There are hundreds of customers in my company’s service area who own their own high voltage transmission, distribution and/or substation electric supply facilities. The service voltages for these customers range from 4 kV to 138 kV.
Many years ago, I investigated an accident involving the death of a farmer on one such facility. The farmer decided to save some money by repairing an anchor guy on his single-phase 14.4 kV distribution line. When the farmer first discovered that a guy wire on his line was broken, he contacted my company (the utility) to see if we would fix it for him. When our trouble-man met the customer, he reminded the customer that the utility did not own the line and that he would have to hire an electrical contractor who is qualified to work on 14.4 kV to repair the guy. The trouble-man asked the customer if he wanted the line de-energized at that time. The customer said no. The trouble-man told the customer to call 24 hours in advance if he or the contractor wanted the line de-energized. Once de-energized, the contractor would have to install their own employee protective ground if they wanted to work on the line as “dead.” The next day, the customer tried to repair the guy himself with the line energized. His aluminum ladder contacted the 14.4 kV phase wire.
Several years later, I was on the stand in a wrongful death lawsuit initiated against my company by the farmer’s wife. The plaintiff’s lawyer was trying to convince the jury that the trouble-man, who spoke to the customer, should have left the line completely safe so that the customer could fix the guy by himself. The lawyer asked me what would have to have been done to the line to make it completely safe. I testified that the only way to make electrical facilities completely safe is to de-energize them. Since the customer told the trouble-man he did not want the line de-energized, we won the case.
There is no way to make energized electrical facilities completely safe. By insulating, guarding, isolating and signing electrical facilities we can limit the likelihood of contact by humans, but we can’t prevent it. When a contractor has to work close to one of our lines, de-energizing the line is one option. The definition of de-energize as far back as 1973 and continuing through the 1997 edition of the NESC was “Free from any electrical connection to a source of potential difference and from electric charge; not having a potential difference from that of the earth” (synonym: dead) (page 5 of the 1997 NESC edition). Sounds completely safe to me. However, in order to make the line safe, we have to turn off the electricity. Customers don’t like that.
Sometimes “de-energized” lines are not completely safe. A large majority of utility engineers, operators, dispatchers, and workers consider a line that is disconnected from all sources of electric supply to be de-energized. This may be true for some low voltage lines, but it is not true for high voltage lines. A high voltage line that is disconnected from all sources of electric supply is dangerous because it can pick up electric charge from nearby energized lines. To eliminate the electric charge, a line that is disconnected from all sources of electric supply must also be effectively grounded, not just grounded. The key word is “effectively.” The effectiveness of a ground is dependant upon many things including location, type, and ground resistance. By effectively grounded I mean, “Intentionally connected to earth through a ground connection or connections of sufficiently low impedance and having sufficient current-carrying capacity to limit the buildup of voltages to levels below that which may result in undue hazard to persons or to connected equipment” (page 6 of the 2002 NESC edition).
To resolve the misuse of the term de-energized, the International Electrotechnical Commission removed the term de-energized from all international standards and the NESC committees elected to change the definition of de-energize in the 2002 edition to what most people think it means, “Disconnected from all sources of electric supply by open switches, disconnectors, jumpers, taps, or other means.” There is also a NOTE reminding people that “De-energized conductors or equipment could be electrically charged or energized through various means, such as induction from energized circuits, portable generators, lightning, etc.” Starting in 2002, a line that is de-energized is not completely safe.
National Electrical Safety Code and NESC are registered trademarks of the Institute of Electrical and Electronic Engineers (IEEE).