George G Karady
The most frequent accident scenario is when a crane works dangerously close to a high voltage line and the crane load line touches the high voltage conductor and produces a short circuit. The short circuit current electrocutes workers handling the load. Fig 1 shows the typical accident scenario.
Crane line
contact Arrows show
typical current path
Tiers flashover
Fig 1 typical crane accident scenario This type of accident can be prevented by using insulating link.
Insulating link description
Fig 2 shows the concept of insulating link application on a crane. The insulating link is a glass fiber rod, which is inserted in the load line just above the hook. In case of a contact between the line and the crane the insulating link insulates the worker from the energized crane and prevents the accident.
Boom Cage
Insulator Link
Figure 2. Concept of crane protection
Figure 3 shows an insulating link made by Hirtzer in California. The link has metallic hardware used to insert the link into the crane line. The metallic hardware parts are insulated by fiberglass and other insulating material. The outer surface has water repellent coating. The manufacturer specifying the rated voltage of the link and tests each links about twice of this voltage. Also the permitted mechanical working load is specified for the link.
he
Figure 3. Typical example for insulator link (Hirtzer) Flashover probability
Several measurements performed by different laboratories on insulator links were reviewed [2, 3, 4, 5, and 6]. The author of this report also performed tests on commercially available links at Arizona State University.
Tests performed by Morgan, Ontario Hydro, and others show that the flashover voltage of the presently available insulated links is dramatically reduced when heavily contaminated. The
same tests also indicate that the dry and wet flashover voltage remains high. As an example the report of Mr. Morgan [1] shows that the tested Miller and Hirtzer links withstand more than 57 kV in dry condition and more than 21 kV in wet conditions. The minimum flashover voltage under salt spray was 25 kV.
Contamination caused insulator flashover is described in literature and is well understood. Dry contamination is usually non-conducting; therefore it has no effect on the flashover voltage. The insulator contamination causes flashover only when drizzling rain and fog slowly wet the
insulator. A flashover in contaminated conditions requires the simultaneous occurrence of the following:
1. Contamination deposit on the link surface in the form of a thin layer (dust layer, mud or tar like deposit from roads, etc.)
2. The insulated link wetted slowly by fog or drizzling light rain. Normal or heavy rain washes down the contaminant and eliminates the danger of flashover.
3. The insulated link energized by an accident.
The simultaneous occurrence of these three events is rare.
The contamination on a utility insulator surface is due to airborne deposit. The wind drives dust, cement powder, industrial exhaust, salt water and other pollutants to the insulator surface. The amount of pollution depends on the local atmospheric and other conditions. Relatively few places in the USA experience the heavy pollution used to test insulator links. Fig 4 exhibits the results of an industrial survey showing the distribution of insulator use at different contamination levels.
14% 7%
36%
47%
Figure 4. Distribution of insulator use at different contamination levels This figure indicates that only 7% of the insulators are operating in heavily contaminated conditions. Cranes are operating all over the USA. Therefore, it is prudent to assume that only 7-10% of the links are subjected to heavy pollution. Furthermore, the manufacturers require that the insulated links be regularly cleaned and maintained by the operator. Therefore, the build up of heavy contamination is unlikely. The most probable scenario is negligent dragging of the link in dust or mud. Experiments performed by ASU show that the rolling of an insulated link in dust or mud results in medium to light contamination. However, for the sake of argument, let us assume that 10% of the insulated links are heavily contaminated.
The second criterion is the wetting of the insulators, which requires fog or light rain. A survey of an US meteorological report shows that the number of days with light rain or fog is about 8-90 per year. This number depends on the location. In Arizona, the number of foggy or light rainy days is significantly less than in Wyoming. Using the maximum (90 days) and minimum (8 days) values, the probability that the link will be wetted is between 8/365 = 2.19% to 90/365 = 24.6%.
The probability that the link is heavily polluted and wetted simultaneously is 10% x 24.6% = 2.46% or less than 3%. This means that, the link will fail to provide protection only less than 3 times out of hundred (100) accidents. Crane to power line contacts, in the USA, cause about 300 fatal accidents yearly. The use of insulated links would reduce this number to 8-10.
The results of this risk analysis re-enforces the conclusion that the insulating link improves crane safety and that failing to use insulating link, when a crane is working in the vicinity of a power line is negligent.
Leakage current
The wet insulating link conducts leakage current for a short period of time when the carne touches a high voltage line. The accident triggers the line protection, which typically switch of the within 4-10 cycles, which corresponds to a time 83-167 milliseconds. The worst case the protection operation is delayed when the line is de-energized less than 0.5 second.
The clean or wet insulating links conduct less than 5 mA current when energized without flashover. ASU measured the leakage current of polluted insulating links [10]. The leakage current of a Hirtzer link 5T, 50kV, was 0,3 mA at 15kV and in wet and polluted condition the leakage current was 3.325mA at 1.5kV and 44.9mA at 30 kV. It can be concluded that the leakage current is less than 50mA below 30kV at light pollution.
The international standard 479 IEC 1994 provides data on current effects on humans. The standard specifies four zones; AC 1, AC 2, AC 3 and AC 4. The first three zones the sinusoidal, 60Hz current has only transient temporally effects. Fig 5 and explanation is word by word copy of the IEC 479.
“AC 1 Usually no reaction
AC 2 Usually no harmful physiological effect
AC 3 Usually no organic damage is expected. Like hood of cramp like muscular
contractions and difficulty of breathing for duration of current flows more than 2s.
Reversible disturbances of formation and conduction of impulses in the heart, including arterial fibrillation and transient cardiac arrest without ventricular fibrillation increasing with current and time.
AC 4 Increasing with magnitude and time dangerous pahtophysiological effects such as cardiac arrest, berthing arrest and sever burns may occur in addition to the effects of zone AC 3.
AC 4.1 Probability of ventricular fibrillation increasing up to about 5%.
AC 4.2 Probability of ventricular fibrillation increasing up to about 50%.
AC 4.3 Probability of ventricular fibrillation increasing above 50%.
Fig 5 Current effects on human being (copy from 479 IEC 1994)
The measurement indicated less than 0.5 mA pollution at dry condition, when the link is energized to 15 kV voltage, this is in AC 1 zone, no harmful effect.
The polluted link current was less than 50mA, when the link is energized to 30 kV voltage for less than 0.5 second, this is in AC 3, “Reversible disturbances of formation and conduction of impulses in the heart, including arterial fibrillation and transient cardiac arrest without
ventricular fibrillation increasing with current and time”.
At 15 kV the leakage current is about the half value, which practically eliminates the danger of deadly electrocution at distribution level.
References
H. B. Hamilton, J.D. Morgan, “Final Report on Evaluation of Mobile Crane Safety Devices”, a report to Bucyus Eric Inc., 1982.
J. Derald Morgan, Howard B. Hamilton, “Evaluation of Link for Safety Applications”, a report to R.O. Corporation and National Crane Company, 1982.
J. Derald Morgan, “Evaluation of Insulated Links for Cranes”, unpublished test data prepared for Grove Manufacturing Inc. 1985, (Not reviewed).
Martin N. Kaplan, “Test of Insulating Links Used on Cranes Under Field Conditions”, 1990.
J. Derald Morgan, Howard B. Hamilton, “Insulating Devices for Cranes. Test Results”, manuscript containing the summary of tests performed in [1, 2, 3].
J. Derald Morgan, “Insulated Link and Standoff Evaluation for Grove Manufacturing Inc.”, A.B.
Chance Test Laboratories, Centralia, Missouri, 1990.
J. Derald Morgan, “Insulated Link Test Using IEC 507 Artificial Pollution Methods”, A.B.
Chance Company, Centralia, Missouri, 1992.
Ontario Hydro Report No. ET91-94-P, “Electrical Tests on Insulating Crane Links”, August 13, 1991.
George G. Karady, “Efficiency of Insulating Links for Protection of Crane Workers”, IEEE Transactions on Power Delivery, Vol. 6, No 1, Jan. pp. 316-323, 1991.
George G. Karady, “Test of Insulator Links”, report, Arizona State University, 1993.
George G. Karady, Minesh Shah, D. Dumora, “Probabilistic Method to Assess Insulating Link Performance for Protection of Crane Workers”, IEEE Transactions on Power Delivery, Vol. I 1, No 1, Jan. pp. 212 -323, 1996.
Ben J. Lehman
Rear Admiral [Engineering] US Navy Ret.
Professional Engineer - President, MechElexTex Inc.
David V. MacCollum, Chairman Hazard Information Foundation, Inc.
P.O. BOX 3962
Sierra Vista, AZ 85636 Dear Mr. MacCollum:
I have read your outline for a study of electrical power line contacts with great interest.
As an expert consultant and witness in at least fifty electrical power line matters since 1976, I have found them to involve many types of equipment. One equipment with whose hazard of electrical injury I was very familiar was a type of recreational boat.
The passengers in this type of boat could have been protected by known and economical
means from the time of its original design. However, the argument against the use of protective devices included the "false sense of security theory" and apparently it had some influence. The use of the word "theory" is misleading. The argument is simply the hypothesis that persons who believe themselves to be protected by any device which decreases the probability of injury to themselves are more likely to expose themselves to situations involving that risk. I spent many hours in libraries trying to find reports of incidents in which this hypothesis was correct: I did not find any. On the contrary, I did find papers reporting psychologist’s efforts to validate the hypothesis: all concluded that it was incorrect.
Both the major manufacturer of the type of sailboat involved in these mishaps and the standards and regulatory bodies concerned have taken action to reduced this risk of injury. As a result, so far as I know, there has been no litigation at all involving this product since the manufacturer’s last actions about 10 years ago
Briefly, the use of a small catamaran boat having a higher aluminum mast than existed on previous boats of that size permitted contact between the mast and power lines in locations where such incidents had not occurred previously. The standards and
regulations for the minimum height of electrical power lines over lakes, bodies of water, and launching sites did not require sufficient elevation.
First, the standard was changed. While not legally required to, most power companies quickly raised their lines over water. Second, the manufacturer instituted an aggressive campaign to notify and educate the owners of existing boats about the hazard. Third, the manufacturer changed his design to include a protective device.
These three actions have apparently eliminated injuries from this hazard. Similar actions by crane manufacturers and electric power utilities would be very likely to accomplish the same result.
Original Signed Ben J. Lehman
Melvin L. Myers, MPA 1293 Berkeley Road
Avondale Estates, Georgia 30002-1517 February 23, 2004
David V. MacCollum
Chairman, Board of Governor
Hazard Information Foundation, Inc. (HIFI) P.O. Box 3902
Sierra Vista, AZ 85636-3962 Dear Mr. MacCollum:
Powerline contact is a significant cause of death among construction as well as other workers, and I have had an opportunity to review a draft of your report entitled, Safety Interventions to Control Hazards Related to Power Line Contacts by Mobile Cranes and Other Boomed Equipment. I am in full agreement with the need and necessity for a “Prevention of Powerline Contact Plan” as presented in the report.
The elimination of the hazard is possible and should be required on construction sites. Indeed, many sites such as industrial parks and housing develops are burying their electric lines, which eliminates the hazards. Likewise, this approach for burying (or relocating) power lines has become a recognized practice in industrial construction, such as by Jacobs Construction on their joint venture in the Spallation Neutron Source Project at Oakridge, Tennessee. The HIFI report addresses the improved technology (boring equipment) and approaches (underground mapping and pre-dig programs) that augment placing electrical lines underground.
The elimination of the hazard as a first priority is consistent with engineering safety and public health approaches for controlling hazards. Fundamentally, this priority is to control the problem at the source as a first line of protection. In safety engineering, this means get rid of the
possibility of electrical contact, and in industrial hygiene, it is to substitute the technology with something less dangerous.
The second priority in engineering safety and public health is to place a barrier between the source of the hazard and the person at risk. In the report, distance is a barrier by relocating the overhead power line. Another barrier is the use of insulating guards to obstruct the path of current flow in case of an overhead power contact. This secondary line of protection must be used in anticipation of a power line contact that may arise during the transport of equipment or where lines are obscured by overcast days or other circumstances.
The third line of protection in engineering safety and public health is to warn of the hazard. The use of the “Powerline Danger Zone” to mark the ground near overhead power lines is a
recognized warning procedure, and the use of proximity warning devises is another approach to alert the crane operator to the nearby hazard of power lines.
Every worker has a right to safe and healthful working conditions, and the duty to provide a worksite free of recognized hazards such as overhead power lines is placed upon the employer.
However, the duty chain traces back to the sources of the hazard and its prevention: the planners of the project, the manufacturers and purveyors of the equipment used, and the utilities that place the power lines.
No matter the line of protection provided, those planning and engaged in working with or near mobile-boomed equipment need training related to eliminating, guarding against, and warning of electrical power line contact. The use of a “Prevention of Powerline Contact Plan,” especially as related to controlling the problem at the source, is a necessary step in assuring that the worksite be free of the possibility of power line contact.
Melvin L. Myers
Jeff Speer
SUBJECT: Review of Safety Interventions to Control Hazards Related to Power Line Contracts by Mobile Cranes
This review of the “Safety Interventions to Control Hazards Related to Power Line Contracts by Mobile Cranes and Other Boomed Equipment” by Hazard Information Foundation
Incorporated (HIFI) incorporates a system safety perspective at time facility and equipment design. The system safety process involves the application of engineering and management principles, criteria and techniques to identify and eliminate hazards, recommend risk reduction techniques and document system hazards, which assist in optimizing all aspects of safety within the constraints of cost, schedule and design requirements throughout all phases of the system life cycle. The very nature of construction project activities can be considered a system since it involves a composite of people, procedures, materials, tools, equipment, facilities, software and design being used together in an environment to perform a given task or achieve a specific production, support, or project requirement. Incorporating the system safety process into construction project management and mobile crane operations will assist in maximizing the creation of a safer work environment and lessen the potential for power line contracts.
The HIFI study uses both an extensive timeline with selected case studies to provide a systematic evaluation of the powerline contact hazard and control of the hazard. These design activities follow the Hazard Reduction Precedence Sequence (HRPS) used within system safety. There is an order of precedence that HRPS follows to satisfy system safety requirements and resolving identified hazards: design to eliminate hazard, design to reduce hazard, provide safety devices, provide warning devices, and provide special procedures and training. The most effective involves design to eliminate the hazard. Throughout the study, HIFI has shown the powerline contracts occurrence happen due to poor preplanning and reliance on “thin air clearances”
(procedural control). In the ‘Recommendations’, the combination of ‘Organizational’ and
‘Managerial’ categories identified by HIFI reflect an understanding of how the principles of HRPS incorporated into effective Construction Safety Plan will eliminate the powerline hazard contact. .
Elimination of overhead powerlines is the most effective means for reducing powerline contacts available to management to use. A key element identified in the HIFI study is the role played by various management levels, summarized in the ‘Recommendations’ under the category of
‘Managerial’. The incorporation of system safety principles within construction and facilities
maintenance management programs allows examination of the interrelationship of all components within these programs, identified hazards and resolution, made available the management review process for automatic consideration in a total program or project perspective. It provides management with an effective means of identifying what hazard elements exist and means of implementing solutions to eliminate or control the hazard prior to requiring crane operations.
Another key element within system safety is the performance of hazard analyses, both during design but especially during the operational phases. As HIFI timeline illustrated, an evolution of a variety of engineering design elements, such as the insulated link and the electrostatic proximity alarm has occurred within the crane industry to increase operational safety.
Accomplishment of hazard analyses allows identification of hazards, which will require resolution. HIFI has identified several elements, which need to become integral parts of an effective safety program, i.e. Powerline Contact Prevention Plan, providing a safe environment for crane operations. A planned and implemented safety program must integrate safety analyses with other factors to influence management decisions to ensure a safe work environment prior to the arrival of equipment and personnel.
An important element within any safety program is feedback. Feedback allows for the
verification of the implementation and effectiveness of hazard control and it can be iterative until the prevention of an incident or accident event successfully occurs. The HIFI study provides critical knowledge allowing the reader to learn from the various elements presented and to translate and implement this knowledge to any activities requiring crane operations to support or complete required tasks without an incident or accident occurrence. This study represents a system safety process and recommends implementation of a number of corrective actions before the task (crane operations) may proceed.
Original signed Jeff Speer
System Safety Engineer
Human Factors Perspective
The research study, with its Thirty (30) recommendations, addresses the frailties of the ten foot clearance rule, which is the current OSHA guideline for operating personnel. The key issue lies in gaining the involvement and cooperation of all management; working together to
The research study, with its Thirty (30) recommendations, addresses the frailties of the ten foot clearance rule, which is the current OSHA guideline for operating personnel. The key issue lies in gaining the involvement and cooperation of all management; working together to