Establishing an equipotential zone at the work site—thereby protecting linemen working aloft and linemen, groundmen and equipment operators working on the ground during transmission and distribution (T&D) construction and maintenance operations—usually requires the use of a temporary grounding jumper assembly (TGJA). TGJAs appear to be very simple devices but are in fact very complex, both electrically and mechanically. Like any safety equipment, TGJAs must be inspected and tested regularly to ensure they are physically intact and capable of protecting workers when needed.
ASTM international TGJA standards
There are two American Society for Testing and Materials (ASTM) International standards that address TGJA specification and testing. These two standards are as follows:
F 855 04 Standard Specifications for Temporary Protective Grounds to Be Used on De-Energized Electric Power Lines and Equipment
F 2249 03 Standard Specification for In-Service Test Methods for Temporary Grounding Jumper Assemblies Used on De-Energized Electric Power Lines and Equipment
ASTM F 855 provides the requirements for the materials and equipment that make up TGJAs and their manufacture. This standard is not only used by TGJA manufacturers but also the users of manufactured TGJAs, including electrical contracting firms and utilities. The tables in ASTM F 855 are used in the field to determine the minimum size TGJA conductor based on the predicted work site fault characteristics that include fault current, clearing time and X/R ratio. While ASTM F 855 does provide specifications for inspection and testing of TGJAs by the manufacturer, it does not provide any direction for inspection and testing of TGJAs once they have been put into service in the field. In fact, ASTM F 855 paragraph 1.3 specifically states that the uses and maintenance of this equipment is beyond the scope of these specifications.
ASTM F 2249 covers the visual inspection prior to testing and the electrical testing of TGJAs. This standard is not as well known in the industry as ASTM F 855. It provides a discussion of the various methods that can be used to test TGJAs. It also offers pretesting preparation requirements, electrical testing requirements, pass/fail criteria, and post-testing cleaning and reconditioning requirements prior to placing the TGJA back in service. It should be noted that ASTM F 2249 only addresses in-service TGJA test methods and does not cover the application, care, use or maintenance of TGJAs as stated in paragraph 1.4.
A TGJA is an assembly, consisting of a ground cable, ferrules and ground clamps, that is manufactured in accordance with ASTM F 855. A more precise definition that embodies the purpose of a TGJA is provided in paragraph 3.1.1 of ASTM F 2249, which states it is, “Grounding cable with connectors and ground clamps attached, also called a ‘grounding jumper’ or a ‘protective ground assembly’ installed temporarily on de-energized electric power circuits for the purpose of potential equalization and to conduct a short circuit current for a specified duration (time).”
Potential equalization is achieved with TGJAs by using them to provide a low-impedance parallel path for fault current to flow around the worker. This low-impedance parallel path limits the voltage across and subsequent current flow through the worker’s body to a safe level. In its simplest form, an equipotential zone can be established using a TGJA by creating a low-impedance shunt across the worker to protect him or her from hazardous touch and step voltages resulting from induced voltages, contact with a live conductor, or accidental reenergization. In a more complex job site arrangement, an equipotential zone may be established using TGJAs to bond together structures, conductors, permanent and temporary grounds, personal and equipment ground grids, construction equipment, and other conductive objects that could become energized in the work zone. In either a simple or complex job site equipotential arrangement, TGJAs play a critical role of protecting workers aloft and on the ground by bonding conductive objects together to eliminate dangerous voltages between them and provide a path for fault current to flow. As a result, it is critical that in-service TGJAs be inspected regularly and electrically tested as needed to ensure that they have not been damaged by use or that their electrical performance has not been degraded by corrosion.
TGJA visual inspection
TGJAs should be visually inspected both right after they have been used and before they are going to be used. Inspecting TGJAs right after use ensures that the TGJA has not been damaged in use and is suitable for future use. The crew using a TGJA may know that a cable has potentially been damaged during use and where the damage may have occurred, making it easier for that crew to spot a problem through visual inspection than the follow-on crew that may not know of the potential damage. Similarly, TGJAs should always be thoroughly visually inspected prior to use to ensure that they have not been physically damaged or electrically degraded in storage or transit to the job site. Also, there is no guarantee that the crew that previously used the TGJA inspected it for damage at the end of the job or that the person performing the visual inspection saw everything.
Section 11.1a of the Institute of Electrical and Electronics Engineers (IEEE) Standard 1048-2003, “IEEE Guide for Protective Grounding of Power Lines,” recommends that cables and clamps be visually inspected for the following indications of damage or deterioration:
• Cracks or broken cable insulation
• Broken cable strands particularly in the vicinity of the ferrule
• Clamps that have sharp edges, cracks or other defects
• Smooth clamp operation that is not excessively loose
• Clean and tight connection between the ferrule and clamp In addition to those indicators listed in IEEE 1048, ASTM 2249 also includes the following indications of damage or deterioration that should be looked for:
• Badly smashed or flattened cable
• Swollen cable jacket or soft spots indicating internal corrosion
• Cable strands with black deposits on them
If any of the above conditions are noted, the TGJA or the cable and clamps that make it up should be permanently marked, tagged or destroyed to prevent reuse before the damaged assembly or component is repaired.
TGJA electrical testing
Electrical testing of TGJAs can be used to detect damage or deterioration that may not be able to be detected visually or may not be readily apparent visually. There is no standard for when electrical testing of TGJAs should be performed. Electrical contracting firms and utilities that use TGJAs usually establish their own requirements for electrical testing, and these requirements vary throughout the industry. Maintenance intervals for TGJAs, which includes both visual inspection and electrical testing, should depend on exposure, manner of use, individual company policy and field inspection per section 11.1 of IEEE Standard 1048.
The electrical contracting firm should establish a policy for TGJA electrical testing that includes both a test method and testing interval. As noted above, the test interval should be based on the firm’s work, TGJA use and experience. Whatever interval is used, the TGJA should be tagged to indicate its last electrical testing and when it is due for its next testing. Crews should be aware of the tags and encouraged to check the tags as part of their visual inspection procedure after and before use to ensure the firm’s testing interval has been exceeded. TGJAs close to the prescribed interval should be tagged and sent to the electrical contracting firm’s shop for testing prior to further use.
In addition to performing TGJA electrical testing at regular intervals, the electrical contracting firm should also perform electrical testing on any TGJA tagged as possibly damaged or deteriorated during a visual inspection and prior to putting a TGJA back into use after repair of maintenance.
TGJA electrical test methods
The two most common methods of TGJA electrical testing are covered in ASTM F 2249 and are direct current (DC) resistance measurement and alternating current (AC) impedance measurement.
A number of manufacturers produce test equipment for in-service TGJA electrical testing that uses either the DC resistance measurement method or the AC impedance measurement method. Both methods are recognized as acceptable by ASTM F 2249, and each manufacturer that produces TGJA test equipment using resistance or impedance has selected either the DC resistance method or AC impedance method as the basis for their equipment. Testing procedures and pass/fail criteria are different for the two methods because the DC method measures the resistance of the TGJA, and the AC method measures impedance of the TGJA, which includes both resistance and inductive reactance. The electrical contracting firm should choose one testing method, which can usually be accomplished by standardizing on one manufacturer’s test equipment. This will not only ensure consistent test results company-wide that can be tracked over time but also reduce the possibility of bad tests due to the use of improper testing procedures or the need for training on different test equipment.
Proper TGJA application and uses
Inspection and testing is no substitute for proper TGJA application and use in the field. Selection of TGJA cable and clamp types and ratings should be based on the job site physical and electrical conditions and the work to be performed. Clamps should be installed in accordance with the manufacturer’s torque recommendations with proper tools. At the end of the job, TGJA components should be cleaned as required and carefully stored to protect them from physical damage or corrosion prior to their next use.
This article is the result of a research project titled Transmission & Distribution Work Site Shock Protection that was sponsored by ELECTRI International. The author would like to thank ELECTRI- International for its support.
GLAVINICH is an associate professor in the Department of Civil, Environmental and Architectural Engineering at the University of Kansas. He can be reached at 785.864.3435 and firstname.lastname@example.org.