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I define safety as identifying and managing hazards to prevent incidents. That is accomplished using a broad array of tools and rules for the employer and workforce. That is where preambles to the standards, interpretations, CPLs and consensus standards are needed. In 29 CFR The intent of the standard is to provide specifications for manufacturers.
The value to safety professionals is that the standard clearly identifies what clamps do, how they are rated, and which ferrule and cable they are to be mated with. About Testing Grounds assembled by the employer require continuity integrity testing and inspection in order to comply with If a protective ground installed in an equipotential arrangement has too much resistance, it will not protect workers.
Resistance in the protective ground increases current across the worker in a fault. If the ground is improperly rated and fails before the system protection opens, the fault current will take any other available path, which includes personnel in contact with the faulted conductor. Clamps for temporary protective grounds. Ferrules for temporary protective grounds. Cables for temporary protective grounds. Construction of complete assemblies of protective grounds.
Clamp types I and II are for installation on conductors that have been de-energized and tested. Type I has a hot-stick eye and Type II comes fitted with a hot stick. Type III is usually installed using a formed T-handle and is intended as a first connection to a system neutral, pole bond, or grounded station or apparatus steel that does not require use of a hot stick. Grade 1 to Grade 7 Clamps Clamp grades 1 through 7 establish minimum requirements for physical strength and electrical characteristics in withstand rating, ultimate rating and continuous current, strength yield and ultimate strength.
The withstand rating is the current ratings the clamp can sustain in kA at 15 and 30 cycles without damaging the clamp. It is the rating used to determine the proper application of the clamp to the anticipated fault current. The ultimate rating is the current ratings the clamp can sustain in kA at 6, 15, 30 and 60 cycles.
The clamp must carry the rated current, but it is not required to survive the exposure to the point of being reused. The continuous current rating is the current the clamp must conduct without overheating to failure. Class B clamps have serrations or crosshatch patterns on the contact surface intended to abrade or bite through corrosion products on the surface of the conductor. Connections for Cable to Clamp There are two connections required for cable to clamp — mechanical and electrical.
Mechanical connections secure the cable to the clamp to prevent stress movement damage to the cable, also known as strain relief. All cables must have strain relief. Electrical Connection The electrical connection on the grounding clamp is known as the terminal.
All clamps require a ferrule-to-terminal connection. It is not permissible to make a stranded conductor connection directly to a clamp. There are nine ASTM termination styles designated for ground clamps. While not part of the F standard, the nine styles can be unofficially grouped into three basic terminal types: 1. Pressure-type terminal connections use a plain eye bolt inserted through a hole in the clamp.
A plain stud-type ferrule is inserted in the eye. The nut on the bolt end of the eye is tightened, trapping the ferrule against the clamp. Threaded female terminal connections accept externally threaded ferrule studs.
The threaded ferrule comes with a spring washer and nut. The ferrule is screwed into the clamp terminal connector and locked into place with a backup spring washer and nut. Drilled terminal connections have no internal threads. They use either an externally threaded ferrule or a ferrule that is drilled and internally tapped.
For externally threaded ferrules, a spring washer and two nuts are used. The threaded ferrule is inserted through the drilled terminal. A nut on either side of the terminal is tightened to trap the ferrule in place.
Ferrules Cable ferrules are sleeves compressed onto the cable end to protect the stranding of the cable when installed in a connecting terminal. To meet ASTM standards, temporary personal protective grounds must use multistranded flexible cable conductor installed in a rated clamp using protective cable ferrules. However, when assembling grounds, care should be taken to maintain conventional similarity of metals in the electrical connection to minimize connection corrosion. Plain copper ferrules are usually used with bronze clamps.
Tinned copper ferrules are commonly used with bronze or aluminum clamps, or with aluminum clamps that use bronze pressure terminal connectors.
There are no restrictions for using either copper or aluminum ferrules on high-flexibility ASTM-compliant copper grounding cable. At present, there is no Type II specification. Ferrules are rated Grade 1 to Grade 7 in withstand, ultimate and continuous current just as clamp grades 1 to 7, making electrical ratings easy to match. The conductor size for each grade of ferrule also corresponds to the minimum single cable size specified for the clamp of the equivalent grade rating.
The nonthreaded stud connector of an ASTM-rated ferrule is always half-diameter size for each grade even though each grade of ferrule corresponds to a specific cable size. Ferrule Types Ferrules are categorized according to ferrule-to-clamp connection method and by either shrouded or nonshrouded.
Shrouded ferrules have a two-part compression sleeve at one end of the ferrule. The narrow part of the sleeve is the cable connector that is crimped onto the conductor strands. Extending past the conductor sleeve is a wider sleeve called the shroud, which extends down from the cable connector over the cable insulation where it is also crimped as a method of strain relief. Shrouded-type ferrules are designed to be used with ASTM-type grounding cable. Other highly flexible cable may be used for grounding assemblies.
If the cable is not compliant with the ASTM standard, the insulation may be too thick to fit under the ferrule shroud. The principal difference in each type is the temperature rating of the insulation and stranding of the conductor. Most manufacturers will not use nonstandard cables in delivered assemblies.
The concerns when considering non-ASTM-listed types of cable are insulation performance, proper stranding, quality of the strand lay and alloying of the copper strand materials in the conductor, all of which affect performance in a fault. Most of the cables in the market are constructed to ASTM standards and have performed well in utility use. In addition, the paragraph notes that many of these cables have been tested to the ASTM standard and have been found to meet the requirements of the standard.
Cable Capacity The current ratings for grounding cables are higher than standard service cable ratings because of the specific application, lengths of cable in use and expected duration of applied currents. With the exception of Grade 7 in the ultimate category, all grades of clamps exceed the matching grade ground cable fault current ratings for the 6-, , and cycle durations under the derated scenarios.
Conclusion Over the years the utility industry has recognized that personal protective grounding has saved many lives. Good information and effective training are key to maintenance and continued improvement in this high level of workplace safety performance.
If you have comments about this article or a topic idea for a future issue, please contact Kate Wade at This email address is being protected from spambots. For this article, we revised the guidance according to the revision of F In the edition, F did allow heat shrink for strain relief if approved by the shrink and ferrule manufacturer.
However, it appears that F removed the acceptance of a compatible shrink installed over cable and ferrule as a means of cable restraint at least since the edition. We agree. Calculating Fault Current Calculation of fault current is best accomplished through the services of an engineer experienced in utility systems.
Such calculation also assumes that the resistance between the station and the point of a fault will further limit the amount of power delivered into the fault. This method is commonly referred to as the infinite buss theory. In the first few cycles of a short circuit, there is an asymmetrical factor in the current flow.
If described using a sine wave diagram, instead of the waves equally arcing above and below the line known as point zero, most of the wave would be offset above the zero line. In engineering terms this is known as DC offset. This DC offset asymmetry creates the initial high current in the first cycles of a short circuit. The DC component asymmetry drops off quickly because of I2r losses in the circuit.
These initial fault current levels are the basis for the differences in the 6-, , and cycle ratings of the grounding system components. Over the duration of the fault, the components heat up and the ratings of the components decrease. In the same way, the initial fault current is high because of the DC component asymmetry, but drops off quickly due to I2r loss.
There are several methods of arriving at the total asymmetry of the fault current. This maximum available fault current must be calculated to accurately select the required grounding components. After 25 years as a transmission-distribution lineman and foreman, Jim Vaughn, CUSP, has devoted the last 20 years to safety and training. A noted author, trainer and lecturer, he is a senior consultant for the Institute for Safety in Powerline Construction.
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Temporary But Required
Historical Version s - view previous versions of standard. Work Item s - proposed revisions of this standard. More F These specifications cover the equipment making up the temporary grounding system used on de-energized electric power lines, electric supply stations, and equipment.
Train the Trainer 101: ASTM F855 Grounding Equipment Specs Made Simple
I define safety as identifying and managing hazards to prevent incidents. That is accomplished using a broad array of tools and rules for the employer and workforce. That is where preambles to the standards, interpretations, CPLs and consensus standards are needed. In 29 CFR The intent of the standard is to provide specifications for manufacturers. The value to safety professionals is that the standard clearly identifies what clamps do, how they are rated, and which ferrule and cable they are to be mated with. About Testing Grounds assembled by the employer require continuity integrity testing and inspection in order to comply with