N. Idaho utility worker electrocuted
Reply With QuoteIn recent years, there have been many changes in the pole crew work environment. As our systems age, pole replacement has increased on energized lines, the voltage carried on distribution poles has increased substantially, and the new popularity of steel poles gives us another reason to revisit the work methods and safety procedures for pole crews.Originally Posted by PA BEN
Granted, the necessary steps to prevent an accidental pole contact during an energized pole setting job are well established. Pole crews know, for example, that installing insulated cover-up on the conductors and/or insulated cover-up on the pole and designating an observer/signal person reduces the probability of accidental contact. Operators using modern digger derrick equipment have become skilled at controlling the pole, and they normally have no problem setting a pole between covered up live wires. But even though these steps are in place and are adequate, the steps to protect everyone when Murphy's Law takes over are often less than adequate. If it can happen it will.
Step Potential
When an object such as a pole accidentally makes contact with an exposed live conductor, electrical current flows into the earth (ground fault). At the point where the pole touches the ground, there is a rise in voltage relative to any earth farther away from the base of the pole. This current can take many paths as it flows through the earth. As the current flows away from the pole, the earth acts as a resistor, usually lowering the voltage as it gets father away from the base of the energized pole.
This voltage also has been called ground gradients, potential gradients or step potentials. It behaves like a ripple in a puddle after dropping a stone in the middle. As the ripple moves away from the base, a voltage difference develops between each ring of ripples, typically decreasing with each ring farther from the center.
Step potentials (accidents) occur when a worker gets one foot on a high-voltage ring near the base of the pole and the other foot on a lower voltage ring farther away from the pole.
When a fault-to-ground occurs, the current will flow through the earth back toward the source through the easiest paths available. In other words, during a ground fault there is no way to know where and how far the ground gradients will travel. The return flow can be into the earth, back up any ground wires to the neutral, along fences, creek beds and so on. If the derrick truck is grounded to the neutral, then the return path for much of the current could be back to the truck and up to the neutral.
Everyone in the electric utility business today knows the safety steps to avoid touch and step potentials. Procedures such as, minimum approach distance, truck grounding, truck barricading, wearing of electrical resistant footwear, staying on the operating platform and/or dedicated observers; these are a given.
Most utilities and employers also require the promotion of a quick trip-out of the circuit when accidental contact is made in order to reduce exposure time to a ground fault. There are still other utilities and employers that do not ground the truck and rely instead on keeping people away from the vehicle by using barricades. This is done by grounding the truck to the best ground electrode available, which is usually the neutral on most distribution circuits. A truck ground does not make it safe to touch a truck. Its purpose is only to promote a quick trip out. But as any field personnel will tell you, it does not help the worker controlling the butt of the pole.
The Pole Butt Position
The worker most at risk while setting a pole in a live circuit is the worker controlling the pole butt. While the signal person and boom operator can stand clear of any objects that may become energized the person handling the butt is at risk. Traditionally, the pole butt worker handled the pole using rubber gloves to protect from touch potentials and tongs to keep his feet away from the highest potential ground gradients near the pole. But, with the higher voltages today, whether it is a higher distribution voltage or a subtransmission voltage, this protection may not be enough.
The derrick operator is safe while on the operating platform; other observers or supervisors on the ground can keep a safe distance from the truck, truck ground cables and any attached trailer, but the person handling the pole butt is in a vulnerable location.
The worker on the ground should take measures to prevent touch and step potentials should a pole (and therefore the truck, truck ground cables and any attached trailer) become accidentally energized. Other observers or supervisors standing near or touching the truck, trailer, grounding cables and/or ground rod are obviously also at high risk. Depending on the voltage, the conductivity of the pole and the type of earth, the ground gradients around the pole can spread out at some unknown distance from the pole.
The development of procedures for setting transmission poles could reduce those risks. One idea would be to have crew members steady the butt with ropes tied off in three or four directions to control the pole butt without touching the pole during placement. This would keep workers at a safe distance from the pole. This work method might be difficult to implement for distribution because of the additional manpower required and the limited space on most distribution jobs.
Ground Mats
Again, borrowing from other energized work applications, there is a common bonding principal: If your feet stay at the same potential as your hands, there can be no electric current flow. If you cannot stay totally away from it, then totally bond to it.
To protect the truck boom operator, he stands on a metal operating platforms. He is protected because the matt is bonded to the operating controls the operator's feet and hands remain at the same voltage if the truck becomes energized. Similarly, a person operating a disconnect switch in a station keeps their feet on a grid bonded to the operating handle or workers at the tensioners in a tension stringing operation stay on a grid.
This same bonding matt principle could add safety to the distribution line crew setting poles and particularly to the very vulnerable worker position of handling the butt of a pole. Setting up a ground-gradient matting grid bonded to the truck ground and bonded to the pole would insure that the worker controlling the butt will then be protected, should the pole become accidentally energized when it is being raised.
There is bound to be resistance to the introduction of ground-gradient matting, but newer fabric-style ground-gradient matting may make this added procedure a little easier to swallow. Keep in mind that other than the use of butt ropes, there is no other assurance that a person is out of range from hazardous step potentials.
Reducing Exposure
Another important step that is not practiced everywhere is setting circuit breakers or reclosers in a non-reclose position and placing a tag or card to reduce exposure time when things go wrong. This added step would ensure that the circuit remains isolated if it trips-out. Not only does this reduce worker exposure time, it is a safer way to carry out a rescue if necessary.
Pole Tongs
The use of epoxy pole tongs also should be revisited, given the higher voltages on today's lines and the common misconception as to what the tongs are used for.
Pole tongs or "pole handlers" were designed to keep the worker away from the pole, away from the highest step potentials - the voltage difference between a worker's feet on the ground and hands or another part of the body such as a shoulder touching the pole.
While pole tongs do provide additional insulation, they cannot be considered a means to provide insulation unless they are stored, maintained and retested like a live-line tool. The perception is that using pole tongs along with rubber gloves will provide additional insulation for higher distribution or subtransmission voltage. But, in practice, their only function is to protect the worker from step potentials.
Steel Poles
Steel poles, which are common in transmission line construction and in distribution in other countries, is beginning to migrate toward the distribution side for replacement and new construction. This advent of steel poles highlights the fact that all poles, regardless if they are wood or steel, should be handled the same. From a safety and operational perspective, no pole can be guaranteed to be safe to handle when in contact with a live circuit. The work method used for protection from a pole with a high conductivity should be the same as discussed here. There may be a "bark" from a wood pole contacting a line, but just because the line does not trip-out, does not mean that it would have been safe to touch during the contact.
The effects of step potential may vary depending on several factors, such as the voltage of the circuit, the conductivity of the pole and the type of soil. However, if a pole (wood, concrete or steel) makes contact with a live circuit, there will be touch potentials.
Interestingly, the more conductive the pole, the lower the potential difference or touch potential between where the pole is touched and where the pole is touching the ground. This almost sounds like its safer to touch a steel pole than a wooden pole that is in contact with a live line, but no pole is safe to touch in an energized work environment.
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