SALTEK Voltage Limiting Devices (VLDs) in DC Railways

Jiri Vykydal, Product Manager, SALTEK

Reasons and Effects of Stray Currents

Electric railways are using rails as a robust electrical conductor for return currents from the trainsets back to the power supply substations. As the electrical conductivity of even a massive rail is limited (i.e. the rails feature a specific value of electrical resistance and inductance), and the rails cannot be perfectly isolated from the earth (in addition to regular leakage currents arising in the track via limited impedance of track isolation a significant occurrence of current leakage can be observed e.g. at places with contaminated ballast, railroad crossings etc.), a part of the return current, also known as stray current, escapes from the rails to the environment and flows outside of the rails through parallel paths with the lowest resistance, i.e. through the surrounding soil and, in particular, nearby conductive structures back to traction power substations, in accordance with the principles of Ohm’s and Kirchhoff’s circuit laws.

The conductive structures are mostly steel constructions or reinforcements embedded in iron-concrete buildings along the railroad (bridges, tunnels, …), pipelines, metallic cables of railway signalling systems, telecommunication cables, etc.

Alternating currents do not imply a risk for the above structures, but the more dangerous is the long-term influence of direct currents. In principle it is an electrochemical reaction similar to the zinc electroplating (galvanization) process in which metallic material from a zinc anode is transferred to a zinc-plated object (cathode) by the interaction of direct current, causing the volume of the zinc anode to decrease.

Similarly, stray DC currents cause corrosion at the anode area of the conductive component (i.e. at the area where the stray current leaves the steel structure lying in parallel with the railway), causing loss of metallic material. Long-term exposure to even small currents may result in material losses that eventually destroy the affected structure.

Concerning the rate of material loss, Faraday’s law may be used. Let us consider an average traction current of 1000 A and a stray-current share of 0.1 per cent (i.e. 1 A). Over the period of one year this stray current can transfer up to 9 kg of iron or more than twice that amount of copper.

Since stray currents may reach multiples of the value considered above, the associated risks can be very significant. In reinforced-concrete structures, corrosion is linked with local expansion of the corroded reinforcement, causing cracks in the concrete due to internal pressure and consequently reducing stiffness and load-bearing capacity.

In a better-case scenario, this results in periodic inspections and costly repairs, leading to substantial OPEX (Operating Expenses) and CAPEX (Capital Expenses). In extreme cases, it may result in catastrophic failures causing major property damage and even loss of human life.

Figure 1: DC stray currents and their effect on steel structures, resulting from earthed or improperly insulated rails.

Safety of Persons and Animals First

Most DC-powered electrical railways worldwide (tramways, underground railroads and DC railways) use an insulated return circuit (rails) as part of the earthing system.

A simplified diagram of such a system is shown in Fig. 2, where Ri represents the quality of insulation between the rail and the surrounding soil, RGs is the quality of earthing of the traction power substation, and the series combination ROCL + LOCL + Σ(RR + LR) represents the total impedance of the electric traction loop (overhead contact line and rails in the section under consideration). The impedance of the locomotive (RL + LL) can usually be neglected.

Figure 2: Simplified model of insulated DC railway traction.

The objective of this engineering approach is to limit, as far as possible, the leakage of stray currents into the vicinity of the railway, where metallic structures are frequently present, particularly in urban areas. These structures may suffer severe damage as a result of long-term exposure to even relatively small DC stray currents.

Achieving high values of Ri requires the track superstructure to be designed and constructed to an appropriate quality level and subsequently maintained accordingly. Dirt within the ballast and damage to insulated fastening elements can significantly deteriorate the Ri impedance value over time and consequently increase corrosion caused by stray currents.

This concept of insulated rails, however, has one significant disadvantage. The operation of railway systems with a floating return conductor (rails) causes voltage drops to occur on the return conductor due to its impedance Σ(RR + LR).

Under certain conditions, the voltage appearing on insulated rails relative to earth may reach relatively high values and become life-threatening for people and animals in the railway vicinity, including passengers, railway personnel, farm animals and wildlife.

Such dangerous voltages may occur as a result of:

• long distances between traction substations,
• a high number of trainsets operating within the same power-supplied section,
• the use of regenerative braking,
• slow operation of protective disconnectors at traction substations,
• the inability of substation disconnectors to distinguish fault current from normal operating current in some situations.

This is particularly relevant for faults occurring at a greater distance from the power substation, such as:

• an overhead contact line (OCL) falling onto the rail, or
• a short circuit occurring within a locomotive.

In these cases, the total loop impedance can limit the fault current to values similar to normal operating current ranges.

Similar situations may arise when the ground resistance of the MEB earthing system at the traction substation deteriorates (increases), or when the overhead contact line falls outside the rails.

…on earthed parts of the railway infrastructure. In such a case, the higher resistance of the impedance loop could cause the fault current to drop below the tripping threshold of the disconnectors.

From the above examples, it can be seen that dangerous touch voltages along a railway line may arise not only during fault conditions but also during normal operation. All such risks must be mitigated through the use of a voltage limiting device (VLD).

The maximum permissible voltage values with respect to earth (i.e. earthed, electrically conductive structures installed around the railway) as a function of time are specified by the EN 50122-1 standard. Railway operators are required to ensure that these limits are not exceeded at any point of the line.

The most commonly applied limits are:

• 120 V DC for general railway environments
• 60 V DC for railway workshops

(Some national regulations may specify lower values.)

Predicting the occurrence of such unwanted voltage peaks is often difficult, particularly on modern railway systems where numerous factors influence their occurrence, including:

• the impedance of the overhead contact line,
• the impedance of the return circuit,
• train power consumption,
• train density and position along the route,
• regenerative braking systems,
• modern AC traction drives using IGBT DC/AC inverters.

Compliance with these voltage limits may be achieved in several ways, for example:

• reducing the length of traction power sections,
• increasing the number of traction substations,
• reducing the impedance of the power supply circuit,
• increasing rail conductivity,
• installing auxiliary return conductors in parallel with the rails.

However, such solutions often require substantial additional investment and introduce operational complexity.

In these situations, voltage limiting devices (VLDs) provide an effective and economically efficient method of mitigating risks. By balancing all available measures, it is possible to achieve a functional and safe traction system while optimising both CAPEX and OPEX.

SALTEK Voltage Limiting Devices Used in DC-Powered Railway Lines

Voltage limiting devices (also referred to as VLD, OVPD, RPCD and similar designations) are designed to fulfil two conflicting requirements at critical locations within electric railway systems:

1. Ensure the safety of people, animals and often critical technologies against excessive voltages that may arise between insulated rails and their surroundings by temporarily connecting the rails to earth or to earthed conductive structures.
2. Minimise, as far as possible, the duration of that temporary grounding so that stray currents are allowed to flow only for the shortest possible time, thereby reducing their corrosive effects.

The EN 50526-2 standard divides voltage limiting devices into four classes according to their design while defining the fundamental operational and performance requirements for each class.

The EN 50122-1 and EN 50526-3 standards define VLD types according to their intended function (VLD-F and VLD-O) and establish the recommended methods for their application.

SALTEK offers three VLD product families that collectively cover the full range of practical railway applications.

Figure 3: Typical failure situations arising in traction circuits of a DC-powered railway line.

1. This is the case where a short circuit occurs between the power substation output towards the overhead contact line (OCL) and the substation structure itself, for example when a feeder conductor falls onto the surrounding ground.

During such a fault, dangerous voltages appear on electrically conductive parts of the substation, while hazardous step voltages may also occur in the surrounding area.

Properly located and dimensioned voltage limiting devices (VLDs) provide a current path for the fault current, which initiates the operation of the protective disconnector installed at the traction substation.

In the event of a feeder wire falling onto the ground, the grounding impedance (RGs) of the substation also becomes part of the fault current loop. The protection principle, however, remains unchanged.

In such cases, it is essential that the RGs value does not limit the fault current below the tripping threshold of disconnector B. Consequently, the Main Earthing Bar (MEB) and its earthing system must be designed and maintained to an appropriate quality standard.

2. When the overhead contact line falls onto the ground or onto grounded steel structures located along the railway line, such as passenger shelters, catenary masts and similar infrastructure, dangerous touch and step voltages arise in the vicinity of the fault.

Without a VLD-F installed near the fault location, the fault current loop becomes heavily influenced by:

• the impedance of the overhead contact line (OCL),
• the value of the RGs grounding impedance,
• the resistance of the soil between the fault location and the traction substation.

These factors can significantly reduce fault current levels. As a result, the total impedance of the fault loop may be affected by:

• the distance between the fault location and the traction substation,
• the conductivity of the soil return path,
• the settings of the VLDs installed at substations,
• other local network conditions.

The resulting impedance may become so high that disconnector B is unable to react quickly. Consequently, dangerous voltages may remain present for extended periods, posing serious risks to both people and railway equipment.

Installing and activating a VLD-F along the railway line replaces the high-impedance grounding and soil return paths with the rails themselves, which provide a much more conductive return path.

This ensures a reliable and rapid operation of the protective disconnector installed at substation B and significantly improves overall safety.

3. In the case of the overhead contact line falling directly onto the rail, or when a short circuit occurs inside a locomotive, the protection system associated with disconnector B operates under normal conditions.

It is necessary, however, to properly define the protective settings and operating characteristics of the entire protection system in order to ensure reliable fault clearance under all operating conditions.

It is essential to properly define the relationship between the overall impedance of the traction circuit, the rated traction voltage and the tripping current of disconnector B.

If the overhead feeder conductor falls at a remote location where the safe touch voltage generated on the rail impedance between the fault location and the substation, Σ(RR + LR), may be exceeded due to fault-current flow, the voltage level must be reduced by temporarily grounding the rails using VLDL devices. In practice, this creates an additional parallel current path.

By selecting an appropriate response voltage for the VLDL and positioning these devices strategically along the railway, it is possible to protect even long railway sections and the conductive structures installed along the line.

However, it must always be ensured that the sum of:

• the response voltage of the VLDL, and
• the voltage drop occurring between the VLDL and the fault location

remains below the maximum permissible voltage specified in Clause 9.3.2 of EN 50122-1 for the longest possible operating time of disconnector B.

Comparing the operation of VLDL devices in situations 2 and 3 shows that, depending on the operating condition, the VLD may be exposed to voltages of either polarity. Consequently, bipolar VLDs are required.

As noted previously, dangerous voltage potentials may also arise on rails, and therefore on trainsets, during normal railway operation. These voltages primarily result from the voltage drop caused by return current flowing through the rail impedance Σ(RR + LR) between the train and the traction substation.

Figure 4 illustrates an example of the rail-potential profile during train acceleration after departing from a station at which no traction substation is installed, meaning that the impedance of the return path (the rails) plays a significant role.

During intensive acceleration, or during regenerative braking, rail potentials may exceed safe values. These conditions must be managed by VLD-O devices installed at critical locations where people may be exposed to excessive touch voltages, most commonly at railway stations.

The red-highlighted sections indicate periods during which dangerous touch voltages are present and must therefore be mitigated using VLD-O devices.

In railway systems equipped with regenerative braking, the use of bipolar VLD devices is mandatory.

Figure 4: Example of a rail potential increase (Volts) during train acceleration and regenerative braking (m/s²), occurring at a greater distance from a traction power substation, indicating the period during which the VLD-O becomes active.

Simple Class 1 VLD (VLD-F)

This cost-effective SCG-series voltage limiting device has been developed to provide basic protection for railway sections affected by:

• an overhead contact line (OCL) falling onto conductive structures,
• an OCL falling directly onto the ground,
• insulation failures between the OCL and conductive structures located within the Overhead Contact Line Zone (OCLZ).

In addition to these applications, the VLD-F can also be used in further railway protection scenarios where rapid fault-current diversion and the reliable activation of protection equipment are required.

The VLD-F can also be used wherever a combination with protection against atmospheric overvoltages is required.

The electrical characteristics of these devices are designed so that the VLD enters a conductive state whenever the selected ignition voltage is exceeded. At the same time, it remains capable of restoring its original high-impedance state after short-duration lightning surge currents have been discharged.

In practice, this means that while the electric charge associated with a lightning current is being dissipated, the VLD automatically returns to its high-impedance condition.

Conversely, if a fault occurs on the overhead contact line (OCL) and a sustained short-circuit current begins to flow, the integrated short-circuiting bypass creates a permanent short circuit between the SCG terminals.

This action initiates the correct response of the disconnectors installed at the associated traction substation and therefore fulfils the primary function of a VLD-F.

As an example, the tripping currents of DC disconnectors used in 600 V (740 V) tramway traction systems typically range from 3 kA to 4.5 kA, depending on:

• the length of the energised section,
• the expected number of tram vehicles operating within that section.

The patented SCG series of short-circuiting devices is capable of safely disconnecting the power supply over the entire range of these fault currents.

The overall objective when designing a traction system is to reach a state in which the VLD operates only in genuinely necessary situations while keeping stray-current flow to an absolute minimum.

Where extremely short time intervals can be achieved between the occurrence of an OCL fault and the disconnection of the power supply, for example through the use of fast-acting disconnectors, SCG-series VLD-F devices with higher ignition voltages may be used.

This requirement is addressed by VLD-F devices with ignition voltages of:

• 250 V (SCG-250-250)
• 480 V (SCG-250-500)

In this configuration, unwanted activation of the VLD due to short-term voltage peaks occurring on the return conductor (the rail) is significantly reduced, and consequently stray-current flow is minimised.

When selecting the ignition voltage, the time-dependent permissible touch-voltage limits defined by EN 50122-1 must be considered, particularly in relation to the maximum operating time of disconnector B.

Figure 5: VLD-F (SALTEK SCG series) and its application in tramway traction systems powered with <750 V DC.

It should be noted that the Class 1 VLD-F principle, based on a permanent short-circuit, has one significant disadvantage resulting directly from its operating principle and the requirements of EN 50526-2.

Once the short-circuiting device (earthing switch) has operated, the Class 1 VLD remains permanently in a low-impedance state, even after the original cause of the fault has been removed and normal operating conditions have been restored.

After the original fault has been cleared and normal railway traffic has been restored, a Class 1 VLD-F remains permanently in a conductive state.

This means that the affected railway section remains permanently grounded and a continuous path for stray currents remains open.

In locations where this may present a risk to buildings or infrastructure, the condition of VLD-F devices must be checked regularly, particularly at locations where a fault has been reported. Any permanently short-circuited VLD-F must be replaced with a new unit.

For this reason, it is often more efficient, especially from an operational perspective, to use Class 2 voltage limiting devices at VLD-F locations. Recoverable VLD-O+F devices provide a practical alternative because the slightly higher initial CAPEX is often rapidly offset by reductions in OPEX.

Where communications infrastructure is available, the condition of installed VLDs can be monitored using current sensors connected to a SCADA system.

Such monitoring enables the operator to receive timely information when a VLD becomes overloaded and requires replacement.

Once a VLD-F remains permanently conductive, the statistical behaviour of the current flowing through the device changes significantly. This allows maintenance personnel to detect the fault condition and replace the affected device before excessive stray-current flow causes long-term damage.

As a result, stray currents can be reduced without the need for costly physical inspection and electrical testing of all VLDs installed throughout the railway system.

Sophisticated Class 2 VLD (VLD-O+F)

SALTEK Class 2 VLDs, represented by the bi-directional BVL product family, are autonomous voltage limiting devices that do not require an external power supply and are widely used in modern DC railway systems.

These devices have been designed to:

• withstand very high fault and short-circuit energy in repeatable VLD-F mode,
• respond rapidly to traffic-related overvoltages in VLD-O mode,
• operate as a non-repeatable VLD-F in critical situations such as delayed disconnector operation or extremely high short-circuit currents.

The repeatable operating capability of the SALTEK BVL series makes it particularly suitable for systems where both operational safety and limitation of stray-current leakage are equally important design objectives.

Where specified, the BVL series achieves significantly better results than the SCG series and provides performance advantages compared with other Class 2 products available on the market.

The performance of the BVL family is further enhanced by an integrated Type A2 surge arrester (MOV), which reacts extremely quickly to:

• atmospheric overvoltage impulses,
• steep fault transients generated within the traction system,
• switching phenomena caused by inductive components of the traction network.

At the same time, the MOV protects the semiconductor electronics within the VLD against damage.

Such steep voltage spikes may occur in modern DC transportation systems employing regenerative braking, energy exchange between moving trainsets, or traction drives based on IGBT DC/AC inverter technology.

SALTEK Class 2 VLD devices use sensitive electronic control circuitry (EDC) that continuously evaluates the voltage present at the VLD terminals.

When the permitted voltage threshold is exceeded, the control electronics activate a power thyristor after a short delay of approximately 1.5 ms.

The thyristor then assumes the electrical load from the power varistors, which themselves begin conducting within approximately 10 μs after the permissible voltage has been exceeded.

The unidirectional UVL version uses a single EDC + thyristor + A2 MOV arrangement and therefore responds only to one voltage polarity.

This economical solution is suitable for older and simpler traction systems where:

• negative-polarity voltage peaks do not occur, or
• they occur only very rarely, and
• the complete VLD-F functionality is not required.

The bidirectional BVL version uses an anti-parallel arrangement of two power thyristors, each controlled by a separate EDC system and supplemented by an A2 power varistor for rapid response to both atmospheric and traction-related overvoltages.

Figure 6 illustrates a typical UVL/BVL response to an overvoltage pulse.

Figure 6: Oscillogram showing the response of BVL/UVL during the simulation of a steep high-energy pulse.

Figure 7: Load chart of UVL/BVL.

Figure 7 illustrates the load characteristics of SALTEK Class 2 voltage limiting devices from the UVL/BVL product family.

The chart demonstrates that, in addition to withstanding very high short-duration currents, these devices are capable of repeatedly switching current values that lie within the short-circuit tripping range of modern disconnectors used in substations supplying lightweight tram and railway systems.

It is evident that when the disconnector responsible for interrupting fault currents is configured with a tripping threshold of approximately 4 kA to 4.5 kA, which is a typical overcurrent protection setting for smaller tramway substations, the BVL technology is capable of operating together with fast-acting disconnectors in a repeatable mode.

Under such conditions, BVL devices can be used as repeatable VLD-F units not only along railway tracks but, in some cases, also directly within traction substations where significantly more expensive Class 4 VLDs are commonly recommended.

An additional advantage of this approach is that it is generally unnecessary to equip every individual mast or conductive structure located within the Overhead Contact Line Zone (OCLZ) with a dedicated VLD.

Instead, multiple conductive structures can be electrically bonded together and connected collectively to the rail through a suitably selected VLD.

This approach makes it possible to provide effective protection for an entire DC tramway or light-rail traction network while achieving substantially lower CAPEX compared with alternative protection concepts based on large numbers of high-cost devices.

A minimum deployment configuration should include:

• a VLD installed between the negative power-supply pole (the rail) and earth (MEB) at every traction substation,
• a VLD installed between the rail and earth at every railway stop or station.

Naturally, different maximum fault-current values must be considered depending on the location of the fault.

Faults occurring close to a traction substation require consideration of the characteristics and power capabilities of the substation itself, while faults occurring at greater distances along the line are governed by different current maxima and different impedance conditions of the traction network.

Different maximum fault-current values must be expected for faults occurring at locations far from the traction substations, where the maximum available short-circuit current is limited by the impedance of the traction power loop, particularly the overhead contact line (OCL).

When using VLDs in underground railway systems or DC railways operating heavy trainsets, it should be noted that the operating characteristics of heavy trains are shifted towards significantly higher current levels.

In practice, this means substantially higher currents during train acceleration and braking, and consequently higher short-circuit tripping thresholds for the disconnectors installed at traction substations.

In a carefully designed and properly implemented traction system, overvoltage protection may still be achieved using Class 2 VLDs (BVL) installed at selected, relatively lightly loaded locations.

Such a solution should, however, always be modelled using suitable simulation software and evaluated against every possible operating scenario, including concurrent train operation within the same electrically supplied railway section.

Alternatively, its effectiveness should be verified by practical testing.

The simultaneous acceleration and braking of multiple trainsets at different positions within the same route section may generate significant voltage and current pulses of relatively long duration.

These pulses may reach hundreds of amperes and can approach 1 kA for periods lasting several tens of seconds.

Under such operating conditions, more powerful Class 3 or Class 4 VLDs become necessary.

Smart Class 4 VLD (VLD-O+F)

SALTEK offers the PVL product family, representing powerful Class 4 VLD solutions.

The design of these devices incorporates all currently known requirements associated with modern electrified railway systems and advanced voltage limiting technology.

During product development, SALTEK selected a unique architecture based on the combination of:

• an autonomous Class 2.2 VLD,
• an electronically controlled current bypass circuit.

This concept provides the highest possible level of operational reliability and protection of persons in both normal operating conditions and emergency situations while simultaneously minimising the energy associated with stray currents.

The integrated BVL unit (a bi-directional Class 2.2 VLD with integrated Type A2 surge protection) is the first component to respond whenever the permissible touch-voltage limit is exceeded.

It is capable of conducting the high initial energy associated with:

• atmospheric discharge currents,
• fault currents resulting from short circuits.

This represents a major advantage because the fundamental VLD-O+F functionality remains available even if another part of the Class 4 device should fail, for example:

• the control microprocessor,
• the operating software,
• the mechanical bypass system.

Compared with conventional solutions that permanently short-circuit the VLD during maintenance or fault situations, thereby leaving the return circuit permanently connected to earth and allowing stray currents to flow continuously, the PVL remains in a high-impedance state whenever possible.

The PVL therefore behaves as a conventional Class 2 VLD and becomes active only when the permissible safety voltage threshold is exceeded.

A permanent short circuit across the PVL terminals is established automatically only when:

• auxiliary power is lost, or
• manual activation is performed.

SALTEK’s emphasis on maximum operational reliability is further reflected by the possibility of automatic power-supply redundancy for the VLD system.

This backup arrangement may be implemented using an integrated 1+1 redundant power-supply configuration supporting:

• dual AC 230 V supplies,
• combinations such as AC 230 V + DC 48 V from station batteries,
• other dedicated auxiliary power sources.

Any failure of the power supply can be indicated remotely.

Under normal operating conditions, when auxiliary power is available, the PLC control logic continuously monitors voltage and current values present at the VLD terminals.

According to these measurements, the controller can connect the parallel bypass to the integrated Class 2 VLD and thereby increase the long-term current-carrying capability of the entire protection system.

The bypass is activated only when required, specifically when the energy capacity of the solid-state Class 2 VLD has been exceeded.

This operating philosophy significantly extends the service life of the complete VLD installation.

The possibility of intentionally short-circuiting the VLD terminals, either:

• through forced electronic bypass activation from the PVL control panel, or
• through a manually operated lockable earthing switch,

can be used to provide complete isolation and safe maintenance of the traction system when required.

The forced short-circuit functionality can also be used to ensure the complete protection of maintenance personnel performing work on the traction system.

Figure 8: Limiting curves of load-carrying capacity of PVL-1000.

From the maximum load curves shown above, it is evident that the PVL-1000 is capable of carrying currents of up to 3.5 kA for periods of approximately 30 seconds.

This corresponds closely to the typical acceleration or braking interval of modern trainsets, during which the Class 4 VLD must be capable of handling significant electrical loading.

As a result, the PVL-1000 provides adequate performance for virtually all practical railway operating scenarios.

The bypass disconnection process is electronically controlled in order to prevent relay pumping, meaning repeated switching ON and OFF of the bypass contactor.

At the same time, the control system minimises the total ON time, that is, the period during which a conductive path for stray currents exists.

The complete VLD4 operating behaviour is optimised by software to satisfy common railway operating requirements.

Should special project requirements arise, SALTEK can adapt the response characteristics and operating logic of the VLD to suit the customer’s specific transport conditions and operational needs.

Interactivity forms an essential part of the PVL-1000 concept.

Monitoring and remote control are provided through communication with the control centre via a standard Ethernet communication interface using the widely adopted MODBUS protocol over TCP/IP.

This communication architecture enables remote monitoring of:

• instantaneous voltage values,
• current flow through the VLD,
• the status of critical internal components,
• the overall operational condition of the VLD.

Integration with SCADA systems allows operators not only to supervise the device but also to configure performance parameters and activate specific operating modes, including remote forced activation of the VLD.

The collected operational data can subsequently be analysed using software tools.

Consequently, the PVL-1000 serves not only as a protection device but also as an advanced monitoring component within the traction power network.

Local operation remains available through the integrated interactive control panel installed on the device.

Compared to many similar products, the PVL-1000 stands out because of its compact design, low weight and high level of integration.

These characteristics make it particularly suitable for installation in:

• containerised traction substations,
• railway tunnels,
• other locations where installation space is limited.

The PVL-1000 is most commonly used at locations where dangerous voltage potentials may arise during the acceleration or braking of heavily loaded trainsets, especially at greater distances from traction substations.

Typical applications include:

• railway systems employing regenerative braking,
• systems in which regenerative energy is transferred to other trainsets,
• systems that feed recovered energy back into the supply network,
• DC traction substations,
• sectioning disconnectors,
• railway stops and passenger stations,
• railway workshops and maintenance depots.

Several representative examples of VLD deployment are presented in Figure 9, although many additional application scenarios are also possible.

Figure 9: Typical installation of VLDs along the railroad.

Conclusion

The application of voltage limiting devices (VLDs) has become an essential element of modern DC-powered railway infrastructure.

These devices provide effective protection against hazardous touch voltages while simultaneously reducing the long-term impact of stray currents on surrounding metallic structures and railway infrastructure.

By selecting the appropriate VLD class and implementing it at strategically important locations within the traction system, railway operators can significantly improve operational safety, minimise infrastructure degradation and optimise both capital and operating expenditures.

SALTEK offers a comprehensive portfolio of voltage limiting devices ranging from simple Class 1 VLD-F solutions through sophisticated Class 2 BVL systems to fully interactive and highly intelligent Class 4 PVL platforms.

This portfolio enables railway designers and operators to choose the most appropriate solution for virtually any DC traction application, from urban tramway systems to demanding heavy-rail and underground railway installations.

In addition to the products themselves, SALTEK provides comprehensive technical support covering:

• system design and engineering consultation,
• application studies and modelling,
• testing and commissioning support,
• long-term operational assistance.

SALTEK guarantees high reliability, quality and long service life of its voltage limiting devices, combined with comprehensive customer support throughout the design, testing and operational phases of railway projects.

For further information about SALTEK voltage limiting devices and railway protection solutions, please visit:

www.saltek.eu

Alternatively, additional technical information can be obtained directly from the SALTEK technical support team.