Stringing wire in any environment can quickly go wrong. Dropped conductors can wreak havoc if precautions are not taken. In an energized environment, the result of losing control or dropping conductors has a greatly magnified risk.
Guard structures are the first type of protection conventionally used to prevent contact with energized lines. Ideally, guard structures are positioned so that whether it’s the unexpected loss of stringing tension or something as major as a dropped conductor, the conductor being pulled will not make contact with the energized lines. There are other requirements, too, one being non-automatic setting of breakers for the lines being crossed if it is not possible to de-energize and ground them.
No matter how good your guard structures are, if you are pulling across or parallel to energized lines, you must ground the conductors being pulled. The revised language of 29 CFR 1910.269(q)(2), “Installing and removing overhead lines,” has added requirements focusing on induction as well as contact with other energized lines during pulling. Induction is voltage or current that can build up on the conductors being pulled from sources other than direct contact with an energized line. The rules of 1910.269(q) are mirrored in 1926.964(b) of the construction standard. To save space and time, we will refer only to the 1910.269 standard in this article.
Paragraph 1910.269(q) has always been the guide for stringing and worker protection and formally included specific instruction for placing grounds. The reader is now referred to paragraph 1910.269(p)(4) for grounding for the protection of personnel from inadvertent electrical contact. Paragraph 1910.269(p)(4) has always contained the requirements for protecting workers from injury when booms or equipment may become energized, but the language in the standard has been revised. Conductor being strung in an energized environment is now included, and the protective expectations provided by grounding are the same for conductor during stringing as for boom trucks, cranes and derricks that may become energized.
Placement of Grounds
The expectation of the rule has always been mat, ground and barricade the equipment, traveling ground at the tensioner, grounds at the puller, grounded travelers at each break-over, traveling ground at each crossing and no more than two miles apart.
In 1910.269(q)(2), OSHA requirements for grounding the pulled conductors do not differentiate between transmission and distribution; however, grounded travelers are an issue in distribution wire pulling. While manufacturers have designed studs for attaching a ground to a typical distribution traveler that perform well under electrical tests, they do not have travelers designed for grounding the moving conductor like they do for transmission pulling. The only device designed as a ground for distribution-sized conductors is the running ground installed at the wire tensioner. The running ground is not practicable for use out on the line as a ground for pulling because you cannot pass a grip or swivel through it.
The language in the OSHA standards prior to this year’s revisions had specific requirements for placement of grounds on conductors during stringing. OSHA standards have always required certain protections utilizing grounds to protect workers from the risk of electrical injury should conductors sag into energized lines. The rules still require the employer to protect the workers, but they have removed the specific requirements such as where traveling grounds must be placed. Even though OSHA has removed the specific requirements about where to place grounds when stringing in an energized environment, the practices remain the same, specified by consensus standards. The applicable consensus standard, “IEEE 524: IEEE Guide to the Installation of Overhead Transmission Line Conductors,” contains procedural language, including location and placement of grounds in 22.214.171.124. Article 7 of “IEEE 1048: IEEE Guide for Protective Grounding of Power Lines,” deals with transmission and distribution stringing. Article 7 refers the reader to IEEE 524.
Grounding in stringing has two functions: to cause immediate operation of circuit protective devices and to equalize potentials for the protection of personnel. The more grounds you place along the line, the better path to ground you create. Grounding, however, does not protect all personnel. Planners must be aware that grounding to a structure electrically couples current to them in a fault as well as current from induction. If the conductor being pulled is energized, the structure and the earth around the structure will be energized until the circuit protective device operates to clear the voltage from the line. Grounded travelers out on the line shunt those currents to earth along every available path to ground. Current flowing in those paths is inversely proportional to the resistance of the circuit. This means, of course, that every ohm of resistance built into the grounding connections limits the current that can flow to ground. The net result is that the circuit protections will not operate as quickly. It also means that current will flow in higher levels to other places, such as back to the tensioner where your people are. The message is that those remote grounds have a very important role to play, and the integrity of the connections matters.
Grounding at Equipment Setups
In most cases, the tensioner is the first piece of equipment that will be affected by energizing the conductor being pulled. A worker on the tensioner body is at risk from the conductor being energized, both from arching across the tensioner wheels and at the reels. Here is where the often-neglected traveling ground plays a very important role. We repeatedly see traveling grounds tied off to a tensioner but grounded to a rod. This misses an important role for the traveling ground. If the traveling ground is bonded to the tensioning trailer, and that trailer is bonded to the reel trailer, all of the equipment will be at the same or close to the same potential as the conductor being pulled. This bonding serves to protect operators from potential differences across conductor and trailer.
A very important consideration here are the potential differences between earth and trailers. During pulling, several workers need to be in attendance at pullers, tensioners and reel trailers. If the ground around the equipment is not at potential with equipment, risks are high not only from faults, but also from induction. Individual bonded mats at equipment access points or a large ground mat upon which all the equipment sits protects workers from those risks.
Grounding moving conductors is accomplished using travelers (wire-pulling dollies or sheaves) designed and equipped with attachments to provide a ground on the moving conductor. The design of the ground connection must be sufficient to carry steady-state induced currents as well as operate a circuit protective device if the conductor makes contact with an energized line. The ground is also expected to survive the contact long enough to cause breaker operation.
In the final rule, OSHA has removed the requirement for grounds no more than two miles apart. OSHA has also removed the requirements for grounds at the first structure at each end of the pull and at the nearest structure at either side where conductors being pulled cross energized conductors.
There was always an issue with two miles being considered a rule because, as performance language, OSHA does not cite procedures, which is what they did in the original rules. The grounding rule was primarily based on the ability to trip a circuit in an inadvertent contact with an energized circuit while pulling, not necessarily the risk of induction. You may be grounding to trip, but if there is a source of induction, installing grounds between phases and static or neutral creates a closed electrical loop or cell for current to flow in. If the cell is not bonded to earth, the current circulates in the cell can be very high. If the cell is grounded to earth, the current in the cell splits inversely proportional to the two paths available, earth and flowing in the low-resistance cell. So either way, the risk is still there. I have personally measured transmission induction on a grounded circuit at more than 160 amps and 1800 volts. I know it can be higher.
A two-mile-long cell can be very dangerous, and someone who just followed the rule could be building a trap. In our transmission business, we use clamp-on amp meters to check for current as a backup means to understand risks. If currents in a grounded circuit are too high, they result in nuisance shocks and can be a lot worse – even deadly if the work location is not bonded. The solution is to split those two-mile-long cells with additional sets of ground to cut the current in half and create opposing flows, reducing risks.
Rule 1910.269(q)(2)(iv) has a particular reference for employers regarding protecting workers from the risk of induction. It states, “Before employees install lines parallel to existing energized lines, the employer shall make a determination of the approximate voltage to be induced in the new lines, or work shall proceed on the assumption that the induced voltage is hazardous. Unless the employer can demonstrate that the lines that employees are installing are not subject to the induction of a hazardous voltage or unless the lines are treated as energized, temporary protective grounds shall be placed at such locations and arranged in such a manner that the employer can demonstrate will prevent exposure of each employee to hazardous differences in electric potential.”
Note the requirement that the employer must demonstrate that employees are protected. This is not about following rules. Employers must understand and determine the risks to employees, and they must train employees to understand the risks and know how to protect themselves.
Now for the citation criteria: OSHA has added a note to 1910.269(q)(2)(iv) establishing a method of measuring risk to workers by citing current flow through a 500-ohm resistor as the threshold level of exposure. The 500 ohms represent the (conservative) resistance of the body, and the 1 milliamp of current in the standard establishes an employer action level for risk to the employee.
Finally, Incident Prevention has had readers ask if the new rule requires an engineering study before pulling to protect employees from the hazard of induction. My opinion is no, that is not required. The rule states that “… the employer shall make a determination of the approximate voltage to be induced in the new lines, or work shall proceed on the assumption that the induced voltage is hazardous.”
If you assume induction is present and take the appropriate precautions, you have met the requirements of the rule. The new language is to put teeth and citable violation criteria (the notes) in place. The new rule also includes Appendix C with guidance to employers about grounding, largely as a result of requests by commenters during the hearings on the proposed rule.
KNOWLEDGE, INSIGHT & STRATEGY FOR UTILITY SAFETY & OPS PROFESSIONALS
Incident Prevention is produced by Utility Business Media, Inc.
© 2004 - 2020 Incident Prevention™. All Rights Reserved.