|
U.S. Department of Labor | |
||||
| Occupational Safety & Health Administration | ||||||
 |
|
Standard Interpretations
09/27/2005 - Determining voltage ratings for electrical insulating equipment used during electrical power distribution and transmission work. |
Standard Interpretations - Table of Contents |
| • Standard Number: | 1910.137(b)(2)(i); 1910.269(a)(3); 1910.269(c); 1910.269(l)(1); 1910.269(l)(2); 1910.269(l)(3); 1910.269(l)(4); 1926.951 |
|
September 27, 2005 Mr. Edwin Hill International President International Brotherhood of Electrical Workers 1125 15th St., N.W. Washington, D.C. 20005 Dear Mr. Hill: This is in response to your correspondence dated November 10, 2004, to the Occupational Safety and Health Administration's (OSHA's) Directorate of Enforcement Programs. This letter constitutes OSHA's interpretation of only the requirements discussed and may not be applicable to any questions not delineated within your original correspondence. You requested information about the appropriate voltage ratings for electrical insulating equipment used during electrical power distribution and transmission work. The relevant requirements are found in the electrical protective devices standard (at 29 CFR 1910.137(b)(2)(i) and Table I-5) and in the electric power generation, transmission, and distribution standard (at 29 CFR 1910.269(l)(2)).1 As you are aware, the individual physical configurations of transmission and distribution systems and circuits are variable. This, and the fact that each worksite has unique physical features, means that there are different hazards at each work location. This underscores the importance of determining existing conditions, such as nominal line and equipment voltages, maximum transient voltage level, locations of circuits and equipment, and environmental conditions relative to safety. This assessment, which is required by 29 CFR 1910.269(a)(3), must be done before an employee starts working on or near electric lines and equipment. After the pre-work assessment is performed, 29 CFR 1910.269(c) requires the employee in charge to brief the involved employees on a variety of topics, including the hazards associated with the work, the work procedures involved, any special precautions that need to be taken, energy source controls, and the personal protective equipment required. Your letter requests information about when rubber insulating equipment must be rated for phase-to-phase exposure, as opposed to phase-to-ground-exposure.2 What types and ratings of insulation must be installed, as well as whether insulation is required at all, will depend on several factors:3
Phase-to-ground exposure, on the other hand, exists whenever it is foreseeable that an employee or the longest conductive object he or she may handle can simultaneously breach the electrical component of the MAD of an energized conductor and a grounded surface. The path to ground includes, but is not limited to, an uninsulated aerial lift, a neutral conductor, a protective ground, or a utility pole. In your correspondence you raised specific questions about the voltage ratings required for insulation in certain scenarios. We have paraphrased your question and scenarios below, and our answers follow. For purposes of our responses, it is assumed in all respects, other than the required voltage ratings, that these scenarios are compliant with 29 CFR 1910.137 and 29 CFR 1910.269, including, in particular, that: 1) the employee is qualified, as required by §1910.269(l)(1); 2) he or she is wearing appropriate insulating gloves, protectors, and insulating sleeves, if required, by §§1910.269(l)(2) and 1910.269(l)(3); and 3) he or she is working from a position where a slip or a shock will not bring him or her into contact with an exposed, uninsulated energized line or part as required by §1910.269(l)(4). Please note that we have addressed only the configurations you specifically mentioned in your correspondence, although there are other circuit configurations in common use. Additionally, while it is possible that conductor spacing in some of these scenarios is sufficient to preclude the possibility of phase-to-phase exposure, this is usually not the case in distribution-voltage circuits. Please keep in mind that the configuration of the energized components and the layout and condition of the surrounding terrain make each situation unique, and therefore it is impossible for us to provide conclusive answers to these hypothetical scenarios. It must be stressed that it is critical for the employer to perform the assessment required by 29 CFR 1910.269(a)(3) and that a pre-job briefing be conducted as required by 29 CFR 1910.269(c), before any work is performed. Question: For each of the following scenarios, what rating is required for the rubber insulating line hose and blankets that are used to cover the energized lines and equipment that the employee is not working on? Specifically, does the insulating equipment need to be rated for phase-to-phase or phase-to-ground exposure? Scenario #1: A worker is in an insulated aerial device and approaches a horizontally-configured multiphase primary distribution circuit mounted on a crossarm. The approach to the circuit is from a position horizontal to the outside conductor; the work will only be performed on that outside conductor; and no other circuits are in the work area. There will be no anticipated phase-to-phase exposure while performing the work. Response: If the employee is isolated from the other phase conductors, i.e., the conductors not being worked on, and he or she or the longest conductive object he or she may handle cannot come within the electrical components of the MADs to those other phase conductors, then the conductors not being worked on would not need to be covered. This scenario was addressed in the preamble to the final rule on electric power generation, transmission, and distribution, and electrical protective equipment [59 FR 4328 n. 8; January 31, 1994]: Depending on the configuration of the system, an employee could be isolated from two of the phases on the pole by approaching one of the outside phase conductors and working on it from a position where there is no possibility of coming too close to the other two phase conductors.Scenario #2: A worker is in an insulated aerial device and approaches a multiphase primary distribution circuit from underneath the conductors. Once the rubber insulating equipment is installed, the worker will move into a position to perform tasks that will require him or her to be between the covered conductors. Although the conductors are covered, contact with either the rubber gloved hand, parts of the body covered with rubber insulating sleeves, or uncovered parts of the body is possible because of the close proximity of the conductors. Response: Given your specific reference to the close proximity of, and possibility of accidental contact with, the conductors, we assume that the worker can simultaneously breach the electrical components of the MADs of live parts energized at different phase potentials. Therefore, it appears that the insulating line hose or blankets used to cover the energized equipment would need to be rated for the phase-to-phase voltage. Scenario #3: A worker in an insulated aerial device is covering up a multiphase primary distribution circuit mounted on a crossarm for the purpose of working above the circuit. Even though the worker installs rubber insulating equipment from below the conductors, the required task will position the worker above all of the phase conductors. Response: In this scenario, there would be multiphase exposure, and the insulating line hose or cover-up would need to be rated for the phase-to-phase voltage, if it is foreseeable, considering the factors previously described, that the employee or the longest conductive object he or she may handle could simultaneously breach the electrical components of the MADs of live parts energized at different phase potentials. This could occur, for example, if the employee falls from a bucket or drops a conductive object, e.g., a jumper, onto energized conductors below. Scenario #4: A worker is in an insulated aerial device and approaches a vertically constructed primary distribution circuit. The worker will cover up the conductors starting with the bottom phase and working upwards to cover up the other phases. During the cover-up procedure and during work, after all the cover up is installed, the distances from the worker to the covered conductor will be less than the minimum approach distances (MADs). Response: Assuming this is a multiphase circuit, there would probably be multiphase exposure based on the fact that the distance from the worker to the conductors will be less than the MADs. The insulating line hose or cover-up would need to be rated for the phase-to-phase voltage as long as it is foreseeable, considering the factors previously described, that the employee or the longest conductive object he or she may handle couldsimultaneously breach the electrical components of the MADs of live parts energized at different phase potentials. Scenario #5: A worker is climbing a wooden pole to perform work above a multiphase primary distribution circuit. The worker must cover up the circuit in order to climb by the conductors and perform the required tasks above the conductors. Response: Just like in scenario #3, there would be multiphase exposure, and the insulating line hose or cover-up would need to be rated for the phase-to-phase voltage, if it is foreseeable, considering the factors previously described, that the employee or the longest conductive object he or she may handle could simultaneously breach the electrical components of the MADs of live parts energized at different phase potentials. Thank you for your interest in occupational safety and health. We hope you find this information helpful. OSHA requirements are set by statute, standards, and regulations. Our interpretation letters explain these requirements and how they apply to particular circumstances, but they cannot create additional employer obligations. This letter constitutes OSHA's interpretation of the requirements discussed. Note that our enforcement guidance may be affected by changes to OSHA rules. Also, from time to time we update our guidance in response to new information. To keep apprised of such developments, you can consult OSHA's website at http://www.osha.gov. Sincerely, Richard E. Fairfax, Director Directorate of Enforcement Programs 1 In your correspondence, you asked about both the general industry and construction standards. The requirements under the construction standard, 29 CFR 1926.950, differ from those of 29 CFR 1910.269, in that the Minimum Approach Distances in Table V-1 of §1926.950 are different from those in Table R-6 of §1910.269. Additionally, OSHA's construction standard for electrical protective equipment, 29 CFR 1926.951, does not contain a table like I-5 in 29 CFR 1910.137, which specifies maximum-use voltages for the different classes of electrical protective equipment. Instead, the construction standard refers employers to relevant consensus standards, which contain voltage requirements for insulated equipment similar to those in §1910.137, Table I-5. Although the answers provided in this letter apply exclusively to general industry, the guidance provided should be useful for employers in construction as well. In addition, please note that OSHA has proposed revisions to the construction standards in an effort to make them consistent with the more recently updated general industry requirements. See 70 FR 34822 (June 15, 2005). [ back to text ] 2 The highest maximum use voltage for rubber insulating equipment is 36 kV, phase-to-ground or phase-to-phase. [ back to text ] 3 Section 4.2.4.4 of the Institute of Electrical and Electronic Engineers' (IEEE) Standard 516-2003, IEEE Guide for Maintenance Methods on Energized Power Lines, states: The use of tools and equipment results in a composite insulation system composed of the tool(s), equipment, and air gaps. When air gaps are in series, the resultant dielectric strength can exceed that of the tool or equipment itself, but it cannot be determined from direct addition of the material thickness and air gap sizes.Consequently, unless testing has been performed to determine the rating for a particular combination and configuration of insulation and air gap sizes, at least one of the air gaps or layers of insulation must be rated for the voltage between two parts for the insulation to be considered sufficient for the voltage involved. In short, in the absence of testing, the insulating properties of two individual pieces of insulating material are not additive. [ back to text ] 4 To determine the electrical component of the MAD for either phase-to-phase or phase-to-ground exposure, subtract 2 feet from the value listed for the system voltage in the relevant columns labeled "phase-to-phase exposure" and "phase-to-ground exposure," respectively, in Table R-6. For example, the electrical component of the MAD for two phase conductors operating at a phase-to-phase voltage of 36.0 kV is 7 inches. [ back to text ] Enclosures
Figure 1 - Phase-to-Phase (Multiphase) Exposure
Figure 2 - No Multiphase Exposure  |
|
Standard Interpretations - Table of Contents |
 Back to Top |
www.osha.gov | www.dol.gov |
Contact Us | Freedom of Information Act | Customer Survey Privacy and Security Statement | Disclaimers |
||
| Occupational Safety & Health Administration 200 Constitution Avenue, NW Washington, DC 20210 |
||
