Q: What is meant by the phrase “circulating current” as it pertains to transmission towers? Does it have something to do with the fact that there is no neutral?
A: We’re glad you asked the question because it gives us an opportunity to discuss one of the basic principles of the hazard of induction. More and more trainers are teaching with a focus on principles instead of procedures, and we often overlook some of these basic definitions. The concept of circulation is associated with what happens in any interconnected electrical system. Refer to the basic definition for parallel paths: Current flows in every available path inversely proportional to the resistance of the path. That means that current flows through every path, and the path with the least resistance has the most current flowing in it. Inversely, the path with the most resistance has the least current flowing in it.
When you ground a circuit to the structure, you are making an electrical connection to the tower. Current will flow in every available path. If there is any source for current, including induction, there will be current flow. The greater part of the current will flow in the lower-resistance pathways. If the tower is well grounded, the majority of the current will flow in the tower to ground. In a distribution system, the majority of the neutral current flows in the neutral. Pole bonds to ground rods have much higher resistance and therefore lower current that usually can’t be measured by a typical clamp current meter, so some people think there is no current flowing in them. There is, and under the right conditions – such as a fault or open in the neutral – the level of current flowing in a pole bond can be deadly.
Now, back to circulation. The individual resistances in each ground from phase to pole on a transmission circuit can create a path that has lower resistance between the individual phase connections than the path to ground. This is more likely to occur in wood structures than steel, but if the resistance to the earth through the structure is higher than the resistance between the phase connections, a current can circulate between those connections. In our experience, current circulation between phase connections is usually the reason for overheating of transmission grounds as opposed to current freely flowing through a good low-resistance ground path connection. Of course, if your induction source is high enough, even grounds on a good path to earth will heat.
Paragraph 6.2.8 of IEEE 1048, “IEEE Guide for Protective Grounding of Power Lines,” has a good discussion about the best electrodes for protective grounding and recommends the transmission static as a preferred ground point. Like the distribution system neutral, the static has very low resistance because of numerous ground connections to earth through adjacent structures. Those additional paths also serve to lower the voltage rise at the foot of a structure at the worksite that is created by induction or fault current in the case of accidental energizing.
Q: We are a communications contractor working for a utility. Our job is to install the power company’s self-supporting fiber-optic cables somewhere in the vicinity of the utility’s system neutral, usually 24 to 30 inches below. What is our responsibility for “demonstrated proficiency” that the utility is asking us for?
We are not sure your workers fall under the requirements for “demonstrated proficiency,” a new phrase found in the recently revised 29 CFR 1910.269(a)(2)(viii), which has certain criteria for testing of worker skills. The rule covers both power-line workers and tree trimmers. However, that may not relieve you of what the utility is asking for. The problem is that the wording of the standard has not caught up with the electric utilities that are now installing fiber for retail sales as cable television operators. As a cable installer, your workers are covered by 1910.268, OSHA’s telecommunications standard. The standard specifically exempts cable installations under the control of utilities for the purposes of communication or metering. But the language at that time was for cable systems used for the purposes of supervisory control and data acquisition (SCADA) and metering, not retail delivery of internet or cable TV. In addition, the cabling was usually installed by power company employees or power-line contractors. We are not aware of any enforcement notices or definitions that would add or include internet or cable TV in the original definition of cables for metering or SCADA communication as described above.
The 1910.268 standard also excludes construction activities, relegating those activities to the 1926 construction standard, which is problematic. With the exception of the general requirement found in 1926.21(b)(2) to train workers “in the recognition and avoidance of unsafe conditions and the regulations applicable to his work environment to control or eliminate any hazards or other exposure to illness or injury,” the 1926 standard does not include a specific telecommunications section.
We believe all of this leads us back to the 1910.268 and 1910.269 standards to gather the intent of OSHA for protection of the worker. Paragraph 1910.268(c) does have specifications for training. The intent of the requirement is to ensure that employees do not engage in the activities until such employees have received proper training in the various precautions and safe practices required. The telecommunications standard does have a requirement, which is not specifically required in 1910.269, for detailed records of training, including that the employer certify that employees have been trained by using a certification record. The certification record must contain the identity of the person trained, the signature of the employer or the person who conducted the training, and the date the training was completed. The certification record also must be maintained for the duration of the employee's employment. For experienced employees, the employer can certify the skills based on an interview and records of prior employment.
All of this goes back to OSHA’s intent that employers ensure their employees’ safety. One of the elements for a safe workplace is making sure the employee is qualified. Whether you call it certification of skills – as in 1910.268 – or demonstrated proficiency – as in 1910.269 – must be determined by the owners and contractor. As long as the outcome for safety is assured, we believe either methodology should be defensible as compliance. The secondary compliance issue will be documentation. The 1910.268 standard requires detailed documentation for the term of employment; 1910.269 does not. However, any employer that doesn’t maintain those records is probably taking a risk of not being able to meet the demonstration requirements.
Q: During our search for proper documentation on standard live-line tool testing procedures, we’ve found that many in the industry use IEEE 978 criteria in testing live-line tools. It looks like OSHA 1910.269 only refers to IEEE 516, rather than IEEE 978. Are we overlooking something?
A: IEEE 978 was withdrawn during the past revision of IEEE 516 because it was redundant. The two essentially are the same except that 516 has more detail. OSHA refers to IEEE 516 in the agency’s related references found in 1910.269. IEEE 978 is a little easier to follow, but it is not the reference document, so you should use 516. A precaution for those using IEEE 978: If for some reason technology or materials should prompt a change to IEEE 516, that same change would not be found in the withdrawn IEEE 978.
Q: If you are working out of an insulated bucket truck with phases grounded, do you still need to use equipotential grounding? Our assumption is that if you are touching the pole or cross-arm, you are at risk. The bucket is nonconductive. Do we still bond it?
A: It’s a good question and you are right. Any difference in potential can result in a current path and should be equalized with a bonding jumper. From a fiberglass bucket there is very little chance you could create a difference in potential high enough from wire to bucket to cause injury, but that is not the same between pole and wire. If the circuit is de-energized, it should be bonded to the pole to prevent an exposure to potential differences between the two. We say always look for the gaps, including wood poles and wire, where current could flow and bond them. You may not have high enough induction to create a hazardous potential between wire and wood, but that would not be the case if unexpected energizing occurred.
KNOWLEDGE, INSIGHT & STRATEGY FOR UTILITY SAFETY & OPS PROFESSIONALS
Incident Prevention is produced by Utility Business Media, Inc.
© 2004 - 2020 Incident Prevention™. All Rights Reserved.