Occupational Exposure to Electric and Magnetic Fields While Working at Switching and Transforming Stations of 110 kv

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1 Ann. Occup. Hyg., Vol. 55, No. 5, pp , 2011 Ó The Author Published by Oxford University Press on behalf of the British Occupational Hygiene Society doi: /annhyg/mer013 Occupational Exposure to Electric and Magnetic Fields While Working at Switching and Transforming Stations of 110 kv LEENA KORPINEN 1 *, HARRI KUISTI 2, RAUNO PÄÄKKÖNEN 3, PAULI VANHALA 4 and JARMO ELOVAARA 2 1 Environmental Health, Tampere University of Technology, Box 598, FIN Tampere, Finland; 2 Fingrid Oyj, Box 530, FIN Helsinki, Finland; 3 Finnish Institute of Occupational Health, Theme Well-being Solutions for the Workplace, Box 486, FIN Tampere, Finland; 4 Helsingin Energia, Helen, Helsinki, Finland Received 22 August 2010; in final form 10 January 2011; published online 30 March 2011 The aim of the study was to measure occupational exposure to electric and magnetic fields during various work tasks at switching and transforming stations of 110 kv (in some situations 20 kv), and analyze if the action values of European Union Directive 2004/40/EC or reference values of International Commission on Non-ionizing Radiation Protection (ICNIRP) were exceeded. The electric (n 5 765) and magnetic (n 5 203) fields were measured during various work tasks. The average values of all measurements were 3.6 kv m 1 and 28.6 mt. The maximum value of electric fields was 15.5 kv m 21 at task maintenance of operating device of circuit breaker from service platform. In one special work task close to shunt reactor cables (20 kv), the highest magnetic field was 710 mt. In general, the measured magnetic fields were below the reference values of ICNIRP. Keywords: electric fields; electrical workers; EMFs; magnetic fields; Physical agents directive INTRODUCTION *Author to whom correspondence should be addressed. Tel: þ ; fax: þ ; leena.korpinen@tut.fi The consumption of electricity has been growing for several decades. During the past 25 years, studies of public and occupational exposure to electric and magnetic fields (EMFs) and possible health effects have also increased for extremely low frequencies (ELF). Guidelines have been published for safe occupational exposure to power-frequency EMFs. The International Commission on Non-ionizing Radiation Protection (ICNIRP) has published new guidelines, ICNIRP statement guidelines, for limiting exposure to time-varying EMFs (1 Hz 100 khz) (ICNRP, 2010). According to the guidelines, the reference levels (50 Hz) for general public exposure to varying EMFs [unperturbed root mean square (RMS) values] are 5 kv m 1 and 200 lt. For occupational exposure, the reference levels (50 Hz) are 1000 ltand 10 kv m 1 (ICNIRP, 2010). Directive 2004/40/EC of the European Parliament and of the Council on the minimum health and safety requirements regarding the exposure of workers to the risks arising from physical agents (electromagnetic fields) was published in the European Union on 30 April Although the effective date of the directive was later postponed, it is not known if the action values given for power frequencies will change. The Directive includes both exposure limit values and action values. Action values at 50 Hz are electric field (10 kv m 1 ), magnetic flux density (500 lt), and total contact current (1 ma). The exposure limit value (50 Hz) for the current density for the head and trunk is 10 ma m 2 (European Parliament and Council, 2004). In our article, the evaluation was based on the directive which was published on In the scientific literature, the conclusion from ELF electric field exposure was that the highest 526

2 Occupational exposure at 110-kV stations 527 electric fields at ground level from overhead lines were typically 10 kv m 1 (NIEHS, 1995; AGNIR, 2001a,b; WHO, 2007). The average magnetic fields to which workers can be exposed in the electric power industry were as follows: lt for workers in substations, lt for workers in power stations, lt for workers on lines and cables, and lt for electricians (NIEHS, 1998; AGNIR, 2001b; Helhel and Ozen, 2008). In the measurements of Ozen (2008), the maximum magnetic field inside the 380/154-kV substation under normal load conditions was 20 lt for outdoors and in the substation s outdoor magnetic field was lt (Ozen, 2008). According to Gobba et al. (2004) based on monitoring with an EMDEX Lite for three whole work shifts (8 h 3 days, 1 measure each 10 s), in seven electrical cabin maintenance workers, the arithmetical mean of individual time-weighted average (TWA) was 0.35 lt [standard deviation (SD) 0.25], while the geometric mean of TWAs was lt; in eight power line maintenance workers, the arithmetical mean of individual TWA was 0.15 lt (SD 0.10), while the geometric mean of TWAs was lt (when the maintenance power was off); in five substation workers, the arithmetical mean of individual TWA was 0.12 lt (SD 1.18), while the geometric mean of TWAs was lt (Gobba et al., 2004). In Finland, one aim of the studies of occupational and public exposure to ELF EM fields was to examine EMFs of power lines, cables, and substations and to evaluate their possible biological effects on workers (Korpinen and Partanen, 1994, 1996; Korpinen et al., 2009; Korpinen and Pääkkönen, 2010). There are 400-, 220-, and 110- kv power transmission lines in Finland, and the total length of this transmission system is km. In Finland, workers do not perform live work tasks at 400- kv or 110- kv substations. The live working zones of different levels of voltages were presented by standard SFS 6002 electric work safety (SFS 6002), which is based on EN operation of electrical installations standard of CENELEC (EN ). In substations of the Finnish transmission system, electric fields are closer to limit values than magnetic fields, because 10 kv m 1 is exceeded in many substations while exposure to magnetic fields is usually sufficiently limited (Korpinen and Pääkkönen, 2010). The distribution of the electric field in a substation is usually very complex due to the several overhead busbars crossing each other as well as the proximity of vertical and lateral grounded supporting structures. Therefore, the electric field distribution is already very inhomogeneous without the presence of man. The earlier publication (Korpinen et al., 2009) presented the investigation concerning the current densities in the neck and the total contact currents in occupational exposure at 400-kV substations and power lines. In the study, the workers simulated 15 of their normal work tasks (n 5 151). At the tasks of 400-kV substations, the maximum electric fields were the following: (1) main transformer inspection from maintenance platform: 18.5 kv m 1, (2) maintenance of contacts of reach disconnect or from a man hoist: 8.5 kv m 1, (3) maintenance of operating device of disconnector from service platform: 8.5 kv m 1, (4) maintenance of operating device of circuit breaker at ground level: 15.5 kv m 1, (5) inspection of primary terminals of current transformer from a man hoist: 19.2 kv m 1, (6) inspection of secondary terminals of busbar voltage transformer using ladder: 43.5 kv m 1, (7) changing a bulb by climbing to a pylon: 35.0 kv m 1, (8) walking in the substation: 15.2 kv m 1, (9) maintenance of operating device of circuit breaker from ladder: 44.3 kv m 1, (10) maintenance of operating device of circuit breaker from service platform: 36.3 kv m 1, (11) breaker head maintenance from man hoist: 44.3 kv m 1, and (12) inspection of secondary terminals of current transformer from ladder: 47.0 kv m 1. The study shows that although the action values are exceeded, the maximum average current densities and the total contact currents (caused by electric field) in occupational exposure at 400-kV substations and power lines do not exceed the limit and action values (10 ma m 2 and 1 ma) of the new EU-Directive 2004/40/EC (Korpinen et al., 2009). The other earlier publication (Korpinen and Pääkkönen, 2010) described the investigation concerning occupational exposure to EMFs during work tasks at 110-kV substations in the Tampere region. The aim of the study was to investigate occupational exposure to EMFs during various work tasks at 110- kv substations, and analyze if the action values of Directive 2004/40/EC were exceeded. The EMFs were measured at seven 110-kV substations in the Tampere region. The measurements were planned from the perspective of a worker s operating devices at different height levels. At 110-kV substation, the work tasks and highest EMFs were the following: (1) walking or operating devices on the ground (7.4 kv m 1 and 43.0 lt), (2) working from a service platform (16.6 kv m 1 and 21.0 lt), (3) working around the power transformer on the ground or using a ladder (0.4 kv m 1 and 260 lt), and (4)

3 528 L. Korpinen et al. changing a bulb from a man hoist (4.9 kv m 1 and 2.7 lt). At 110-kV substations, the action value (10 kv m 1 ) was only exceeded in work tasks from service platforms. Since the magnetic field in every exposure case conforms with the action value, 110- kv substations should not be an EMF problem for workers (Korpinen and Pääkkönen, 2010). The aim of the study was to investigate occupational exposure to EMFs during various work tasks at 110-kV switching and transforming stations, and analyze if the action value of Directive 2004/40/EC was exceeded, because in the earlier study (Korpinen and Pääkkönen, 2010), only the 110-kV substations of one company were studied. In addition, the aim was also to analyze the electric fields at 110-kV substations together with the electric fields at 400-kV substations, which were measured in an earlier study (Korpinen et al., 2009), using the logistic regression models. MATERIALS AND METHODS Measured substations During the summer of 2009, altogether 793 electric field values and 203 magnetic fields values were measured at 20 substations (110 kv) of four transmission or distribution companies. In all, 28 electric field measurements were removed from the analysis with an expert because of the error (the measurement place was chosen incorrectly measurement height too high). This was the only reason to remove the measurement. Altogether 765 electric field values and 203 magnetic fields values were accepted to the database. At Company 1, the measurements were done at six substations. At Company 2, one substation was studied. At Company 3, nine substations were studied, and at Company 4, four substations were studied. The substations were at different transforming stations and places in Finland. One substation (at Company 1) was a gas-insulated substation (GIS) and all others were air-insulated substations. The GIS substation was in a building. Two substations (one of Company 1 and one of Company 3) were airinsulated substations in a cave inside rock and all others were outside. At all substations, the measurements were done in the normal working area. Some of the substations were only 110-kV switching stations without power transformers. Substations where electric fields were expected to be high were chosen for measurements. Based on our earlier publications, we know that electric fields are quite low in GIS, but higher in air-insulated substations. Therefore, the material included only some GIS and it is not possible to make analyses on differences between GIS/air-insulated substations, indoor/outdoor substations, and with/without power transformers. Some of the substations were 110-kV transforming stations, where the secondary voltage could be 20 kv, 10 kv, or something else. Some of the substations were 400/110- and 220/110-kV transforming stations, where there could be shunt reactors. In those substations, 400- and 220- kv busbars or conductors could influence exposure to electric fields. Some tasks are related to components on the voltage level of 20 kv. Those measurements are also added to the material from 110-kV substations, because those 20-kV areas were in the 110-kV substations. Figure 1 shows an example of 110-kV substations. Only workers are allowed to go to substations (no children or other persons). The work tasks EMFs were measured from the following work tasks: (1) maintenance of contacts of reach disconnector from a man hoist, (2) maintenance of an operating device of disconnector from a service platform, (3) maintenance of an operating device of a circuit breaker at ground level, (4) inspection of primary terminals of a current transformer from a man hoist, (5) walking in the 110-kV substation, (6) maintenance of an operating device of circuit breaker from a service platform, (7) breaker head maintenance from a man hoist, (8) working near the power transformer on the ground, (9) working near the capacitors on the ground, (10) inspection of a top, and bushings of a power transformer, (11) maintenance of an operating device of disconnector at ground or floor level, (12) working near cables at a GIS substation, (13) working outside a fence of a switchyard, (14) inspection of a distribution cabinet on the ground, (15) inspection of secondary terminals of a current transformer from a man hoist, (16) inspection of primary terminals of a voltage transformer from a man hoist, (17) inspection of secondary terminals of a voltage transformer from a man hoist, (18) inspection of the operating device of an earthing switch on the ground, (19) maintenance of a lamp from a man hoist, (20) inspection of an operating device of an earthing switch from a service platform, (21) working near a fence of a reactor (20 kv), (22) maintenance of the operating device of a circuit breaker at floor level, (23) reading of an oil pressure gauge of a cable terminal on the ground, (24) maintenance of an operating device of a circuit breaker from below at ground level, (25) maintenance of an operating device of a circuit breaker from below

4 Occupational exposure at 110-kV stations 529 Fig. 1. from a platform, (26) maintenance of an operating device of a disconnector of a reactor from a service platform (20 kv), (27) walking near reactor cables (20 kv), and (28) walking in the 20-kV substation. Instantaneous EMF measurement The instantaneous RMS values of EMFs were measured in places where workers generally do their tasks. During the measurement, the worker was not in the work area. This was done to eliminate his influence on the electric fields. The work tasks (meter reading, oil pressure check, etc.) were only simulated and the measurements therefore were of a relatively short duration. It is not possible to measure electric fields with personal dosimeters. We did not measure the TWA value or actual peak exposures, but only instantaneous RMS values. The electric field meters always stood on the floor, ground, platform, or on a man hoist. The magnetic field meter was always in the measurer s hand. He also walked with the meter. The electric field strength was measured with two 3-axis commercial electric meters: EFA-3 meter (Wandel and Coltermann GmbH, Eningen U.A., France) (accuracy 5%, RMS) that has a frequency range of 5 Hz 30 khz and EFA-300 meter (Narda An example of 110-kV substations. Safety Test Solutions GmbH, Eningen U.A., France) (accuracy 3%, RMS) where the frequency range was 5 Hz 30 khz. The EFA-3 meter was used at only one substation. The magnetic field density and the exposure ratios of ICNIRP occupational guidelines (1998) were measured with a Narda ELT-400 meter (L-3 Communications, Narda Safety Test Solutions, Hauppauge, NY, USA) (accuracy 4% RMS) that has the frequency range of 1 Hz 400 khz. The exposure ratios of magnetic field take into account the harmonic components of the field. The measurements were done at the same area where workers normally worked. Optical fibers of electric field meters are very vulnerable to mechanical forces. Therefore, we used two meters in case of a problem. The meters were of similar type and they were compared to give the same values. In the case of the mobile service platform, the electric fields were measured at a level where the measurements were generally where the workers normally worked. At tasks with a man hoist the meter either stood on the bottom of the hoist or was fitted at the end of a long stick held by the measurer. Generally, the meter stood on the floor, on the ground, or on the service platform. The measurement height depended on the worker s height. The

5 530 L. Korpinen et al. measurement places and the measurement environments (e.g. ground) also varied. The measurements were based on realistic work situations. We wanted to simulate the exposure of workers at different heights. Therefore, we used measurement heights 1.7 and 1.8 m. Statistical analyses (logistic regression models) The statistical analyses were done using the PASW Statistics 18 (formerly known as SPSS Statistics, Chicago, IL, USA). First, we put the measured data from tasks at 400 kvand from tasks at 110 kv in the database. In the database, each measurement was presented as one case (a row in the table). The database also included the work tasks related to components of the voltage level 20 kv, because those components were in the 110-kV substations. We tested several different logistic regression models, but in this article we only present the two models that best describe the measurement results. One model is based on all data (measurement cases from 110- to 400-kV substations) and the other is based on the data from task (6) maintenance of an operating device of a circuit breaker from a service platform (400- and 110-kV substations). The analysis started so that a new dummy variable was created the limit value (10 kv m 1 ) is exceeded or is not exceeded (0 5 No handrail, 1 5 Handrail). Then, we generated different logistic regression models, in which covariates were, e.g., measuring distance, measuring height, number of substations, and a handrail around the platform. The measuring height was modified so that at tasks from platforms, the height was from a platform or from an assumed platform. Hence, all heights were comparable. A similar model was also made out of the data of work task (6) maintenance of the operating device of a circuit breaker from a service platform (400- and 110- kv substations). RESULTS Fig. 2. A histogram of all the measured electric field values. Figure 2 shows the histogram of the electric field values and Fig. 3 shows the histogram of the magnetic field values. The average value (arithmetic mean) of all electric field measurements was 3.6 kv m 1 and SD was 2.9 kv m 1. The average value (arithmetic mean) of all magnetic field measurements was 28.6 lt and SD was 95.9 lt. Because Figs 2 and 3 clearly show that the exposure data are not normally distributed, the geometric means were also calculated from data. In Table 1, there are the maximum value, mean, and SD of electric fields at different work tasks. Table 2 shows the similar results of the magnetic fields at different work tasks and the measured exposure ratios of ICNIRP occupational guidelines (1998) are in Table 3. The measured values of EMFs are instantaneous RMS values. The frequency range of the electric fields was 30 Hz to 30 khz (Table 1) and for magnetic fields the frequency range was 30 Hz to 400 khz (Table 2). Tables 1 3 only included those substations where the electric or magnetic fields were measured. In none of the tasks were both EMFs measured. The highest electric fields (15.5 kv m 1 ) were measured at task (6) (maintenance of operating device of circuit breaker from service platform). In one special work task, the highest magnetic field was 710 lt at work task (27) [walking near cables of reactor (20 kv)]. Statistical analysis (logistic regression models) The best logistic regression model for all data (400- and 110-kV substations) included measuring distance, measuring height, and number of substations, and the best logistic regression model for the data of work task (6) maintenance of an operating device of a circuit breaker from a service platform (400- and 110-kV substations) included the measuring height, a handrail around the platform, and number of substations. In the model of all data, the percentage corrects were 97.0% (no) and 91.8% (yes). The overall percentage was 95.3%. Based on these percentages, the model seems to work fairly well. In the Hosmer and Lemeshow test, the significance was 0.999, so

6 Occupational exposure at 110-kV stations 531 Table 1. Maximum values, means, and SDs of all the electric field measurements at different work tasks at 110-kV substations Tasks (n) Amount of substations Electric field (kv m 1 ), max Electric field (kv m 1 ), mean the model was accepted. In the model from task (6), the percentage corrects were 88.1% (no) and 88.0% (yes). The overall percentage was 88.1%. In the Hosmer and Lemeshow test, the significance was 1.000, so the model was accepted. Tables 4 5 show the results of logistic regression models. In the model from all data, the measuring height was significant and in the model from task (6) there was significance with substations 1, 9, 11, 13, 18, and the measuring height. Electric field (kv m 1 ), SD Electric field (kv m 1 ), geometric mean DISCUSSION Height of sensor above ground/floor (m) 1 (11) (18) a or 1.8 a 3 (32) or (9) (175) (255) a or 1.8 a 7 (21) (5) Varying 10 (1) (140) or (11) (17) (9) (3) (3) (1) (3) (33) a or 1.8 a 22 (9) or (3) (2) (1) (2) or (1) Tasks: (1) maintenance of contacts of reach disconnector from a man hoist, (2) maintenance of an operating device of disconnector from a service platform, (3) maintenance of an operating device of a circuit breaker at ground level, (4) inspection of primary terminals of current transformer from a man hoist, (5) walking in the 110-kV substation, (6) maintenance of an operating device of circuit breaker from a service platform, (7) breaker head maintenance from a man hoist, (8) working near the power transformer on the ground, (10) inspection of a top, and bushings of a power transformer, (11) maintenance of an operating device of disconnector at ground or floor level, (13) working outside a fence of a switchyard, (14) inspection of a distribution cabinet on the ground, (15) inspection of secondary terminals of current transformer from a man hoist, (16) inspection of primary terminals of a voltage transformer from a man hoist, (17) inspection of secondary terminals of a voltage transformer from a man hoist, (18) inspection of operating device of an earthing switch on the ground, (19) maintenance of a lamp from a man hoist, (20) inspection of an operating device of an earthing switch from a service platform, (22) maintenance of operating device of circuit breaker at floor level, (23) reading of an oil pressure gauge of a cable terminal on the ground, (24) maintenance of operating device of circuit breaker from below at ground level, (25) maintenance of an operating device of a circuit breaker from below from a platform, (26) maintenance of an operating device of disconnector of a reactor from a service platform (20 kv), and (28) walking in the 20-kV substation. a From service platform or same level without the platfrom. Exposure to electric fields For this study, we collected all known tasks performed on 110-kV substations. All the tasks were not done frequently. However, sometimes a worker does the same task for a whole week. Due to the climatic condition in Finland, not much work is done in air-insulated substations (outside) in winter. All the actual work involves some walking in the 110-kV

7 532 L. Korpinen et al. Table 2. Maximum values, means, and SDs of all the magnetic field measurements at different work tasks at 110-kV substations Tasks (n) Amount of substations Height of sensor above ground/floor (m) Magnetic field (lt), max Magnetic field (lt), mean substations (task 5). When the meters are read, it only takes a few minutes (e.g. task working near the power transformer on the ground ). Work with a man hoist normally takes at least a few hours. Generally, at 110-kV substations, the exposure of workers to electric fields was,10 kv m 1 for different tasks. Only at two work tasks, task (6) maintenance of the operating device of a circuit breaker from service platform and task (7) breaker head maintenance from a man hoist, were the maximum values.10 kv m 1. At task (7), breaker head maintenance from a man hoist only in one case was the maximum value (11.5 kv m 1 ).10 kv m 1 while the mean value was 3.0 kv m 1. We had some problems with the optical fibers of electric field meters. Therefore, we had use of two meters. Meters were of the same type and their readings were compared to ensure that they gave the same values. However, it is still possible that these problems slightly influenced our results. The measurements were done at a height of 1.7 or 1.8 m or both. The measurement height influenced the results, which is important to take into account when analyzing the results. Magnetic field (lt), SD Magnetic field (lt), geometric mean 5 (45) About 1.7 or (46) a or 1.8 a 8 (18) Varying 9 (2) (1) (55) or (3) About (1) About (7) (1) (3) (6) a 21 (6) or (3) (1) (5) Varying Tasks: (5) walking in the 110-kV substation, (6) maintenance of an operating device of circuit breaker from a service platform, (8) working near the power transformer on the ground, (9) working near the capacitors on the ground, (10) inspection of a top, and bushings of a power transformer, (11) maintenance of an operating device of disconnector at ground or floor level, (12) working near cables at a GIS substation, (13) working outside a fence of a switchyard, (14) inspection of a distribution cabinet on the ground, (18) inspection of operating device of an earthing switch on the ground, (19) maintenance of a lamp from a man hoist, (20) inspection of an operating device of an earthing switch from a service platform, (21) working near a fence of reactor (20 kv), (23) reading of an oil pressure gauge of a cable terminal on the ground, (26) maintenance of an operating device of disconnector of a reactor from a service platform (20 kv), and (27) walking near reactor cables (20 kv). a From service platform or same level without the platfrom. At task (6) maintenance of operating device of circuit breaker from service platform, the maximum value was 15.5 kv m 1 and the mean value was 6.3 kv m 1. At task (6), the measuring height was quite an important factor. The live parts were near the measurement point, and the electric field increased quickly if the measuring height was too high (safety distances). So it is important that the working zones are based on standard SFS 6002 electric work safety (SFS 6002) or EN operation of electrical installations standard of CENELEC (EN ). In addition, distances from structures influenced the measured results. In the work tasks at service platforms, the workers generally open the door of the operating device of a circuit breaker. The opened door slightly reduces the worker s exposure to the electric fields. However, in this work, the results were quite similar to the results of the earlier paper (Korpinen and Pääkkönen, 2010). In the earlier study, the action value (10 kv m 1 ) was only exceeded in work tasks from service platforms. However, in this article the task (7) breaker head maintenance from man hoist was not studied. However, a prior study (Korpinen

8 Occupational exposure at 110-kV stations 533 Table 3. Maximum values, means and SDs of the exposure ratios (magnetic field) of ICNIRP occupational guidelines measurements at different work tasks at 110-kV substations Tasks (n) Amount of substations Exposure ratios (%), max Exposure ratios (%), mean Exposure ratios (%), SD et al., 2009) shows that although the action values are exceeded, the maximum average current densities, and the total contact currents (caused by electric field) in occupational exposure at 400-kV substations and power lines do not exceed the limit and action values (10 ma m 2 and 1 ma) of the new EU Directive 2004/40/EC. In some situations, the present action values were exceeded in the work places. However, in our earlier measurements where electric fields were even higher at times, we found that the induced current density in the central nervous system area did not exceed the limit value in any of the examined cases (the helmet measuring system described in Korpinen et al., 2009). Exposure to magnetic fields In general at 110-kV substations, the magnetic fields to which workers are exposed were,500 lt at different work tasks. The measured values were quite low (Fig. 3). In addition, the exposure ratios based on ICNIRP occupational guidelines (Table 3) were in most cases,100%. However, at one substation, there were some cables of reactor (20 kv) and Magnetic field (lt), geometric mean Height of sensor above ground/floor (m) 5 (30) About 1.7 or (33) a or 1.8 a 8 (18) Varying 9 (2) (52) or (3) About (1) About (6) (1) (3) (6) a 21 (5) or (3) (1) (4) Varying Tasks: (5) walking in the 110-kV substation, (6) maintenance of an operating device of circuit breaker from a service platform, 8) working near the power transformer on the ground, (9) working near the capacitors on the ground, (11) maintenance of an operating device of disconnector at ground or floor level, (12) working near cables at a GIS substation, (13) working outside a fence of a switchyard, (14) inspection of a distribution cabinet on the ground, (18) inspection of operating device of an earthing switch on the ground, (19) maintenance of a lamp from a man hoist, (20) inspection of an operating device of an earthing switch from a service platform, (21) working near a fence of reactor (20 kv), (23) reading of an oil pressure gauge of a cable terminal on the ground, (26) maintenance of an operating device of disconnector of a reactor from a service platform (20 kv), and (27) walking near reactor cables (20 kv). a From service platform or same level without the platfrom. near these cables the magnetic field values were.500 lt. Workers can walk near the cables, so they might be exposed to magnetic fields exceeding the action value. Around some cables, there were aluminum plates. Near those cables, the measured values were,500 lt. So with the aluminum plates or other shielding materials, it is possible to mitigate the magnetic fields. At other substations (without reactors), the highest magnetic fields were measured at task (8) working around the power transformer on the ground. The maximum magnetic field was lt and the average value was 40.3 lt. At other tasks, the maximum magnetic fields were,100 lt, so the values were clearly below the action value (500 lt). In Table 3, the exposure ratio based on ICNIRP occupational guidelines are also quite low. The exposure ratios of ICNIRP occupational guidelines (1998) take into account the harmonics of the magnetic fields. Consequently, from the exposure ratios we can see that at 110-kV substations, the currents did not contain much harmonics. Safigianni and Tsompanidou (2009) investigated the EMF values in the vicinity of an outdoor electric power substation 2 50 MVA, 150/20 kv, in Xanthi,

9 534 L. Korpinen et al. Table 4. The results of logistic regression model of all data from dummy variable the limit value (10 kv m 1 ) exceed or not (0 5 no exceed and 1 5 exceed) Variable B SE Wald df Sig. Exp (B) Substation Substation (1) Substation (2) Substation (3) Substation (4) E126 Substation (5) E113 Substation (6) E9 Substation (7) E33 Measuring height ** Measuring distance Constant E12 B, regression coefficient; SE, standard error; Wald, Wald chi-square value; df, degrees of freedom; Sig., two-tailed P-value; **, significant at P, 0.05; Exp (B), the odds ratios for the predictors. Table 5. The results of logistic regression model of task (6) from dummy variable the limit value (10 kv m 1 ) exceed or not (0 5 no exceed and 1 5 exceed) Variable B SE Wald df Sig. Exp (B) Handrail around platform Measuring height ** Substations Substation (1) ** Substation (2) E11 Substation (3) E8 Substation (4) E8 Substation (5) E8 Substation (6) Substation (7) Substation (8) Substation (9) ** Substation (10) Substation (11) ** Substation (12) Substation (13) ** Substation (14) Substation (15) Substation (16) Substation (17) Substation (18) ** Substation (19) Constant B, regression coefficient; SE, standard error; Wald, Wald chi-square value; df, degrees of freedom; Sig. 5 two-tailed P-value; **, significant at P, 0.05; Exp (B), the odds ratios for the predictors. Greece. They presented the following maximum measured magnetic flux density values for different substation areas: (i) 150 kv buses, 24.4 lt, (ii) transformers 26.3 and 28.5 lt, (iii) capacitor banks 707.6, 705, and 20.0 lt, (iv) SF 6 switches area 61.4 lt, and (v) wider substation area 42.7 lt. The maximum electric field strength value was 4.3 kv m 1. It was measured near the high voltage side

10 Occupational exposure at 110-kV stations 535 of the transformer. They concluded among other things that the measured field values are substantially within recognized guidelines. Therefore, there was no cause for concern among the public or working personnel (Safigianni and Tsompanidou, 2009). When we compare our results to the study of Safigianni and Tsompanidou (2009), the results are quite similar in some work areas. For example, near the transformers (task 8), the geometric mean was 16.8 lt, when their values were 26.3 and 28.5 lt. However, we did not have as high values near capacitor banks as they had. We did not carry out broadband measurements, because based on the measurements performed by Fingrid there are usually no significant harmonic contents in the currents and voltages of the 110-kV grid. Therefore, we can draw a conclusion that in the EMFs, the harmonics are not an important factor. Dimbylow (2000, 2005) calculated the internal electric fields and current densities caused by EMFs using the human model. Based on calculations, the highest measured magnetic field densities were well below the level which would lead to induced current densities exceeding the limit value of Directive 2004/40/EC. Logistic regression models The logistic regression models analyzed the covariates, which associated with the question if the action value exceed 10 kv m 1. In general, the measuring height and distance influenced the electric fields. Based on our models, the measuring height was also a significant factor, which is logical. However, at substations there are different structures which influence the electric fields. Therefore, it is difficult to know which factors had an influence on the results. Based on our data, the measuring distances did not determine whether the results exceeded 10 kv m 1 or not. It is important that we can generalize our results when we analyze other 400- or 110-kV substations in Finland, because it is not possible to measure all substations. Based on the logistic regression model from all data (400 and 110 kv), a number of substations did not have a significant influence on the result. In other words, it is also possible to get similar results from other substations, because two substations did not have an influence on the results. On the logistic regression model from task (6), there was significance with substations 1, 9, 11, 13, and 18. Substation 1 was a 400-kV substation and others were from the 110-kV level. At substations 11 and 18, the height of the platforms was only 0.6 or 0.55 m. In general, the heights of the platforms were Fig. 3. A histogram of all the measured magnetic field values. 1.2 m. This can be one reason for the results. At Substation 9, the height of the structures was perhaps slightly more than at other substations. In addition, we did not study other substations from the same company (Company 2). At Substation 13, the height of the structures was perhaps slightly less than at other substations. However, those logistic regression models were only based on the question if the result exceeded 10 kv m 1 or not. Thus, the analysis did not show how much some factors which influenced the values only gave the information to the dummy variable the limit value (10 kv m 1 ) is exceeded or is not exceeded (0 5 no and 1 5 yes). Our study dealt extensively with the 110-kV substations from Finland. In Finland, the situation is quite good, because only in some cases can the electric or magnetic fields be near or over the action values. So we will not need to do shielding or other changes to the substations in order to comply with Directive 2004/40/EC. It is important to take into account that the measurements were only done at Finnish substations using Finnish working practices. CONCLUSIONS In conclusion, it can be stated that generally at 110-kV substations, workers are not exposed to EMFs higher than 10 kv m 1 and 500 lt. Only at two work tasks, maintenance of operating device of circuit breaker from service platform and breaker head maintenance from man hoist, were the maximum measured electric fields.10 kv m 1. In addition, at one substation in one special work

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