High voltage

January 9, 2014 by Tim McAdams

The flight characteristics of a helicopter make it suitable for a variety of interesting missions. One such job is the repair of live high voltage lines. The voltage on these lines is typically between one hundred thousand to one million volts.

A typical configuration uses a platform mounted to the helicopter’s skids with a wire attached to the helicopter’s airframe. The lineman sits on the edge of the platform as the pilot hovers the helicopter next to the line that needs repair. In some cases, the pilot must maneuver the lineman within several inches of the power line. Because this is considered an external load operation, the platform can be jettisoned. However, the lineman’s harness is attached to the helicopter.

The helicopter and the high voltage wire have different electrical potentials, so to equalize them a metal wand is brought close to the wire. When the wand is close enough the voltage jumps across causing an arc. Once the wand makes contact with the wire, a clamp is connected to the platform with a 5 or 6 foot cable that is attached to the helicopter insuring the voltage potential remains equal. The wand is then removed and the repairs can begin. In the event of an emergency the clamp will break away from the power line. The helicopter now has a high electrical potential and the pilot must be careful to not let the helicopter get to close to an object (a tree, for example) that will allow the voltage a path to ground. This will significantly increase the current flow through the helicopter causing high heat and serious damage to equipment and personnel.

Several accidents have happened from engine failures or the rotor system coming in contact with part of the power line infrastructure. One such accident happen in August of 2013 and according to the NTSB the helicopter was conducting an electrical power line construction operation with a lineman standing outside on the skid. The wire was temporarily suspended by a hoist and the lineman was inserting a fiber shoe to attach the wire to the arm of the tower. While the helicopter was hovering next to the wire at about 200 feet above ground level the hoist slipped and the wire fell onto the top of the helicopter’s skid. Control was briefly lost and four of the helicopter’s main rotor blades impacted the tower resulting in substantial damage to the main rotor blades. The pilot quickly regained control and made an emergency landing in tall corn about 200 feet from the accident location.  Fortunately, the pilot did an excellent job and no one was injured.

Even when everything goes right, high voltage power lines create a very strong electromagnetic field. This field produces an induced current that anyone close to the line will feel along their skin. As such, the pilot and lineman wear a special suit with a metal weave that allows the current to flow around the skin. Even with the suit, the sensation has been described as a feeling of pins and needles.

One Response to “High voltage”

  1. Michael Decker Says:

    Many years ago, I was driving my motorhome cross-country when a mechanical problem arose. I pulled off the highway, grabbed my tools, and slid under the vehicle. When I put wrench to bolt, I got zapped by an umistakable 60-hz shock. I lay there stunned, trying to reason a way this could have happened. Cautiously, a grazed the bolt with the wrench, and again felt the (milder) AC shock. Was there some kind of wire touching the vehicle? I crawled out, looked around, and saw nothing. Then I looked up and saw, far above me in the sky, an array of high-tension wires with towers marching from horizon to horizon. Got it. I donned insulating gloves and completed my repair, and learned from this vivid demonstration of EMF induction.

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