see ‘Risk addressed’ section) occur, electrical current seeks the fastest path to the ground. Temporary protective grounding (a set of low resistance conductors) can provide this path before electrical current reaches workers’ body.
The Bureau of Reclamation has enumerated the ways in which ground sets reduce risks to linemen:
- Limiting body contact or exposure voltages at the worksite to a safe value if the lines are accidentally energized from any source of hazardous energy
- Maximizing personal safety while working on de-energized high-voltage equipment through the use of appropriate protective grounding, procedures and training
- Providing a parallel circuit where a low-resistance grounding cable is in parallel with the worker to shunt current away from the body.
IMPORTANT: While the idea of creating a low resistance path is simple, many details are involved in assembling protective groundings on job sites and only trained and qualified workers should apply and remove temporary personal protective groundings.
Effects on Productivity:
The assembled personal protective grounding equipment, sized correctly, can increase productivity because the lineman won’t need to mount the equipment every time he needs to use it.
Also, because it eliminates the risks of electrical shock, the personal protective grounding equipment may increase productivity because of the reduction of accidents.
The standard specifications for temporary protective grounds (ASTM F855-15) cover the equipment making up the temporary grounding system used on de-energized electric power lines, electric supply stations and equipment. This document offers complete instructions and regulations governing in detail the correct use and maintenance of protective grounding equipment (clamps, ferrules, cables and completely assembled protective grounds).
The OSHA Regulation Part 1926 – Safety and Health Regulations for Construction – Subpart V provides minimum requirements for safety and health related to the construction of electric transmission and distribution lines and equipment. The main topics discussed are: tools and protective equipment, grounding for protection of employees, overhead and underground lines, construction in energized substations, lineman’s body belts, safety straps and lanyards.
The IEEE (Institute of Electrical and Electronics Engineers) Standard 978 provides detailed recommendations for in-service maintenance, inspection and electrical testing of live-line tools.
The Facilities Instructions, Standards, & Techniques (FIST) series provided by the Bureau of Reclamation relates to the operation and maintenance of hydroelectric equipment. Reclamation no longer sells publications, but most of the Bureau’s significant scientific, technical and engineering works are available from the National Technical Information Service (NTIS).
- The FIST Volume 3-29 (Facilities Instructions, Standards, and Techniques) also provides procedures for operating equipment near energized equipment, hot-stick work, live-line bare head work, and insulated tools for low voltage work. It is important to emphasize that personnel using techniques covered in this FIST on equipment rated 600-volts and above shall be trained in power system clearance procedures, FIST Volume 1 - 1, and, depending on the type and use as detailed in subsequent sections, specialized training, testing and certification may be required.
- The FIST Volume 5-1 (Personal Protective Grounding for Electric Power Facilities and Power Lines) establishes clear and consistent instructions and procedures for temporary grounding of de-energized and isolated high-voltage equipment (over 600 volts) for the purpose of bare-hand contact. The focus is to enable workers to perform their duties with maximum confidence and safety.
Based on ASTM F 855, double-point grounding on de-energized distribution lines, in addition to a personal ground at the worksite, is recommended. On a system without a neutral, down leads should be connected to screw ground rods installed at least 20 feet from all structures and barricaded. The single-point grounding method is acceptable only for maintenance tasks during which grounds need not be replaced (Source: http://www.hubbellpowersystems.com).
The practice of grounding only the phase being worked is possible only when adequate phase-to-phase clearances permit.
In general, the process of grounding powerlines has three steps.
Step One: Testing
With a test instrument, confirm the circuit to be worked has been de-energized before ground sets are applied.
Step Two: Cleaning
For a good connection, scrub oxides and contaminants from conductor, buswork or lattice contact points. Wire brushes make this easy. Serrated-jaw clamps also aid by penetrating surface contaminants.
Step Three: Connecting
Insulated clamp sticks are the proper tools to apply grounding clamps. To help achieve correct connection tightness, various clamp stick lengths and styles are available.
For safety purposes, each utility needs to re-examine its temporary grounding practices on a regular basis to ensure:
- clamp designs are specific to each application
- cable is sized for fault-current potential and minimum-slack lengths
- how construction affects placement of grounds
- work procedures have been completed as outlined above
- each grounding set has been Inspected and tested.
For the last item, see Grounding-Set Tester in Tester solutions.