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Contact of heavy vehicles with overhead power lines
Safety Science 49 (2011) 951–955
Contents lists available at ScienceDirect
Safety Science journal homepage: www.elsevier.com/locate/ssci
Contact of heavy vehicles with overhead power lines J.D. Koustellis ⇑, S.D. Anagnostatos, C.D. Halevidis, F.S. Karagrigoriou, A.D. Polykrati, P.D. Bourkas Electrical and Computer Engineering, National Technical University of Athens, 9 Iroon Polytechneiou Str., Polytechneioupoli Zografou, Athens, Greece
a r t i c l e
i n f o
Article history: Received 28 October 2010 Received in revised form 3 February 2011 Accepted 7 February 2011 Available online 21 March 2011 Keywords: Contact with overhead power lines Heavy vehicle Occupational accidents Health and safety
a b s t r a c t Contact with overhead power lines is regarded worldwide as one of the leading causes of electrical fatalities. The analysis carried out in this paper refers to a real accident that occurred in Greece a few years ago, involving the contact of a heavy vehicle (concrete pump) with the medium voltage lines of the national distribution network. For a better understanding of the incident, a description of how the protection system of medium voltage distribution lines works when a fault occurs is given. Furthermore, emphasis has been laid on possible ways to avoid similar accidents in the future. Ó 2011 Elsevier Ltd. All rights reserved.
1. Introduction Electrocution is one of the most significant causes of fatal occupational accidents in the world (Cawley and Homce, 2003, 2008; Crow, 2009; Paques, 1993). For example, in the United States, between 1992 and 2002, fatal incidents involving electricity rank sixth among all causes of occupational fatalities, while contact with overhead power lines caused 41% of all occupational fatal electrocutions (Cawley and Homce, 2008). It has also been reported that in the years 2003–2006, in the construction industry the percentage of such fatalities amounted to 47.2% and in other production industries (agricultural, manufacturing, transport, etc.) the number was 45% (Janicak, 2008). Despite the great effort that has been made worldwide to avoid such accidents, the rates of fatal electrocutions involving overhead power lines are still high, with an average of 130 deaths per year. The same rates apply for Canada and Europe (Paques, 1993). Contact with overhead power lines can occur during installation, maintenance, or other work on the distribution network, when heavy goods vehicles carry construction material or even during tree pruning. Contact may be direct (a worker touches the line with a part of his body) or indirect (a vehicle or a moving part of it touches the line or contact with ladders, pipes etc.). In the construction industry in particular, the unintentional contact with overhead power lines is by far the most common type of heavy vehicle accidents (Paques, 1993). The accident analysis in ⇑ Corresponding author. Tel.: +30 6937449696. E-mail addresses: [email protected] (J.D. Koustellis), [email protected] (S.D. Anagnostatos), [email protected] (C.D. Halevidis), fkaragrhgoriou@central. ntua.gr (F.S. Karagrigoriou), [email protected] (A.D. Polykrati), pbourk@as. ntua.gr (P.D. Bourkas). 0925-7535/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.ssci.2011.02.011
relation to the type of vehicles and the voltage level involved is of great interest. It appears cranes and boom trucks are the vehicles most often involved in accidents where unintentional contact with power lines occurs, because of their nature and difficulty in accurately controlling extending moving parts. In relation to the voltage level, the majority of fatal accidents involve medium voltage power lines (Paques, 1993). A possible explanation for this is the fact that low voltage power lines are positioned in a vertical arrangement and in case of crane contact, the crane is most likely to touch two lines simultaneously, which results in a short circuit and the immediate melting of the electrical fuse. However, medium voltage power lines have a horizontal arrangement and in case of contact, the crane is most likely to touch one line and remain under voltage without triggering the protection system. This paper refers to a real accident that happened in Greece, where a concrete pump came in contact with the medium voltage power lines of the national distribution network system. An attempt is made to explain the causes of the accident and to propose measures to prevent not only the accident itself but also its deadly outcome.
2. Description of the accident In this accident a concrete pump was reversing to a slope to wash out the remaining cement. While the vehicle was reversing, its raised rear came into contact with a medium voltage overhead power line. According to on the spot eyewitnesses, the vehicle became enveloped in smoke and the characteristic noise of tires exploding was heard clearly. While attempting to exit the vehicle, the driver touched the metal of the cabin and on touching the
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ground suffered an electric shock and lost consciousness. One eyewitness, not realizing what had actually happened, tried to help the unconscious driver but, as he ran to him, he fell unconscious to the ground, too. According to the evidence of a second eyewitness both individuals were lying on the ground, the driver with severe burns. The site of the accident had originally been characterized as non-accessible to this type of vehicle and according to the national specification, in such places the safety clearance of power lines is determined at a height of 4.5 m (Public Power Corporation, 1982). According to the annual inspection carried out by qualified personnel of the national distribution company Public Power Corporation (PPC) in the year before the accident, the safety distances were as should be. On the day of the accident though, the overhead power lines were found to be at a distance less than 4 m from the ground. The explanation for this is that repeated illegal dumping of rubble had occurred at this spot since the previous inspection had been carried out, resulting in the leveling of the sloping ground and subsequent raising of ground level shown in Fig. 1. According to the technical report released by the PPC, who conducted the investigation of the accident, it turned out that at the moment of the accident a power cut initiated by the protection system (oil circuit breaker) was recorded, followed by a reclosing of the auto reclosing circuit breakers (ARCB). The sequence of voltage interruption followed by an electricity restoration (or line energizing), occurred repeatedly approximately every minute until the permanent power cut. The recorded evidence seems to indicate that the network’s protection system failed to operate properly since the power supply was not interrupted immediately. However, this is not the case as will be shown below.
3. Description of the auto reclosing circuit breakers (ARCB) functionality The overhead power lines of the medium voltage network are protected with simple devices called auto reclosing circuit breakers (ARCB) (ABB, 2001; Bourkas, 2004). Thus, when the recloser detects a fault, it opens within 0.15 s and breaks the circuit (immediate fault interruption). Then, after a lapse of 0.45 s, it closes and automatically restores the energizing of the line. If the fault continues, the recloser breaks the circuit in a time frame ranging from 0.5 to 6 s (1st cycle). The next attempt to reenergize the line occurs 5 s later. This sequence (energizing for 0.5–6 s, break for 5 s) is repeated once more (2nd cycle). The auto reclosing circuit breaker closes one last time (3rd cycle) and if the fault remains, then the power is cut permanently by the primary circuit breaker (oil circuit breaker) (ABB, 2001; Bourkas, 2004). The whole operation of the auto reclosing circuit breaker appears in simplified form in Fig. 2. The auto reclosing circuit breakers are set as described above to avoid a power cut in a whole area in the event of a non-permanent fault, such as the contact of a tree branch with a power line in strong winds. 4. Causes of the accident At this point it must be mentioned that rubber as a material behaves as a perfect insulator and can stand voltage to the order of 400 kV (Bourkas, 2004). The vehicle tires however, also have a fine metallic net to reinforce their durability, which also lends them antistatic. The electrical resistance of the tire has been measured in the laboratory and found to be to the order of 2 MX.
Fig. 1. Power lines clearance before and after the landfill.
J.D. Koustellis et al. / Safety Science 49 (2011) 951–955
Fig. 2. Complete operation cycle of an auto reclosing circuit breaker.
When the concrete pump came into contact with the overhead power line, the vehicle came under voltage, which initiated partial discharges in the tires, so that at some point tire-breakdown (tirefission) occurred. Partial discharges are in fact small leakage currents, which in this particular case were smaller than the interruption current of the auto reclosing circuit breaker and that is the reason why the circuit breaker remained closed. But at the moment of breakdown, the leakage current was strong enough to cause the opening of the circuit breaker. When the auto reclosing circuit breaker closed again, as it is set to function, the fault no longer existed as the vehicles tire acted as an insulator again. The circuit breaker remained closed until another breakdown occurred in some other section of the tire. So when the next tire-breakdown occurred, one minute later in fact, as has been recorded by the Public Power Company (PPC) and can be calculated theoretically, the power line’s protection system recognized the new leakage current as a new fault situation and the three cycle sequence began all over again. This sequence would have probably repeated itself many times more until the total breakdown of the tires caused a short circuit. But as the driver attempted to exit the vehicle and touched the metallic parts of the cabin and the ground simultaneously, he caused a short circuit and the permanent opening of the recloser after the three cycle sequence, as described above. 5. Discussion and proposals From the above presentation it is obvious that this accident would not have happened if the ground level hadn’t been raised, thus reducing the actual distance to the medium voltage overhead
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power line. Hence it must be underlined at this point that the rubble drops at this site were illegal and those responsible failed to notify any public authority. Thus, although the annual inspection by the PPC had been carried out as specified, the subsequent illegal landfill could not have been foreseen. To avoid such unfortunate accidents, a series of structural changes must be implemented, not only on an organizational level but on a technical level as well. The purpose of all the proposed measures listed below is the general adoption of a more workbased way of thinking, the promotion of a safety at work ethic and increasing the general public’s awareness. Because in accidents like the one examined in this paper, those involved are not only specialized workers, but civilians as well, the changes that must be adopted concern both legislation and the actions of public authorities. Therefore, all the responsible public authorities (Department of Environment, Ministry of Labour, Prefectures, Municipalities, Police Department, City Planning Department etc.) must carry out inspections in all such areas known for unauthorized rubble drops, land-filling, etc. In this particular case, of course, the Safety Engineer of the owner company of the concrete pump should have carried out an inspection in the first place and then taken appropriate action. The same applies to the Safety Engineer of the construction company who rented and used the vehicle. Much of the responsibility falls also to the owner of the site where the accident took place after the unauthorized rubble drops. If attention had been paid to simple safety rules and existing legislation adhered to, then it is more than likely that the hazard would never had existed and this tragic accident would never have occurred. The analysis of the accident as described above, also leads to another important conclusion. The regrettable outcome of this incident could possibly have been avoided if the driver of the vehicle had had the necessary knowledge and experience to know how to act in such a situation. Immediately after the vehicle had become enveloped in smoke and sound of exploding tires could be clearly heard, the driver should have tried, if possible, to drive the concrete pump away from the overhead power line. If this proved impossible, he should have stayed in the vehicle supposing of course that the cabin had the necessary dielectic strength as should be the case. If escape from the vehicle was absolutely necessary because of fire, then he should have tried to avoid the metallic parts of the cabin and jumped, with limbs folded in, as far as possible away from it. In this way he could have avoided the step voltage, which is found in a small radius around the vehicle according to Eq. (1) (Bourkas, 2008) and in any case the consequences would have been less dramatic.
VP ¼
qI 2pr
ð1Þ
where VP is the step voltage in (V), at a distance r in (m), while q is the resistivity of the earth in (Xm) and I is the fault current in (A). The same principle as above applies to the first eyewitness who rushed to the aid of the injured driver and suffered an electric shock himself, after entering the area near the vehicle affected by the step voltage. It is obvious that such a lack of basic knowledge in specialized workers, who are frequently exposed to such hazards, is unacceptable. The following proposals concern the implementation of organization and technical measures. 5.1. Proposals concerning organization i. Collaboration with an independent qualified inspection team for the supervision of every project and the inclusion of Safety Officer Services for every production work
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ii.
iii.
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(construction, maintenance, installation, transport, agricultural industry, stock farming, etc.). Introduction of legislation requiring the compulsory evaluation of potential hazards and recommendations for the prevention of accidents caused by contact with overhead power lines as pre-requisites for the issue of any building license. The prefecture, in conjunction with the Technical Chamber and Technical University, should organize compulsory seminars to inform prospective drivers of the dangers of overhead power lines. These educational courses should also be made available for workers of other production industries such as the agricultural, stock farming, transport industry, construction etc. in order to raise awareness of the dangers described above in all relevant sectors of the working community. Every Public Authority (Ministries, Prefectures, Municipalities, Police Departments, City Planning Departments etc.) should launch regular campaigns to inform both workers and the general public, of the potential dangers of overhead power lines, because as aforementioned in Section 1, a large majority of such accidents involve the contact of a power line with a long object (as for example ladders, pipes etc.) and occur not only on worksites but on flat roofs or home balconies as well. On a national level, the authorities should establish inspection teams to monitor not only working conditions on building sites but also the unlicensed rubble dumping in forests and other sites. All Electricity Supply Companies of the country should make a concerted effort to lay all existing overhead power lines and any new extensions of the network, underground. The additional cost of such an undertaking would be more than offset by the avoidance of future accidents with their accompanying toll in human life and property. Furthermore, the gradual replacement of the overhead power lines with underground cabling could deliver reduced fire risk (2009 Victorian Bushfires Royal Commission, 2010).
5.2. Technical proposals i. A high voltage proximity alarm device should be installed in every heavy vehicle that could be possibly involved in an accident with overhead power lines. Such devices detect the presence of electrical fields and sound an alarm if the vehicle should come into close proximity to a high voltage line, in order to alert the operator of the vehicle. A brief look at existing alarm systems on the market, showed that these devices could also provide a warning to alert the operator when he is approaching an alarm set point (Reynolds, 2010). ii. In areas where construction works are in progress, the protection system of the medium voltage network, and the auto reclosing circuit breaker in particular should be set in such a way so as to cut the power of a line the first time a fault is detected. Obviously this would mean additional cost for the Electricity Company, since every time a fault occurs, even if it is not a permanent fault (like the contact of a tree branch with a power line due to strong winds), the company must send a repair team to reenergize the power line. This is a cost, however, that is worth incurring in order to safeguard human life. iii. In areas where no construction activity is expected, the network protection system should be set in such a way so that when three faults occur in succession at a certain power line, the oil circuit breaker should immediately cut off the power. So in a case like the one described in this paper, when tirefission occurs, as a result of a vehicle coming into contact
iv.
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vii.
viii. ix.
x.
xi.
xii.
with power lines, the three-cycle operation will not start all over again and the fault will be recognized as one and the same. A techno-economic study should be carried out for the possible reorganization of the medium voltage network so that the conductors are arranged vertically. This measure will not solve the problem, but it will be possible to reduce the number of fatal accidents. The possibility of placing a powerful insulator on all parts of a heavy vehicle that are likely to come into contact with overhead power lines should be looked into (Paques, 1993). Heavy vehicles like cranes or concrete pumps that can be operated from at distance should make use of a wireless remote control box. In this way, even in case of contact with power lines, casualties will be avoided. The driver’s cabin of every heavy vehicle should be equipped with an insulation mattress that can be used to enable the driver to exit the vehicle in case of a power line contact, and avoid the step voltage around the vehicle. The exact nature and properties of such a mattress must be researched and laboratory tested before being launched onto the market. Brightly-colored and highly visible signs should be placed on every pylon especially in areas with easy access. In areas where works are due to be carried out below overhead power lines, additional signs should be put up to inform workers of the dangers. Before any works commence in areas with overhead power lines, the Safety Engineer should first determine the danger zones and inform the drivers of the vehicles and the rest of the workers of legal safety distances. In case of an alteration to the original building plan whilst works are in progress, both the project manager and the safety engineer should be immediately informed. During the operation of any type of heavy vehicle (crane, concrete pump, transport vehicles etc.) a skilled worker should be put in charge of surveillance of vehicle operation, to monitor their every movement, to aid and warn drivers in case of danger.
The protection system of medium and low voltage networks implemented in Greece by the PPC, is anthropocentric as the clearance requirements are set in such a way as to ensure maximum safety. However, the fact that fatal accidents involving power lines occur so frequently suggests the need to promote new regulations to include, not only the above mentioned suggestions but also those proposed in other papers and studies (Crow, 2009; Hazard Information Foundation Inc. (HIFI), 2002; Homce et al., 2001; Janicak 2008; Paques, 1993; Sacks et al., 2001). In conclusion, all responsible bodies should support a coordinated effort to inform the public and assist in the running of many more safety workshops focusing on safety matters concerning electrical current and overhead power lines. In any case, these electrical accidents have already provoked a response from a great part of the scientific community in Greece and such material is already included in the curriculum of the National Technical University of Athens, as part of the lesson in ‘‘Industrial and Building Installations’’ that is held in the faculty of Electrical and Computer Engineering. Another issue that is of great interest is the contact of other types of vehicles with overhead power lines, like vehicles equipped with aerial lifts, broadcasting vehicles with telescopic antenna masts, surveillance trucks or even cases where the crane itself doesn’t come into direct contact with a power line, but the lifted object or the metal cable stabilizing it does. In any case, it is encouraging that during this research and from personal discussions with representatives from public authorities,
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construction companies, production industries, engineers, machines operators and press editors – it has become clear that this study has deeply sensitized all and there is a general willingness to help to ensure that such accidents be avoided in the future. 6. Conclusions This paper examined an actual fatal accident that happened in Greece, where a concrete pump came in contact with the medium voltage power lines of the national distribution network system. For better understanding of the incident, the protection system of the medium and low voltage distribution lines as well as its specific operation in case of a fault was described. It became clear that the whole protection system is anthropocentric and the clearance distances are set in such a way to minimize the danger. The fact, however, that contacts with overhead power lines occur relatively so often in many production sectors, has led us to make a series of proposals to bring about the necessary changes not only on an organizational level but on technical levels as well, in order to spread a safer work ethic and above all to raise awareness in society in general. Contact with overhead power lines is considered worldwide as one of the leading causes of all occupational fatal electrocutions and the electrical community is duty bound to support a coordinated effort to inform the public and help prevent such electrical accidents. References 2009 Victorian Bushfires Royal Commission, 2010. Victoria’s Bushfire Safety Policy, Final report recommendations, Recommendation 27, Australia.
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royalcommission.vic.gov.au/Assets/VBRC-Final-Report-Recommendations.pdf> (accessed 31.07.10) ABB, 2001. Switchgear Manual, 10th ed. Cornelsen Verlag, Berlin, Germany, pp. 384, 459 and 715. Bourkas, P.D., 2004. Applications of Industrial and Building Installations. National Technical University of Athens Editions, Athens, GR. Bourkas, P.D., 2008. Applications of Industrial and Building Installations, Exercise Book. National Technical University of Athens Editions, Athens, GR. Cawley, J.C., Homce, G.T., 2003. Occupational electrical injuries in the United States, 1992–1998, and recommendations for safety research. Journal of Safety Research 34, 241–248. Cawley, J.C., Homce, G.T., 2008. Trends in electrical injury in the US, 1992–2002. IEEE Transactions on Industry Applications 44, 962–972. Crow, D.R., 2009. Contact with Overhead Power Lines How Can We Prevent This. Electrical Safety Workshop IEEE IAS ESW2009-08, pp. 1–3. Hazard Information Foundation Inc. (HIFI), 2002. Research Report: Safety Interventions to Control Hazards Related to Power Line Contacts by Mobile Cranes and Other Boomed Equipment. Homce, G.T., Cawley, J.C., Yenchek, M.R., Sacks, H.K., 2001. An alarm to warn of overhead power line contact by mobile equipment. In: Industry Applications Conference, Thirty Six IAS Annual Meeting, vol. 2, pp. 1376–1383. Janicak, C.A., 2008. Occupational fatalities due to electrocutions in the construction industry. Journal of Safety Research 39, 617–621. Paques, J.J., 1993. Crane accidents by contact with powerlines. Safety Science 16, 129–142. Public Power Corporation S.A., 1982. Minimum Distances of Medium Voltage Line’s Conductors to the Ground, Specification No. C14, Athens, GR. Reynolds, R.L., 2010. Informational Report 1035: Field Evaluation of a Proximity Alarm Device, Pittsburgh Technical Support Center, Pittsburgh PA, pp. 107–115. (accessed 20.10.10). Sacks, H.K., Cawley, J.C., Homce, G.T., Yenchek, M.R., 2001. Feasibility study to reduce injuries and fatalities caused by contact of cranes, drill rigs and haul trucks with high tension lines. IEEE Transactions on Industry Applications 37, 914–919.
Contents lists available at ScienceDirect
Safety Science journal homepage: www.elsevier.com/locate/ssci
Contact of heavy vehicles with overhead power lines J.D. Koustellis ⇑, S.D. Anagnostatos, C.D. Halevidis, F.S. Karagrigoriou, A.D. Polykrati, P.D. Bourkas Electrical and Computer Engineering, National Technical University of Athens, 9 Iroon Polytechneiou Str., Polytechneioupoli Zografou, Athens, Greece
a r t i c l e
i n f o
Article history: Received 28 October 2010 Received in revised form 3 February 2011 Accepted 7 February 2011 Available online 21 March 2011 Keywords: Contact with overhead power lines Heavy vehicle Occupational accidents Health and safety
a b s t r a c t Contact with overhead power lines is regarded worldwide as one of the leading causes of electrical fatalities. The analysis carried out in this paper refers to a real accident that occurred in Greece a few years ago, involving the contact of a heavy vehicle (concrete pump) with the medium voltage lines of the national distribution network. For a better understanding of the incident, a description of how the protection system of medium voltage distribution lines works when a fault occurs is given. Furthermore, emphasis has been laid on possible ways to avoid similar accidents in the future. Ó 2011 Elsevier Ltd. All rights reserved.
1. Introduction Electrocution is one of the most significant causes of fatal occupational accidents in the world (Cawley and Homce, 2003, 2008; Crow, 2009; Paques, 1993). For example, in the United States, between 1992 and 2002, fatal incidents involving electricity rank sixth among all causes of occupational fatalities, while contact with overhead power lines caused 41% of all occupational fatal electrocutions (Cawley and Homce, 2008). It has also been reported that in the years 2003–2006, in the construction industry the percentage of such fatalities amounted to 47.2% and in other production industries (agricultural, manufacturing, transport, etc.) the number was 45% (Janicak, 2008). Despite the great effort that has been made worldwide to avoid such accidents, the rates of fatal electrocutions involving overhead power lines are still high, with an average of 130 deaths per year. The same rates apply for Canada and Europe (Paques, 1993). Contact with overhead power lines can occur during installation, maintenance, or other work on the distribution network, when heavy goods vehicles carry construction material or even during tree pruning. Contact may be direct (a worker touches the line with a part of his body) or indirect (a vehicle or a moving part of it touches the line or contact with ladders, pipes etc.). In the construction industry in particular, the unintentional contact with overhead power lines is by far the most common type of heavy vehicle accidents (Paques, 1993). The accident analysis in ⇑ Corresponding author. Tel.: +30 6937449696. E-mail addresses: [email protected] (J.D. Koustellis), [email protected] (S.D. Anagnostatos), [email protected] (C.D. Halevidis), fkaragrhgoriou@central. ntua.gr (F.S. Karagrigoriou), [email protected] (A.D. Polykrati), pbourk@as. ntua.gr (P.D. Bourkas). 0925-7535/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.ssci.2011.02.011
relation to the type of vehicles and the voltage level involved is of great interest. It appears cranes and boom trucks are the vehicles most often involved in accidents where unintentional contact with power lines occurs, because of their nature and difficulty in accurately controlling extending moving parts. In relation to the voltage level, the majority of fatal accidents involve medium voltage power lines (Paques, 1993). A possible explanation for this is the fact that low voltage power lines are positioned in a vertical arrangement and in case of crane contact, the crane is most likely to touch two lines simultaneously, which results in a short circuit and the immediate melting of the electrical fuse. However, medium voltage power lines have a horizontal arrangement and in case of contact, the crane is most likely to touch one line and remain under voltage without triggering the protection system. This paper refers to a real accident that happened in Greece, where a concrete pump came in contact with the medium voltage power lines of the national distribution network system. An attempt is made to explain the causes of the accident and to propose measures to prevent not only the accident itself but also its deadly outcome.
2. Description of the accident In this accident a concrete pump was reversing to a slope to wash out the remaining cement. While the vehicle was reversing, its raised rear came into contact with a medium voltage overhead power line. According to on the spot eyewitnesses, the vehicle became enveloped in smoke and the characteristic noise of tires exploding was heard clearly. While attempting to exit the vehicle, the driver touched the metal of the cabin and on touching the
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ground suffered an electric shock and lost consciousness. One eyewitness, not realizing what had actually happened, tried to help the unconscious driver but, as he ran to him, he fell unconscious to the ground, too. According to the evidence of a second eyewitness both individuals were lying on the ground, the driver with severe burns. The site of the accident had originally been characterized as non-accessible to this type of vehicle and according to the national specification, in such places the safety clearance of power lines is determined at a height of 4.5 m (Public Power Corporation, 1982). According to the annual inspection carried out by qualified personnel of the national distribution company Public Power Corporation (PPC) in the year before the accident, the safety distances were as should be. On the day of the accident though, the overhead power lines were found to be at a distance less than 4 m from the ground. The explanation for this is that repeated illegal dumping of rubble had occurred at this spot since the previous inspection had been carried out, resulting in the leveling of the sloping ground and subsequent raising of ground level shown in Fig. 1. According to the technical report released by the PPC, who conducted the investigation of the accident, it turned out that at the moment of the accident a power cut initiated by the protection system (oil circuit breaker) was recorded, followed by a reclosing of the auto reclosing circuit breakers (ARCB). The sequence of voltage interruption followed by an electricity restoration (or line energizing), occurred repeatedly approximately every minute until the permanent power cut. The recorded evidence seems to indicate that the network’s protection system failed to operate properly since the power supply was not interrupted immediately. However, this is not the case as will be shown below.
3. Description of the auto reclosing circuit breakers (ARCB) functionality The overhead power lines of the medium voltage network are protected with simple devices called auto reclosing circuit breakers (ARCB) (ABB, 2001; Bourkas, 2004). Thus, when the recloser detects a fault, it opens within 0.15 s and breaks the circuit (immediate fault interruption). Then, after a lapse of 0.45 s, it closes and automatically restores the energizing of the line. If the fault continues, the recloser breaks the circuit in a time frame ranging from 0.5 to 6 s (1st cycle). The next attempt to reenergize the line occurs 5 s later. This sequence (energizing for 0.5–6 s, break for 5 s) is repeated once more (2nd cycle). The auto reclosing circuit breaker closes one last time (3rd cycle) and if the fault remains, then the power is cut permanently by the primary circuit breaker (oil circuit breaker) (ABB, 2001; Bourkas, 2004). The whole operation of the auto reclosing circuit breaker appears in simplified form in Fig. 2. The auto reclosing circuit breakers are set as described above to avoid a power cut in a whole area in the event of a non-permanent fault, such as the contact of a tree branch with a power line in strong winds. 4. Causes of the accident At this point it must be mentioned that rubber as a material behaves as a perfect insulator and can stand voltage to the order of 400 kV (Bourkas, 2004). The vehicle tires however, also have a fine metallic net to reinforce their durability, which also lends them antistatic. The electrical resistance of the tire has been measured in the laboratory and found to be to the order of 2 MX.
Fig. 1. Power lines clearance before and after the landfill.
J.D. Koustellis et al. / Safety Science 49 (2011) 951–955
Fig. 2. Complete operation cycle of an auto reclosing circuit breaker.
When the concrete pump came into contact with the overhead power line, the vehicle came under voltage, which initiated partial discharges in the tires, so that at some point tire-breakdown (tirefission) occurred. Partial discharges are in fact small leakage currents, which in this particular case were smaller than the interruption current of the auto reclosing circuit breaker and that is the reason why the circuit breaker remained closed. But at the moment of breakdown, the leakage current was strong enough to cause the opening of the circuit breaker. When the auto reclosing circuit breaker closed again, as it is set to function, the fault no longer existed as the vehicles tire acted as an insulator again. The circuit breaker remained closed until another breakdown occurred in some other section of the tire. So when the next tire-breakdown occurred, one minute later in fact, as has been recorded by the Public Power Company (PPC) and can be calculated theoretically, the power line’s protection system recognized the new leakage current as a new fault situation and the three cycle sequence began all over again. This sequence would have probably repeated itself many times more until the total breakdown of the tires caused a short circuit. But as the driver attempted to exit the vehicle and touched the metallic parts of the cabin and the ground simultaneously, he caused a short circuit and the permanent opening of the recloser after the three cycle sequence, as described above. 5. Discussion and proposals From the above presentation it is obvious that this accident would not have happened if the ground level hadn’t been raised, thus reducing the actual distance to the medium voltage overhead
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power line. Hence it must be underlined at this point that the rubble drops at this site were illegal and those responsible failed to notify any public authority. Thus, although the annual inspection by the PPC had been carried out as specified, the subsequent illegal landfill could not have been foreseen. To avoid such unfortunate accidents, a series of structural changes must be implemented, not only on an organizational level but on a technical level as well. The purpose of all the proposed measures listed below is the general adoption of a more workbased way of thinking, the promotion of a safety at work ethic and increasing the general public’s awareness. Because in accidents like the one examined in this paper, those involved are not only specialized workers, but civilians as well, the changes that must be adopted concern both legislation and the actions of public authorities. Therefore, all the responsible public authorities (Department of Environment, Ministry of Labour, Prefectures, Municipalities, Police Department, City Planning Department etc.) must carry out inspections in all such areas known for unauthorized rubble drops, land-filling, etc. In this particular case, of course, the Safety Engineer of the owner company of the concrete pump should have carried out an inspection in the first place and then taken appropriate action. The same applies to the Safety Engineer of the construction company who rented and used the vehicle. Much of the responsibility falls also to the owner of the site where the accident took place after the unauthorized rubble drops. If attention had been paid to simple safety rules and existing legislation adhered to, then it is more than likely that the hazard would never had existed and this tragic accident would never have occurred. The analysis of the accident as described above, also leads to another important conclusion. The regrettable outcome of this incident could possibly have been avoided if the driver of the vehicle had had the necessary knowledge and experience to know how to act in such a situation. Immediately after the vehicle had become enveloped in smoke and sound of exploding tires could be clearly heard, the driver should have tried, if possible, to drive the concrete pump away from the overhead power line. If this proved impossible, he should have stayed in the vehicle supposing of course that the cabin had the necessary dielectic strength as should be the case. If escape from the vehicle was absolutely necessary because of fire, then he should have tried to avoid the metallic parts of the cabin and jumped, with limbs folded in, as far as possible away from it. In this way he could have avoided the step voltage, which is found in a small radius around the vehicle according to Eq. (1) (Bourkas, 2008) and in any case the consequences would have been less dramatic.
VP ¼
qI 2pr
ð1Þ
where VP is the step voltage in (V), at a distance r in (m), while q is the resistivity of the earth in (Xm) and I is the fault current in (A). The same principle as above applies to the first eyewitness who rushed to the aid of the injured driver and suffered an electric shock himself, after entering the area near the vehicle affected by the step voltage. It is obvious that such a lack of basic knowledge in specialized workers, who are frequently exposed to such hazards, is unacceptable. The following proposals concern the implementation of organization and technical measures. 5.1. Proposals concerning organization i. Collaboration with an independent qualified inspection team for the supervision of every project and the inclusion of Safety Officer Services for every production work
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ii.
iii.
iv.
v.
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(construction, maintenance, installation, transport, agricultural industry, stock farming, etc.). Introduction of legislation requiring the compulsory evaluation of potential hazards and recommendations for the prevention of accidents caused by contact with overhead power lines as pre-requisites for the issue of any building license. The prefecture, in conjunction with the Technical Chamber and Technical University, should organize compulsory seminars to inform prospective drivers of the dangers of overhead power lines. These educational courses should also be made available for workers of other production industries such as the agricultural, stock farming, transport industry, construction etc. in order to raise awareness of the dangers described above in all relevant sectors of the working community. Every Public Authority (Ministries, Prefectures, Municipalities, Police Departments, City Planning Departments etc.) should launch regular campaigns to inform both workers and the general public, of the potential dangers of overhead power lines, because as aforementioned in Section 1, a large majority of such accidents involve the contact of a power line with a long object (as for example ladders, pipes etc.) and occur not only on worksites but on flat roofs or home balconies as well. On a national level, the authorities should establish inspection teams to monitor not only working conditions on building sites but also the unlicensed rubble dumping in forests and other sites. All Electricity Supply Companies of the country should make a concerted effort to lay all existing overhead power lines and any new extensions of the network, underground. The additional cost of such an undertaking would be more than offset by the avoidance of future accidents with their accompanying toll in human life and property. Furthermore, the gradual replacement of the overhead power lines with underground cabling could deliver reduced fire risk (2009 Victorian Bushfires Royal Commission, 2010).
5.2. Technical proposals i. A high voltage proximity alarm device should be installed in every heavy vehicle that could be possibly involved in an accident with overhead power lines. Such devices detect the presence of electrical fields and sound an alarm if the vehicle should come into close proximity to a high voltage line, in order to alert the operator of the vehicle. A brief look at existing alarm systems on the market, showed that these devices could also provide a warning to alert the operator when he is approaching an alarm set point (Reynolds, 2010). ii. In areas where construction works are in progress, the protection system of the medium voltage network, and the auto reclosing circuit breaker in particular should be set in such a way so as to cut the power of a line the first time a fault is detected. Obviously this would mean additional cost for the Electricity Company, since every time a fault occurs, even if it is not a permanent fault (like the contact of a tree branch with a power line due to strong winds), the company must send a repair team to reenergize the power line. This is a cost, however, that is worth incurring in order to safeguard human life. iii. In areas where no construction activity is expected, the network protection system should be set in such a way so that when three faults occur in succession at a certain power line, the oil circuit breaker should immediately cut off the power. So in a case like the one described in this paper, when tirefission occurs, as a result of a vehicle coming into contact
iv.
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viii. ix.
x.
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with power lines, the three-cycle operation will not start all over again and the fault will be recognized as one and the same. A techno-economic study should be carried out for the possible reorganization of the medium voltage network so that the conductors are arranged vertically. This measure will not solve the problem, but it will be possible to reduce the number of fatal accidents. The possibility of placing a powerful insulator on all parts of a heavy vehicle that are likely to come into contact with overhead power lines should be looked into (Paques, 1993). Heavy vehicles like cranes or concrete pumps that can be operated from at distance should make use of a wireless remote control box. In this way, even in case of contact with power lines, casualties will be avoided. The driver’s cabin of every heavy vehicle should be equipped with an insulation mattress that can be used to enable the driver to exit the vehicle in case of a power line contact, and avoid the step voltage around the vehicle. The exact nature and properties of such a mattress must be researched and laboratory tested before being launched onto the market. Brightly-colored and highly visible signs should be placed on every pylon especially in areas with easy access. In areas where works are due to be carried out below overhead power lines, additional signs should be put up to inform workers of the dangers. Before any works commence in areas with overhead power lines, the Safety Engineer should first determine the danger zones and inform the drivers of the vehicles and the rest of the workers of legal safety distances. In case of an alteration to the original building plan whilst works are in progress, both the project manager and the safety engineer should be immediately informed. During the operation of any type of heavy vehicle (crane, concrete pump, transport vehicles etc.) a skilled worker should be put in charge of surveillance of vehicle operation, to monitor their every movement, to aid and warn drivers in case of danger.
The protection system of medium and low voltage networks implemented in Greece by the PPC, is anthropocentric as the clearance requirements are set in such a way as to ensure maximum safety. However, the fact that fatal accidents involving power lines occur so frequently suggests the need to promote new regulations to include, not only the above mentioned suggestions but also those proposed in other papers and studies (Crow, 2009; Hazard Information Foundation Inc. (HIFI), 2002; Homce et al., 2001; Janicak 2008; Paques, 1993; Sacks et al., 2001). In conclusion, all responsible bodies should support a coordinated effort to inform the public and assist in the running of many more safety workshops focusing on safety matters concerning electrical current and overhead power lines. In any case, these electrical accidents have already provoked a response from a great part of the scientific community in Greece and such material is already included in the curriculum of the National Technical University of Athens, as part of the lesson in ‘‘Industrial and Building Installations’’ that is held in the faculty of Electrical and Computer Engineering. Another issue that is of great interest is the contact of other types of vehicles with overhead power lines, like vehicles equipped with aerial lifts, broadcasting vehicles with telescopic antenna masts, surveillance trucks or even cases where the crane itself doesn’t come into direct contact with a power line, but the lifted object or the metal cable stabilizing it does. In any case, it is encouraging that during this research and from personal discussions with representatives from public authorities,
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construction companies, production industries, engineers, machines operators and press editors – it has become clear that this study has deeply sensitized all and there is a general willingness to help to ensure that such accidents be avoided in the future. 6. Conclusions This paper examined an actual fatal accident that happened in Greece, where a concrete pump came in contact with the medium voltage power lines of the national distribution network system. For better understanding of the incident, the protection system of the medium and low voltage distribution lines as well as its specific operation in case of a fault was described. It became clear that the whole protection system is anthropocentric and the clearance distances are set in such a way to minimize the danger. The fact, however, that contacts with overhead power lines occur relatively so often in many production sectors, has led us to make a series of proposals to bring about the necessary changes not only on an organizational level but on technical levels as well, in order to spread a safer work ethic and above all to raise awareness in society in general. Contact with overhead power lines is considered worldwide as one of the leading causes of all occupational fatal electrocutions and the electrical community is duty bound to support a coordinated effort to inform the public and help prevent such electrical accidents. References 2009 Victorian Bushfires Royal Commission, 2010. Victoria’s Bushfire Safety Policy, Final report recommendations, Recommendation 27, Australia.
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royalcommission.vic.gov.au/Assets/VBRC-Final-Report-Recommendations.pdf> (accessed 31.07.10) ABB, 2001. Switchgear Manual, 10th ed. Cornelsen Verlag, Berlin, Germany, pp. 384, 459 and 715. Bourkas, P.D., 2004. Applications of Industrial and Building Installations. National Technical University of Athens Editions, Athens, GR. Bourkas, P.D., 2008. Applications of Industrial and Building Installations, Exercise Book. National Technical University of Athens Editions, Athens, GR. Cawley, J.C., Homce, G.T., 2003. Occupational electrical injuries in the United States, 1992–1998, and recommendations for safety research. Journal of Safety Research 34, 241–248. Cawley, J.C., Homce, G.T., 2008. Trends in electrical injury in the US, 1992–2002. IEEE Transactions on Industry Applications 44, 962–972. Crow, D.R., 2009. Contact with Overhead Power Lines How Can We Prevent This. Electrical Safety Workshop IEEE IAS ESW2009-08, pp. 1–3. Hazard Information Foundation Inc. (HIFI), 2002. Research Report: Safety Interventions to Control Hazards Related to Power Line Contacts by Mobile Cranes and Other Boomed Equipment. Homce, G.T., Cawley, J.C., Yenchek, M.R., Sacks, H.K., 2001. An alarm to warn of overhead power line contact by mobile equipment. In: Industry Applications Conference, Thirty Six IAS Annual Meeting, vol. 2, pp. 1376–1383. Janicak, C.A., 2008. Occupational fatalities due to electrocutions in the construction industry. Journal of Safety Research 39, 617–621. Paques, J.J., 1993. Crane accidents by contact with powerlines. Safety Science 16, 129–142. Public Power Corporation S.A., 1982. Minimum Distances of Medium Voltage Line’s Conductors to the Ground, Specification No. C14, Athens, GR. Reynolds, R.L., 2010. Informational Report 1035: Field Evaluation of a Proximity Alarm Device, Pittsburgh Technical Support Center, Pittsburgh PA, pp. 107–115.