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.