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. Good safety professionals and trainers have to go beyond the OSHA and MSHA regulatory text to completely understand the rules. That is where preambles to the standards, interpretations, CPLs and consensus standards are needed.
In 29 CFR 1910.269 Appendix E, OSHA lists consensus standards that, as the introduction to Appendix E states, can be helpful in understanding and complying with the requirements of 1910.269. One of the referenced standards is ASTM F855-09. ASTM F855, “Standard Specifications for Temporary Protective Grounds to Be Used on De-energized Electric Power Lines and Equipment,” is a manufacturing specification. Procedures for protective grounding are outlined in IEEE Standard 1048, “IEEE Guide for Protective Grounding of Power Lines.”
Under OSHA and ANSI standards, the employer is required to:
• Determine the fault levels that workers may be exposed to.
• Determine what electrical characteristics are required for protective grounds to protect workers at those fault levels.
• Determine what physical characteristics are required of the grounding equipment.
• Determine what methods shall be employed to provide the best protection of employees using the selected equipment.
• Provide training on the procedures and applications of the selected grounding equipment.
Every safety professional who deals with protective grounding should have a copy of ASTM F855. 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.
Grounds assembled by the employer require continuity integrity testing and inspection in order to comply with 1910.269(n)(4)(ii), which states that “[p]rotective grounds shall have an impedance low enough to cause immediate operation of protective devices in case of accidental energizing of the lines or equipment,” and ANSI C2 411(C)(1), which states that “[p]rotective devices and equipment shall be inspected or tested to ensure that they are in safe working condition.” IEEE 1048 11.2.2 describes recommended methods for performing both millivolt and high-current tests on ground assemblies.
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.
The Construction of ASTM F855
The ASTM F855 standard has four parts:
1. Clamps for temporary protective grounds.
2. Ferrules for temporary protective grounds.
3. Cables for temporary protective grounds.
4. Construction of complete assemblies of protective grounds.
Clamps are rated according to three criteria categories in ASTM F855:
• By installation method as Type I, II or III.
• By strength and electrical requirements as Grade 1 to Grade 7.
• By internal jaw contact surface as Class A or Class B.
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 A and B Ground Clamps
Class A clamps have a smooth-finished internal jaw contact surface. 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. It may be accomplished using shrouded cable ferrules or by means of a separate cable clamp that is part of the ground clamp’s design. (See the “Ferrules” section of this article for a discussion of shrouded ferrules.) Strain relief may also be accomplished using the manufacturer’s heat shrink sleeves if the manufacturer has tested and listed the sleeves for strain relief application.
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 F855 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.
2. 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.
3. 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.
If a drilled and internally tapped ferrule is used, a bolt and lock nut are used to connect through the clamp’s drilled terminal into the ferrule.
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.
Similarity of Metals in Terminal Connections
There are no dissimilar metals restrictions in the ASTM F855 standard. 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.
Cable ferrules are classified by two criteria: Grade 1 to Grade 7 and Type I, III, IV, V and VI. 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. Threaded studs come in three sizes: 1/2-inch 13 UNC, 5/8-inch 11 UNC and 3/4-inch 10 UNC.
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.
Grounding cables are specified as Type I, II or III. The principal difference in each type is the temperature rating of the insulation and stranding of the conductor. Type I is elastomer 40 C to 90 C, minimum of 665 strands #30 or #34 AWG; Type II is elastomer 25 C to 90 C, 133 for #2 or 259 strands for 1/0 or larger; and Type III is thermoplastic 10 C to 60 C, minimum 665 strands of #30 AWG.
Type III cable’s upper temperature limit, like many other non-ASTM-rated cables, is lower than would be experienced in fault conditions and could burn under some fault conditions, producing toxic fumes.
Nonstandard Cable Use
ASTM F855 paragraph 32.4 allows, at the user’s discretion, the use of cables that are nonstandard. 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.
ASTM F855 Appendix X1.1 recognizes that much of the grounding cable in service has been constructed for welding use. 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.
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. Continuous current ratings for ASTM-rated grounding-type cable is as follows:
• #2 – 200 amps
• 1/0 – 250 amps
• 2/0 – 300 amps
• 3/0 – 350 amps
• 4/0 – 400 amps
• 250 kcmil – 450 amps
• 350 kcmil – 550 amps
The standard includes ultimate current-carrying capabilities for cables derived from EPRI RP2446. Those capacities are rated/derated in worst-case conditions determined by the X/R factor. You can find a nontechnical explanation of the X/R factor below.
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-, 15-, 30- and 60-cycle durations under the derated scenarios.
Over the years the utility industry has recognized that personal protective grounding has saved many lives. That performance is improved by the effective determination of the workers’ exposure, application of procedures to accommodate the exposure, and proper selection and maintenance of protective equipment. Good information and effective training are key to maintenance and continued improvement in this high level of workplace safety performance.
Correction:The original practical guide to ASTM F855 was included in the 1990 version. For this article, we revised the guidance according to the 1997 revision of F855. In the 1990 edition, F855 did allow heat shrink for strain relief if approved by the shrink and ferrule manufacturer. However, it appears that F855 removed the acceptance of a compatible shrink installed over cable and ferrule as a means of cable restraint at least since the 1997 edition.
Additionally, a Salisbury by Honeywell representative pointed out to us that F855 Table 1 no longer includes the Ultimate 6 cycle rating and that ASTM F2249, “Standard Specification for In-Service Test Methods for Temporary Grounding Jumper Assemblies Used on De-Energized Electric Power Lines and Equipment,” is another excellent resource for safety and equipment maintenance professionals. We agree.
Calculating Fault Current
Calculation of fault current is best accomplished through the services of an engineer experienced in utility systems. Often the assumption is made that the worst-case fault is the current delivered at the substation’s transformer. 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. Infinite buss calculation is a simple Ohm’s law calculation of voltage divided by the circuit resistance measured in impedance.
There are limitations on the acceptability of the infinite buss theory, principally the effect of reactance (X) and resistance (R). This is known as the X/R of the electrical system between the source and the point of the short circuit.
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-, 15-, 30- and 60-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.
Fault current asymmetry is quantified as an X/R ratio or X/R factor. There are several methods of arriving at the total asymmetry of the fault current. In simple terms, the X/R factor, when known, becomes the basis for a multiplier that expresses the highest current available at the short-circuit location.
The X/R factor raises the fault current to the maximum thermal and mechanical stress that can be delivered by the electrical system. This maximum available fault current must be calculated to accurately select the required grounding components.
The Importance of Engineering Assistance in Ground Selection
Using a simple infinite buss calculation, a 40-MVA power transformer with an impedance of 4.775 percent is calculated to be able to deliver a fault current of 24,271 amps. If relying on this basic method for ground selection, a crew might select a Grade 3 ground with a 2/0 cable, rated at 27,000 amps.
Unknown to the crew, the circuit in question has a high reactive component resulting in an X/R ratio of 24.9. As a result of the high X/R ratio, the fault current is 38,809 amps. Instead of a Grade 3 clamp with 2/0 cable, the new information indicates a Grade 5 clamp with 4/0 cable is required.
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