Dynamic stray current measuring methods in urban areas

146 THE BALTICJOURNAL OFROAD AND BRIDGE ENGINEERING 2022/17(4) ISSN 1822-427X/eISSN 1822-4288 2022 Volume 17 Issue 4: 146–170 https://doi.org/10.7250/bjrbe.2022-17.583 * Corresponding author. E-mail: [email protected] Katarina VRANEŠIĆ (ORCID ID 0000-0001-7610-3469) Marijana SERDAR (ORCID ID 0000-0001-8386-7272) Stjepan LAKUŠIĆ (ORCID ID 0000-0002-5407-9290) Václav KOLÁŘ (ORCID ID 0000-0002-4626-9844) Andrea MARISCOTTI (ORCID ID 0000-0002-0096-7305) Copyright © 2022 The Author(s). Published by RTU Press This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. DYNAMIC STRAY CURRENT MEASURING METHODS IN URBAN AREAS KATARINA VRANEŠIĆ1*, MARIJANA SERDAR1, STJEPAN LAKUŠIĆ1, VÁCLAV KOLÁŘ2, ANDREA MARISCOTTI3 1University of Zagreb, Faculty of Civil Engineering, Zagreb, Croatia 2VŠB-Technical University of Ostrava Department of Electrical Engineering, Ostrava, Czech Republic 3University of Genoa, Department of Electrical, Electronic and Telecommunications Engineering, and Naval Architecture, Genoa, Italy Received 4 January 2022; accepted 11 July 2022 Abstract. In areas where urban tracks are used as public transportation, dynamic stray currents cause high maintenance costs for the tracks and metal structures near the tracks. Stray currents caused by rail vehicles depend on many factors (traffic density, vehicle speed, acceleration and deceleration, soil and track moisture), so it is very difficult to get a clear picture of the harmfulness of the stray current based on the results of a single field measurement. However, there are several measurement methods that can be used to determine the presence of stray currents and predict appropriate track maintenance actions. Some of these methods are described in this article, namely the use of stray current mapper, measurement of rail potential and rail 147 Katarina Vranešić, Marijana Serdar, Stjepan Lakušić, Václav Kolář, Andrea Mariscotti Dynamic Stray Current Measuring Methods inUrban Areas current, measurement at the stray current collection system, and the use of nondestructive sensors. In track construction, measuring the electrical potential between rail and ground is one of the most common methods of detecting the damaging influence of stray current. Keywords: rail current, rail potential, stray current mapper, stray current, stray current measurement, stray current corrosion, urban tracks. Introduction Stray current represents current that is flowing into the ground from insufficient isolated conductor resulting with the stray current corrosion (Siranec et al., 2019). Stray current leaks through the path of the smallest resistance and it can originate from different sources. Stray current can be static and dynamic. Sources of static stray current are offshore structures, marine platforms, cathodic protection systems, and the main source of dynamic current is electrical traction system (Siranec et al., 2019). Unlike industrial platforms that produce stray current with a relatively stable intensity in time, stray currents produced by electrical tractions are fluctuating in intensity and duration. They may fluctuate over short or long intervals of time, parallel to the varying load of the power source. The biggest intensity of stray current will be during the highest traffic load (morning and afternoon rush hours) (Chen et al., 2017). Stray current arising from electrified traction systems may be direct (DC) stray current or alternating (AC) stray current. Influence of AC current is much less dangerous than DC current (Chen et al., 2017). Tramways and subways are operated at DC with nominal voltage in the range of 500 to 1500 V, most commonly in the lower rage of it, namely 600 to 750 V (Steimel, 2012; Krajcar et al., 2009). The vehicles receive power through the pantograph and the so-formed current path recloses back to the Traction Power Stations (TPSs) through the running rails (Krajcar et al., 2009; Dolara & Leva, 2009). Rails are not grounded and feature some amount of electrical insulation to prevent leakage of traction current. At the same time, their instantaneous potential may reach higher values than for a fully grounded solution, so that there is always some compromise between stray current protection and electrical safety (touch voltage) (Mariscotti, 2021a). When the traction current flows back to the TPS, a potential is created between the rails and the earth due to the longitudinal resistance (or, in general, impedance) of the rails themselves; such rail potential is variable along the track and, in general, minimum at TPSs (Du et al., 2016). Since it is very difficult to isolate the rails completely, some of the return current leaks away from the rails and reaches the 148 THE BALTICJOURNAL OFROAD AND BRIDGE ENGINEERING 2022/17(4) TPS from alternative paths. This current is called stray current. Such paths attract stray current because they may be conveniently less resistive, e.g., because of their large cross-section (the soil itself) (Du et al., 2016; Hill & Cai, 1993). If a pipeline crosses a railway system, stray current may be picked up at the points of crossing as trains pass. Current will flow in both directions from the crossings and discharge at remote locations. On the other hand, if the pipeline is parallel with the rails, but does not approach a DC substation, it will pick up current as the tram track vehicle passes the parallel section. This current may be discharged from the ends of the parallel section. In finding its way back to the DC substation, stray current may flow from pipeline to pipeline, at crossings, in order to follow the least resistant path (Hartenergy, 2021; Alamuti, et al. 2011). This phenomenon is particularly relevant in an urban context, such as for tram lines because the soil is full of structures and low-resistance paths (pipes, reservoirs, cables, walls, foundations, etc.), tram tracks are almost all built in closed formation, and rails are not completely insulated. At the point where the current leaves the rail or a metal object near the track, the anodic corrosion reaction (oxidation, dissolution of the metal) starts and at the point where the current enters the rail or another metal object, the cathodic part of the corrosion reaction takes place (Zaboli et al., 2017). Stray current magnitude depends on the traction current, the rail resistance and rail-to-earth resistance (Charalambous et al., 2016). Shape and material properties of rails are chosen with mechanical requirements as the main objective, and electrical characteristics are more or less in the background, as pointed out among others by Dolara & Leva (2009) and Mariscotti (2021b). The rail resistance affects the voltage drop and stray current leakage (Mariscotti, 2021b). According to Milesevic et al. (2018), the return current in the rails near electric traction substations and vehicles is about 75% of the supply current and decreases with distance from the contact points. The amplitude of the return current changes significantly only near the traction substations and vehicles. In the middle of the supply segment, return current has almost constant values. It depends on the rail-to-earth resistance. It reaches 53% of the feeder current at a ground resistance of 100 Ω and 59% of the feeder current at a ground resistance of 1000 Ωm (Milesevic et al., 2018). The highest stray current values occur on tracks with discretely fastened rails (Figure 1a) when the fasteners are not insulated from the 149 Katarina Vranešić, Marijana Serdar, Stjepan Lakušić, Václav Kolář, Andrea Mariscotti Dynamic Stray Current Measuring Methods inUrban Areas rail. When these tracks are constructed as part of the roadway surface and closed with various materials (asphalt, concrete slabs, etc.), it is very difficult to provide adequate drainage, so water keeping will increase stray current values. In continuously supported and fastened tracks (Figure 1b), the rails are completely insulated, and a high electrical resistance is obtained between the rail and the ground, so that the current cannot leak from the rail. In this type of track, the rails are laid in the grooves of the precast concrete slab and the free space between the rail and the slab is filled with elastic material so that the rails are separated from the surrounding structure and protected from corrosion processes (Vranešić et al., 2020). Measures to reduce stray current at the source can be divided into two groups: Reducing the longitudinal rail resistance and increasing the resistance between rail and earth (Wang et al., 2020). The electrical resistance of the rail depends on many factors – type of rail, distance between electrical substations, rail cross-section. The value of longitudinal rail resistance is 40–80 mΩ/km (Charalambous, 2005). The values of the stray currents depend on the electrical resistance between rail and earth. This resistance depends on the type of fastenings and sleepers, the use of insulating materials and their performance, the type of track construction and the type of soil (Bongiorno & Mariscotti, 2015). To prevent the current from entering the buried metal object near the track, stray current collection systems are installed on many urban tracks in order to collect the current flowing off the rails and return it to the electrical substation (Xu & Wang, 2013). Figure1. a) Model of discretely supported and discretely fastened track, b) Model of continuous supported and fastened track (Vranešić et al., 2020) a) b) 150 THE BALTICJOURNAL OFROAD AND BRIDGE ENGINEERING 2022/17(4) 1. Rail corrosion: Examples and case studies 1.1. Examples of rail corrosion Rail corrosion is a consequence of stray current and rail contact with the corrosive environment. Robles Hernández et al. (2009), Safa et al. (2015), Isozaki et al. (2016) and Xu et al. (2021) analysed electrochemical corrosion at the rail foot in urban tracks. Rail corrosion failure usually occurs at the rail foot. Tracks with discretely supported rails are most susceptible to corrosion processes, as the corrosive media can easily accumulate in the region of the rail foot. The fastening system acts as a barrier to the flow of the corrosive media, so that the greatest corrosion damage occurs at the contact between rail and fastening (Panda et al., 2008). Corrosion at the rail foot leads to thinning of the rail cross-section and instability of the rail with the risk of displacement and accumulating the large stresses or even failure of the rail (Xu et al., 2021). Loss of material at the rail fastening and at the top and bottom of the rail foot can also reduce fastener toe load and affect traffic safety. This can increase the possibility of rail rollover or track widening on sharp curves (TCRP, 2016). In closed rail formation, the rail foot thickness can only be determined when the track is open, which in some cases is not done until the track is reconstructed. Figure 2 shows examples of rail corrosion noticed during track reconstruction. In closed track systems, it is very difficult to insulate the rail completely, so the corrosion process can be very rapid. Furthermore, if proper track drainage is not provided, water remaining in the track will accelerate corrosion. Urban tracks built into road surface are also exposed to chlorides, which are used in winter to prevent icing of the Figure2. Rail degradation noticed at urban tracks 151 Katarina Vranešić, Marijana Serdar, Stjepan Lakušić, Václav Kolář, Andrea Mariscotti Dynamic Stray Current Measuring Methods inUrban Areas road surface (Safa et al., 2015). These chlorides dissolve easily in water and enter the track. Figure 3 shows the condition within the closed track formation, and this condition is present through the most part of the year. This is noticed during reconstruction, after the track was opened. At the places where current is leaving from the rail, material loss at the rail and rail fastening is much higher due to stray current corrosion. This type of corrosion is different from natural corrosion because it is caused by an external electrical current (WebCorr, 2021; Alar, 2015). Stray currents may have more serious consequences in chloridecontaminated medium due to higher electrical conductance of this medium (Chen et al., 2017). Zaboli et al. (2017) made numerical analysis of stray currents at the fastening point considering different values of rail-to-earth resistance. Three different models were analysed and the only difference between the models was the value of soil resistivity – in model A the soil resistivity was 100 Ωm, in model B – 300 Ωm and in model C – 1000 Ωm. The type of rail fastening system was the same in all three models. The results showed that the stray currents were the highest in model A because the electrical resistance between the rail and the ground was very low due to low soil resistivity. For discretely fastened tracks, these models can be identified with electrical resistance between the rail and the ground at the points where the rail is fixed to the base. At these locations, the resistance between the rail and the ground is a function of the insulation of the fastening system (Charalambous, 2017). When the fastening system is isolated from the rail, the resistance between the rail and the ground is high. If, on the other hand, the fastening system is in direct contact with the rail, current flows from the rail through the fastening system in the lower parts of the track. Figure3. Condition of the closed track formation during the most part ofyear 152 THE BALTICJOURNAL OFROAD AND BRIDGE ENGINEERING 2022/17(4) 1.2. Measurement methods to detect rail corrosion Many studies have been conducted on rail wear and defects, contact impact, and fatigue of rail materials, but most of them do not consider the effects of corrosion on rails (Xu et al., 2021; Ovchinnikov et al., 2021; Haladin et al., 2019). Conditions including sunlight, humidity, temperature, salt deposition affect corrosion (Xu et al., 2021), the change of temperature on rail surface, condensation of water from air and evaporation result in wet/dry cycle that also favours corrosion (Xu et al., 2021). Samal et al. (2011) carried out corrosion tests on rails in different media – 3.4% NaCl at 45 and 65 °C and in sulfate medium (H2SO4, pH 1.49) at 45 and 65°C. It was found that the corrosion rate of the rail in a 3.4% NaCl solution at room temperature was 0.425 mm/year and as the temperature increased, the corrosion rate increased in the saline atmosphere. A sulfate medium with a pH of 1.59 can also cause corrosion. As the corrosion rate increases, the yield strength and tensile strength decrease. The presence of salt on the rails compromises the integrity of the rails because salt from the electrolyte promotes the reaction of oxygen with the rails, which leads to acceleration of corrosion. Most parts of the tracks are usually wet, so there is always contact between the rail, the fastening system and the electrolyte (Ritter et. al., 2018. In a highly corrosive environment and in the presence of stray current, the service life of rails can be reduced to only a few years, so it is important to identify susceptible areas of track infrastructure and stop corrosion on time. For this reason, it is necessary to implement measurement methods to detect rail corrosion (Robles Hernández et al., 2009). Visual inspection is the primary method for corrosion detection at the rail foot, but it is only applicable to the open tracks (Ritter et. al., 2018). Developing non-destructive and contactless technologies of corrosion detection is necessary. Some operators are using ultrasonic detection and infrared inspection to detect rail corrosion. Xu et al. (2021) and Pathak et al. (2019) described a conceptual technique based on finite element simulations of the propagation of laser induced ultrasonic guided waves to detect cracks in the rail foot. High powered laser positioned above rail surface producing ultrasonic waves combined with an air-coupled transducer receiving ultrasonic signal can detect flaws within and under rails on an entire rail section (Xu et al., 2021). Infrared inspection is also used to detect rail corrosion because of its ability to localize defects based on various thermal properties. Since localized corrosion in early stages cannot be detected by visual inspection, ultrasonic defect detection with ultrasound has been used 153 Katarina Vranešić, Marijana Serdar, Stjepan Lakušić, Václav Kolář, Andrea Mariscotti Dynamic Stray Current Measuring Methods inUrban Areas to detect non-visible surface fractures and internal defects and to assess corrosion at the rail foot (Xu et al., 2021). Some other non-destructive detection technologies used for rail corrosion detection are radiographic inspection, eddy current inspection, electromagnetic transducer, ground penetrating radar, etc. (Xu et al., 2021). These techniques have application prospect for rail corrosion, but there are still difficulties on implementation. Robles Hernández et al. (2009) conduced numerical simulations of two corroded rail samples. The digitized image was imported into ANSYS software and used to create a highly reliable model that accurately reproduced the properties of the corroded rail. During the exploitation both rail samples were located on a sleeper and exposed to DC stray current. Corrosion was found to cause very complicated defect shapes that grew in unpredictable directions and caused sharp angles, resulting in significant stress concentration. The stress level can be high enough to easily lead to failure of the rail due to traffic load, as the cyclic stresses can reach values close to or above the yield strength of this steel. 2. Stray current measuring methods The amount of stray current depends on the current in the rail, track construction techniques, and insulation techniques (Mariscotti, 2020). Stray currents are usually measured in buried pipelines to determine whether the pipelines are at risk from static or dynamic stray currents. Measurements of dynamic stray currents in railway infrastructure are not common in the literature and some measurement methods are difficult to perform due to the nature of the railway infrastructure (e.g., in a closed track system) and due to the high traffic load. Results of field measurements of stray currents or some other parameters used to calculate stray currents are subject to external influences. The relationship between the track potential distribution and the resulting stray current is an indicator of track isolation and stray current intensity. The value of rail-to-earth potential mainly depends on the number of vehicles on the track, load current, distance between traction substations, contact resistances between rail and earth, etc. (Zhang, 2012). The distribution of track potential is not intuitive, and several factors affect its waveform (Mariscotti, 2020): − Traction and braking phases alternate, interrupted by coasting and deceleration phases; − Rails and tracks are connected along the line (rail-to-rail and track-to-track), achieving optimal track balancing and allowing a 154 THE BALTICJOURNAL OFROAD AND BRIDGE ENGINEERING 2022/17(4) wide exchange of regenerative braking current towards traction units on the same or different tracks; − Substations are approximately evenly distributed along the line and situations with one-way fed trains do not occur. Ooagu et al. (2019) conduced measurement of rail potential in 7 electrical substations. Of these, 5 substations were located in urban areas and the other 2 were located in suburban areas. The measurement results showed that the rail potential of substations in urban areas was positive during most of the time of train operation and the rail potential of substations in suburban areas was negative during most of the time of train operation. This means that the current flows from the rails to the ground in the high-density train operation area and is sucked from the ground to the rails in the low-density train operation area. Ooagu et al. (2019) also conduced continuous measurement of rail potential in one substation for one year. It was established that the output current increased on hot and cold days and that the rail potential increased towards negative polarity especially on a cold day. By continuously measuring the rail potential in one substation it was confirmed that two factors affected the variation of rail potential. First, as the substation output current increases, the leakage current from the feeder circuit to ground increases and the rail potential becomes negative. Second, when the rail leakage resistance decreases due to rain, the leakage current increases and the rail potential becomes negative. Many different methods are used today to detect dynamic stray current. Stray current can be detected at railway infrastructure or in Rail current measurement Pipe-to-earth potential Rail-to-earth conductance measurement Stray current measurements methods Using the stray current mapper Rail-to-earth potential measurement Using the nondestructive sensors Figure4. Stray current detection methods Measurements in contact with the rail Measurement at the infrastructure near the tram track 161 Katarina Vranešić, Marijana Serdar, Stjepan Lakušić, Václav Kolář, Andrea Mariscotti Dynamic Stray Current Measuring Methods inUrban Areas supplied this section. Unlike many other infrastructures in the world, in Ostrava tram infrastructure, the rails represent the positive pole, and the overhead line is negative. This means that the current to supply the vehicle flows through the rails and the overhead line is used as the return conductor. The measurement was performed on a 1344 km long measuring section and the current was continuously measured in measuring points at the beginning and at the end of the measuring section (measuring points M1 and M3 in Figure 9). The continuous measurement of the current was performed using laptop computers equipped with input analogue-to-digital converters (16-bit resolution and 10 ks/s) and measuring software created in the LabVIEW system. During the measurement, the tram type TANGO NF2 Nova (Figure 10) passed the measurement section. This type of tram vehicle uses 600 V Figure11. Distribution of current in rails at the measuring points 1 and 3 intime period from 11:22 to 11:53 11:22 11:23 11:25 11:26 11:28 11:30 11:31 11:33 11:35 11:36 The vehicle has passed or is approaching the measuring sec�on at measuring point 3 11:38 11:40 11:41 11:43 11:45 11:46 11:48 11:50 11:51 11:53 The vehicle has passed or is approaching the measuring sec�on at measuring point 3 Current, ACurrent, A 550 400 250 100 –50 –200 1000 800 600 400 200 0 –200 Time, s Time, s Current at MM1 Current at MM1 Current at MM3 Current at MM3 162 THE BALTICJOURNAL OFROAD AND BRIDGE ENGINEERING 2022/17(4) current for driving. Figure 11 shows time-current diagrams for a 30-minute interval. Analysis of the data has shown that the diagrams have the same shape at the moments when the vehicle leaves or approaches the measurement section and measuring point 3. As can be seen from Figure 11, the power consumption of the tram vehicle is not continuous. The value of current in the rails and stray current depends on the tramway driving mode – acceleration, deceleration or driving at the same speed, but there are also moments when it is in inertial movement or stops (Chen et al., 2006; Kolář & Hrbáč, 2014). The vehicle consumes the most current when it accelerates, which means that during acceleration the return current in the rails is the highest. Differences between current values in measuring point 1 and 3 represent stray current values in this measuring section. 5. Rail-to-earth potential measurement using thereference electrodes Mariscotti (2020) analysed the relationship between the track potential distribution and the resulting stray current. He stated that operating conditions of the trains, rail and track bonding and substation distribution had an influence on the potential distribution. The EN 50122-2011 standard describes the measurement of the potential between rail and earth using the reference electrodes. In this type of measurement, one electrode must be placed at a distance of at least 1 m from the rail and the other at a distance of 30 m (Fig. 12). The measurement includes the measuring of the potential between rail and electrode E1 and the potential between electrodes E1 and E2. Figure12. Rail-to-earth potential measurement according the standard EN50122:2-2011 R1 – rail 1 E2 – electrode 2 a – distance between rail R2 and electrode E1 R2 – rail 2 uRE(t) – rail-to-earth potential b – distance between rail R2 and electrode E2 E1 – electrode 1 u1-2(t) – potential between electrode E1 and E2stg – gauge width R1R2 E1 stg a b E2 uRE(t) u1-2(t) 163 Katarina Vranešić, Marijana Serdar, Stjepan Lakušić, Václav Kolář, Andrea Mariscotti Dynamic Stray Current Measuring Methods inUrban Areas Using the measured values, rail-to-earth conductance can be calculated. For the single-track lines local conductance per length is calculated as follows Eq. (1):       Gm bb saas RE sr Et gt g ·· ln ()ln ()   2000 (1) For double track electrical conductance can be calculated using the Equation (2):      Gm bsbssas RE sr Etgtgtdt   1000 05 05 05ln (.)( .)ln (. ggtgtd )( .)ass       05 , (2) where msr is stray current transfer ratio; ρE is the soil resistivity, Ω m; a is the distance between the outer running rail and the remote electrode E1, m; b is the distance between the outer running rail and the remote electrode E2, m; stg is the track gauge, m; std is distance between parallel tracks, m. Potential U12 needs to be plotted as a function of the rail potential ΔURE. The slope of the linear regression of this function is the stray current transfer ratio msr. Rail-to-earth conductance is of great significance for stray current assessment. Knowing this value, stray current per length can be calculated according to the equation given in the standard, Eq. (3): IUG SRERE , (3) where URE is rail potential, and it can be calculated using the equation U IR e L L RE C C           05 1. , I is the average value of the traction current in the section, RC is the characteristic resistance of the system running rails/structure RRG CRRE  /, Lc is the characteristic length of the system running rails/ structure LRG CRRE  1/ , ′ RR the longitudinal resistance of the running rails. The rail resistance can be measured and calculated using Ohms law as described by Kolář & Hrbáč (2014). The rail-to-earth potential measurement with reference electrodes was performed in the city of Ostrava on the tram track section showed in Figure 9. The measurement was performed at all three measuring points (M1, M2 and M3). Cu/CuSO4 reference electrodes were used for the measurement. One electrode was placed 7 m and the other 40 m from the track. The measurement was performed continuously for 2.5 h. Soil resistivity was measured using the Wenner’s method. This method is based on putting four electrodes into the ground at equal distances. A current of a certain strength is conducted into the soil 164 THE BALTICJOURNAL OFROAD AND BRIDGE ENGINEERING 2022/17(4) via two outer electrodes and the voltage is measured at the two inner electrodes (“Watts Current” technical bulletin, 2021; Vidov, 2009). Specific soil resistance is a basic parameter at determination of corrosive aggressiveness. The calculation of the resulting specific soil resistance can be performed using the following equation (Siranec et al., 2019), Eq. (4): 2lR , (4) where l is the pitch of electrodes, m and R is the measured resistance, Ω. After this measurement, the average values of the rail-toearth potential URE and the potential between electrodes U12 were calculated. Using the equation defined in the standard, the rail-to-earth conductance was calculated at all three measuring points. Bongiorno & Mariscotti (2018) gave some practical tips for rail potential field measurements. If it is hard to reach the soil due to asphalt area, concrete electrodes of CuSO4 type may be considered. When the tracks are closed, it is difficult to ensure the connection to the rail, so magnets may be used for this purpose, holding a small flat piece of copper to the side of the rail opposite to where the edge of the wheel is. Stray current monitoring systems are based on continuous rail-toearth potential measurements. The rail potential is normally measured in electrical substations and when the measured values change, the railto-earth resistance in the section between two substations also changes. 6. Measurement at stray current collection system If the track is built with a stray current collection system, it can be used to measure the current flow in the collection system. Stray current collection system is providing the low resistance path for the stray current so the current that leaves the rails enters the collection system, rather than nearby infrastructure (Stray DC Current, 2021). Using the collection system corrosion of nearby infrastructure is stopped. The upper layer of reinforcing bars in the concrete track bed may be used as a collection system. Collection system is needed when rail isolation and power-system design themselves cannot keep stray current levels below maximum allowed values (Cotton et al., 2005). According to Charalambous et al. (2016), current can be measured using a shunt resistor installed as part of the transverse bond connection, using the sensor or other suitable means. Since the collection system is “tightly coupled” to the tracks, it can provide a reasonable estimate of stray current leakage. If the transit system is not operated on a 24-hour basis to obtain more accurate data, 165 Katarina Vranešić, Marijana Serdar, Stjepan Lakušić, Václav Kolář, Andrea Mariscotti Dynamic Stray Current Measuring Methods inUrban Areas a test tram can be used. In this case, measurements should be made while a single train is traveling through the area of interest, pausing at passenger stations, and then accelerating at full power to normal speed between stations. It is good practice to perform the tests with the test train running once in each direction through the area of interest. 7. Non-destructive sensors Stray current on metal objects near urban track infrastructure can be measured with non-destructive sensors. Peelen et al. (2011) described the non-destructive measurement of stray current on sheet piles near railway infrastructure. Stray current from the railway enters and leaves the sheet pile wall, resulting in changes in corrosion current density and electrical potential between the steel and the soil. The authors used two types of sensors – a coupon sensor, which provides information about the local stray current density, and a reference electrode, which is used to measure the electrical potential differences between the electrode and the soil near the coupon sensor. A coupon sensor is a small steel plate made of the same material as the sheet pile and attached to the sheet pile in such a way that it is electrically isolated. The plate is electrically connected to the sheet pile wall via a zero-ohm ammeter so that it is possible to measure the current entering or leaving the steel coupon (Peelen et al., 2011). Mariscotti (2009) described a sensor for non-invasive measurement of rail current. This sensor is designed to detect the magnetic field generated by the rail current. Xu et al. (2014) described a stray current sensor for buried pipelines. The sensor is connected to the pipeline through a connector. Through this connection, the stray current from the pipeline can enter the sensor and be detected there. Continuous stray current monitoring systems that are in use in urban tracks worldwide also use non-destructive sensors. For example, continuous potential monitoring with the permanent reference electrode buried during track construction uses a stray current sensor to measure the potential difference between the object and the electrode (Peng et al., 2020). Conclusion Given the high density of rail traffic in urban areas, corrosion from stray currents is an increasing problem for rail infrastructure and adjacent structures. Many operators around the world have recognized this problem and have taken measures to reduce stray currents, which 166 THE BALTICJOURNAL OFROAD AND BRIDGE ENGINEERING 2022/17(4) can be divided into two groups: i) ensuring a low longitudinal electrical resistance of the rail, ii) increasing the electrical resistance between the rail and the ground. The low value of longitudinal rail resistance can be achieved by using rails with larger cross-section and ensuring continuous connection of rails, i.e., welding. When it comes to electrical resistance between the rails and the ground, the most important element is the insulation of the rail. In discretely supported and fastened tracks, where it is extremely difficult to completely insulate the rails, the high value of this electrical resistance can be achieved by efficient track drainage to avoid water accumulation in the track and by choosing a fastening system with high electrical resistance. In discretely supported and fastened track, the fastening points are “discharge points” of current, i.e., locations where current can flow from the rail through the fasteners into the lower layers of the track structure. Thus, by using a better insulated fastening system, this current flow can be reduced. The presence of stray currents on the track and surrounding metal structures can be detected by various field measurement methods. Stray currents due to tram traffic are dynamic and depend on many factors. The most important ones are the density of tram traffic, the mode of operation, and the electrical resistance between the rail and the ground, which depends on the insulation of the rails and fasteners, track maintenance, and soil moisture – moist soil has lower electrical resistance. For this reason, the results of field measurements taken at the same location and at different times can be different. To get a clear picture of the hazard of stray currents on an observed section of track, measurements should be made at several time intervals or continuous monitoring of stray currents on the track should be established. Such monitoring is nowadays mostly based on continuous monitoring of the rail potential. Today, there are no applicable standards prescribing ways to control and monitor corrosion and stray currents on tracks, and all methods used today depend on the operator. The effect of corrosion on track is still an area that does not receive enough attention, and the harmful consequences that corrosion can cause are not taken seriously enough. 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