A shocking development while dangling 80 feet off the ground. Oh my!
My first job out of college was working for the Power Company in Roanoke, VA. I was assigned to the Communications Group, responsible for the company's Carrier Relay, Cable Carrier, Telephone, Microwave Link, and VHF Radio systems.
Years before my arrival at the Power Company, there had been an "incident" at one of the Power Generating Plants where a generator had shorted out and exploded, causing massive damage. They sued the manufacturer and litigation had dragged out for years. Just prior to my arrival, a settlement had been reached. It was an odd case, since the manufacturer had replaced its damaged equipment years earlier, and the Power Company’s labor to rebuild the plant had been paid for. The settlement involved two research projects that the manufacturer had underway: the Zinc Oxide (ZnO) Lightning Arrestor study, and Remote Metering (a new idea in the 1980s). Since they needed to work with the Power Company to test these systems anyway, the settlement included the manufacturer reimbursing them for labor and expenses to support these projects.
I was assigned to the ZnO Lightning Arrestor project. The ZnO arrestor was an insulative tube filled with ZnO, and connected from the 138-kV phase wire to the tower. When lightning struck, the tower would rise to a certain voltage, and the ZnO arrestor would break down and conduct the surge to the phase wire until the strike was over. The circuit would trip out until the surge dissipated, then re-energize. The ZnO would have stopped conducting by then and the circuit returned to normal operation.
Why do this? It was not obvious to me then either. When a tower atop a mountain is struck, it can rise to over 2 MV due to the high earth-ground resistance, way larger than the 138-kV phase voltage. Since the insulators are designed to insulate the phase wire to the tower (not vice versa) they can flash over, damaging the insulators and requiring their replacement. The ZnO arrestor effectively shorts the phase wire to the tower, relieving the strain on the insulators (they are quite a chore to change and require a long outage on the circuit).
I became involved because the sensors on the tower were connected to radios. When a lightning strike occurred, a coil around the tower leg sensed the current and closed a pair of contacts. These, in turn, caused a radio to transmit to the base station in the valley, where a small computer logged the strike on a printer.
One day, I was hanging from the tower crossbar by my safety belt (about 80 feet off the ground) changing the radio batteries. I reached out to steady myself and was surprised to receive a bit of a shock!!! That was odd, since I saw no high-voltage components in the radio in front of me. I leaned close to the tower and tapped it with my finger. As I moved farther away from the tower, the little zap became larger. "Smelling a rat", I turned around to see the 138-kV phase wire only 5 feet from my head. I was in the "E-field" of the circuit…another example of an Engineer acting as a "conductor."
I hope that these old-time stories have some power to wake up people 'on active service'. The safety records are full of examples of repeated accidents following near-misses like the one described.
It should be noted that ionizing fields like RF from a transmitter or mobile phone greatly reduces the hazardous distance, increasing the risk of a strike.
The safe approach distance is one that stops you getting an arc jumping across onto you. Nevertheless the field gradient at that distance would still be considerable, so if you weree not in contact with the metal structure at any time you'd build up quite a charge.
And how about these guys:
Was about to ask the safe distance, thanks David you answered my question.
Still a scary situation, Dwight, not only the HV but dangling 80 feet (25 meters) above the ground. What if you had stood up on the crossbar? (is that possible?) Am surprised that your managers did not inform you ahead of time to expect this effect.
I work for an eletricity utility and although my dept doesn't work on the wires, we have to do assessments every year in which we have to give safe approach distances. We usually only go up to 66KV (another co does the 132KV distribution) but I looked up our tables and it gives the minimum approach disstance for 132KV as 1200mm - about 4 feet, so you were above that. The above is for "instructed persons" ie those who know what they are doing - for others it is 3 metres (about 10 feet). Sounds like you were cutting it a bit fine though....
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