ACanton-based contractor, who had worked on the recently completed First Lieutenant Derek S Hines Bridge project, faces as much as $91,000 in fines after the US occupational safety and health administration accused it of willful and serious safety violations. A worker was injured as a crane struck an overhead power line on March 31.
It has long been known that the main cause of crane fatalities in the USA, is contact of the crane with roadside distribution powerlines. These powerlines, which are difficult to see, have killed or injured between 500 and 1000 workers over the last 18 years.
The personnel at risk are those touching or near the crane (oilers), and those guiding the load (riggers). The crane operator remains safe inside his cab unless the crane catches fire and he exits, risking electrocution by step potential.
When a crane contacts a power line, the ground underneath the crane will become energised as the current flows from the power line down the crane, through its out-riggers and tyres, to the ground.
At some distance from the crane, which is a function of soil type and moisture, the ground will be at zero volts. A person within this area is in direct danger of either touching any conductive part of the crane or even walking across the potential gradient.
This situation can result in fatalities due to the ‘step potential’, which is the voltage difference between the two feet, being sufficient to pass a current through the heart causing fibrillation. The approved and safe manner to exit this danger area is to shuffle or hop. In this way no potential gradient or difference will exist between the two feet, as they will be at the same physical point hence the same voltage. This area of danger may extend to a distance of 1.2m from any conductive part of the crane touching the ground when energised with 25 kV.
Avoiding crane power line contact would appear to be the obvious, and traditional, way of avoiding these risks. Unfortunately it requires 20:13 vision, better than 20:20, (normal vision), to be able to distinguish the distance of cranes rope from a powerline, (which does not allow for lack of concentration). The authors of the recent US Crane Federal rule, 29 CFR Part 1926, recognised that this human frailty was not something that could be removed by training and as a result an alternative solution was sought.
The solution was to define the increasing levels of danger as a crane approaches a power line. At each higher level the crane operator is allowed to choose from a ‘menu of safety’ to add additional ‘layers of protection’, ensuring that if one layer is breached it will not result in a fatality. The point at which these different danger zones are breached is a function of crane size, reflecting the ability of the crane’s fully extended boom to reach a power line.
The menu of safety included insulating links and nonconductive taglines designed to prevent the transmission of power to construction operatives. These devices are called ‘insulating links’ or ‘crane insulators’. It was pointed out during rule making that rope conducts, and that a tag line insulator, inserted in a line, creates a non-conductive tag line. However, OSHA required the public to be protected from dangerous products, so required these products be tested to a standard.
Currently there exists an American National Standard ANSI UL2737 for crane insulators.
This standard is based upon 30 years work by the crane industry, and equivalent to the proposed joint logo IEC/ISO standard. ANSI UL2737 can be safely used to create a nonconductive tagline.
The authors of the federal rule recognized that all ropes are conductive even out-of-the-box, and instantly become more-conductive on a construction site. They expressed the hope that industry may solve this technical issue in the future.
The amount of the current will vary depending on a rope’s construction, length and level of contamination. The effect on the human will vary and may be considered to be below let-go and fatal. The let-go current is the current which will lock the arm muscles so the human is unable to let-go of the object, in this case the rope.
Simple experiments can quickly demonstrate the effects of an energized tagline with or without a tag line crane insulator, as shown in the illustrations.
The results at only 2kV are quite dramatic: the current results in the rope igniting and passing sufficient current that the rigger would be unable to let go.
The cost of using a crane insulator, under the hook of the crane, was calculated by OSHA at $210 per use. This is the typical purchase cost of the crane insulator used in the tagline.
A rigger may not be sure if his employer has invested in a crane insulator to use on his crane.
However at $200 the rigger can obtain his own anti-electrocution Personal Protection Equipment in the same way that he buys his own hard hat and boots.
