Voice of Experience: Understanding Induced Voltage
It has taken the electric utility industry many years to understand induced voltage. When I started working in the 1960s, it was explained to me that voltage remaining on de-energized lines was static voltage that had to be bled off or else it could be deadly. Now, when I speak to groups about temporary system grounding for the protection of employees, I occasionally still hear the term “static voltage” being used to describe what really is induced voltage from a nearby energized line. Even today, not everyone in the industry completely understands induced voltage.
So, what exactly is induced voltage? Here are some things utility safety and operations professionals should understand. The electromagnetic field around an energized conductor produces capacitive and magnetic coupling to all nearby objects within the electromagnetic field. The voltage level of the energized conductor and the physical length of the de-energized conductor that is exposed to the energized (source) conductor will determine the amount of voltage on the de-energized conductor or equipment. A de-energized conductor or piece of equipment will remain energized as long as the source remains energized and de-energized equipment remains ungrounded. Properly installed temporary system safety grounds can be used to create an equipotential work zone for employees.
The induced voltage found on de-energized equipment is not static, and it can’t be bled off. System safety grounds that have been installed simply give the induced voltage a conductive connection to ground. Once grounds are removed, the induced voltage returns to exactly the same amount of voltage instantly. It is voltage of 60 cycles per second in a steady-state condition, because there is no path in which electricity can flow other than the energized, isolated conductor or equipment. If grounds are applied to de-energized conductors, the voltage immediately will collapse to near zero, but now the physics have changed and a current flow is established in the system safety grounds. The amount of current flow in ground sets is determined by the amount of induced voltage on the de-energized equipment before the grounds were installed, and the resistance of the ground set and the ground. In addition, the more ground sets that are applied to a de-energized line, the less current flow there is in each set of grounds.
There have been numerous injuries and fatalities over the past 10 years related to the failure to control induced voltages. In 2014, a couple significant changes were made to OSHA 29 CFR 1910.269 regulations in an attempt to address induced voltage issues.
First, let’s take a look at paragraph 1910.269(m), “Deenergizing lines and equipment for employee protection.” The rule has always stated that the employer must ensure system safety grounds are installed. Specifically, paragraph 1910.269(m)(3)(vii) states the following: “The employer shall ensure the installation of protective grounds as required by paragraph (n) of this section.”
Until de-energized lines and equipment are grounded, paragraph 1910.269(n) requires employees to maintain minimum approach and consider de-energized lines and equipment to be energized. Per 1910.269(n)(3), an equipotential zone must be established. The paragraph states the following: “Equipotential zone. Temporary protective grounds shall be placed at such locations and arranged in such a manner that the employer can demonstrate will prevent each employee from being exposed to hazardous differences in electric potential.”
In an attempt to control hazardous energy and induced voltage, a major change to 1910.269(q), “Overhead lines and live-line barehand work,” went virtually unnoticed when the new 1910.269 rule was published in 2014, and no attention was brought to it during initial webinars about the new rule. An explanation of the change can be found in 1910.269(q)(2)(iv). Until the 2014 update, if crews were working on or installing conductors in parallel to energized lines, system safety grounds were required at a minimum 2 miles apart. So, when working on grounded lines, employees would never be more than a mile from a set of temporary safety grounds. As it turns out, 1 mile from a set of system safety grounds on a 345-kV or 500-kV right-of-way may be too far, possibly exposing employees to a hazardous difference of potential if they contact de-energized lines or equipment.
The updated 1910.269(q)(2)(iv) now states the following: “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 1 to paragraph 1910.269(q)(2)(iv) states, “If the employer takes no precautions to protect employees from hazards associated with involuntary reactions from electric shock, a hazard exists if the induced voltage is sufficient to pass a current of 1 milliampere through a 500-ohm resistor. If the employer protects employees from injury due to involuntary reactions from electric shock, a hazard exists if the resultant current would be more than 6 milliamperes.”
You may have noticed that the text of 1910.269(n)(3) was copied and added to 1910.269(q)(2)(iv) in an effort to ensure the protection of employees from hazardous differences of potential. Methods to determine locations for grounds on conductors can require grounds more frequently than 2 miles apart to mitigate risks of differences of potential. Once conductors are in place, additional system safety grounds will reduce the induced voltage and meet the regulation.
After talking with many workers about induced voltage, the belief is that once grounds are installed, the entire length of line is dead. Science tells us that the system safety ground is the only location on the grounded line where the voltage to ground is zero. In cases of induced voltage, the farther you are from temporary grounds, the greater the possibility of the difference of potential between grounded conductors and other surfaces – hence the change in regulations. Note that when employees are working in a grounded crane basket or JLG on a grounded circuit on the right-of-way or in a substation, there will be a potential difference in the gap between the bus and the platform. Those conductive platforms must be bonded to the grounded conductors to bridge that gap and protect workers in the basket from the difference in potential.
In addition, even when equipment is grounded, and bus or conductors are grounded, there can exist circulating ground currents related to induced voltage and the path to ground. Grounding equipment in a different location, even a large substation, can create a hazardous condition within the grounds.
We must remember that electricity does not take only the path of least resistance, as I was told years ago. Instead, electricity will take any and all conductive paths. Kirchhoff’s law of current division in parallel circuits helps us understand that the amount of current flow in a path is determined by impedance and the resistance of the path. It only takes about 50 volts AC to penetrate human skin, and 30 to 50 milliamperes to be fatal to a human being. Humans are only about a 1,000-ohm resistor in an electrical circuit. All employees should be familiar with the law of parallel resistances and Ohm’s law.
About the Author: Danny Raines, CUSP, safety consultant, distribution and transmission, retired from Georgia Power after 40 years of service and opened Raines Utility Safety Solutions LLC, providing compliance training, risk assessments and safety observation programs. He also is an affiliate instructor at Georgia Tech Research Center OSHA Outreach in Atlanta.