Stray Current Mitigation Measures
Introduction:
Stray current, also known as leakage current, is a type of electrical current that flows through the ground or other unintended paths due to imperfections in the insulation of electrical systems, particularly in railway applications. This phenomenon is especially prevalent in Direct Current (DC) traction systems, in urban rail systems where the tracks are used as a return path for electrical current. If not properly managed, stray currents can cause significant damage to infrastructure, including corrosion of metallic structures and interference with nearby electronic systems.
What is Stray Current?
In railway systems, the propulsion of trains is typically powered by electrical energy supplied through a third rail or an overhead catenary system. The return path for this electrical current is usually through the rails. However, due to imperfections in the insulation and other factors, some of the current may leak into the ground and stray from its intended path. This unintended current is referred to as stray current.
The primary issues associated with stray current include:
Corrosion: The most serious consequence of stray current. When this current enters underground metal structures, it accelerates their degradation through electrochemical reactions, which can lead to failures in underground pipelines, reinforcing bars in concrete, and other structural elements.
Safety Hazards: High levels of stray current can pose safety risks to maintenance personnel and the public.
Electrical Interference: Stray currents can interfere with the operation of nearby electrical and electronic systems, causing malfunctions.
As per the European Standard EN-50122-2:
“All components and systems which can be affected by stray currents should be considered such as:
Running rails,
Metallic pipe work,
Cables with metal armour and/or metal shield,
Metallic tanks and vessels,
Earthing installations,
Reinforced concrete structures,
Buried metallic structures,
Signalling and telecommunication installations,
Non-traction a.c. and d.c. power supply.”
And:
“Factors that can impact on the stray current presence.
The possible impact of stray current corrosion should be investigated, where the following aspects are included, such as:
Insulation from earth of the rails and connected metallic structures,
Humidity of the track bed,
Longitudinal resistance of the running rails,
Number of and distance between the substations,
Effects of inequalities in the no-load voltages of substations,
Substation no-load voltage and source impedance,
Timetable and vehicles,
Neighbouring metallic structures.”
Let’s focus only on examples of specific solutions in Slab Track design that protect the track systems against the appearance of stray current.
First, we should look at the railway track holistically, not only as the road for rolling stock and the linear structure that is meant to withstand and distribute the load from the rails further on the substructure, but also as a structure participating in management of the electrical current and mitigates the possible risks associated with it.
1. Stray Current Collection System
Slab track reinforcement plays a dual role in the whole system, apart from the obvious structural role it also can act as an electrical mesh for current conductivity, which is a main part of the overall Stray Current Collection System. Under-track metal bars are bonded to form a collection mat, connected to a continuous conductor running along the track.
Figure 1: An example of Stray Current Collection System
Stray Current Mesh
Figure 2: Slab Track Mesh Example
Normally on projects, as per the Stray Current Mitigation Plan, the track slab reinforcement should create a continuous mesh that is able to conduct the current. For the purpose of this continuity, the reinforcement mesh can be welded all along the track (in case of a continuous concrete slab) or connected at the slab joints (in case of separate concrete slabs). Cable bonds or earth plates are used as a connection between the separate reinforcement sections.
Earth Plates/Bars and Stray Current Collection Cables
In addition to the above, Earth Plates/Bars are also provided at Parallel Earth Conductor (PEC) connection points.
The requirements for the Earth Plates and Collection Cables include, but are not limited to, the following: minimum and maximum dimensions, corrosion criteria (for example, as per EN 50122-2), ability to withstand the atmospheric pollution of the environment they are used in, welds to the track reinforcement as per conductivity requirements, specific cable lug requirements, and flexibility requirements (for example, at Civil's expansion joints).
At the location of the collection point to PEC, a track-to-track connection is also present. Variable lengths for transversal installation between the inbound and outbound track may be required.
All connections should be subject to a continuity test with a micro-ohmmeter.
Keeping in mind the continuity of the stray current connection system, it must be mentioned that in certain locations this continuity must be stopped. For example, at the boundary of two electrical sections of the railway line or at Switches and Crossings locations. In such cases, a standard slab track with a mesh that can conduct the current should be replaced with high-performance concrete or fibre-reinforced concrete without steel reinforcement. Alternatively, sections of slab reinforced with electrically insulated (coated) mesh could be used.
Figure 3: Coated Reinforcement Mesh
2. Increased Rail-to-Ground Resistance
Using insulated components for rail fastenings helps to prevent current from leaking into the slab and further into substructure. Applying materials with high electrical resistance into the fastening while in between fastenings in regular intervals ensuring minimal conductive paths to the ground can significantly reduce stray currents.
The following insulator elements are normally used within modern fastening systems:
Rail pad (resilient pad)
Insulators (between the holding down clips and the rail)
Shims for vertical adjustment
Baseplate
Insulation of the anchorage system (if not provided as part of the baseplates)
Figure 4: Example of insulated elements of the modern rail fastening system.
In tramways, at the track section built into highways, where rails are embedded in the pavement, rail encapsulation is used to provide additional insulation of the rail and its fastening system.
Figure 5: Rail encapsulation examples.
Other elements of the embedded track that have contact with rails will also require electrical insulation. These could include components such as expansion joints or track drainage components. Special consideration must be given to the insulation of embedded switches and crossings.
All the insulation materials used have different chemical compositions, but they all should ensure the required electrical resistance for the overall track systems. All materials should comply with applicable standards and project requirements, and need to be fit for purpose, including resistance to environmental conditions such as sun exposure and operating temperature. For example, plastic components that may be suitable for a high-humidity climate might not withstand a dry desert climate, where plastic tends to become brittle. It is essential that the testing of components provided by manufacturers is conducted under conditions that represent the actual project environment.
3. Rail Connections / Cross Bonding
Regularly spaced electrical connections between the rails help balance potential differences and provide a controlled return path for the current, reducing leakage. The common names for these types of connections are cross-bonding or equipotential bonding.
Figure 6: An example of cross-bonding connection
4. Rail Joints
There are several types of rail joints that also must be mentioned in the context of stray current and the management of rail conductivity.
Welding joints
The welded joint should have electrical resistance as low as possible to ensure efficient current flow. High resistance can cause voltage drops, heating, and potential damage to the rail and electrical equipment. Various standards specify the electrical requirements for rail welding joints to ensure safety and performance. In Europe, for example, standards such as EN 14587 apply. The most common types of welding joints are:
Flash Butt Welding (FBW): This method typically provides excellent electrical conductivity due to the precise control of the welding process and the quality of the bond.
Aluminothermic Welding: Properly executed aluminothermic welds should also provide good electrical conductivity, although care must be taken to avoid inclusions and voids that can affect performance.
Rail Expansion joint
In railway projects when tracks are laid on engineering structures, there is often a need for using expansion joints in the rail. These types of joints provide a discontinuity in the rails that allows them to expand due to temperature changes or civil structure movements. While it is desirable for the rails to be discontinued from the mechanical point of view, the continuity of the electrical conductivity should be maintained. In such cases, the rails should be connected with a cable allowing for rail movement.
Figure 7: Example of ERJ with electrical connection.
Insulated rail joint (IRJ)
An insulated rail joint (IRJ) is a crucial component in railway systems, designed to electrically isolate sections of the rail track. The primary feature of an IRJ is the use of non-conductive materials, such as epoxy or fiberglass, placed between the rail ends to ensure electrical isolation.
Figure 8: Insulated Rail Joint Example
Track Flooding Mitigation
Water presence in the track is highly undesirable for stray current mitigation in electrified rail systems, as water can facilitate stray current leakage. This can occur after heavy rainfall due to ineffective track drainage systems or groundwater presence in underground tunnels. To prevent water ingress, joints and cracks in the slab should be sealed, and the concrete slab should be designed to protect against groundwater aggressive mineral composition, such as sulphates.
Conclusion
The examples provided are generic and meant to offer an overview of slab track design elements related to stray current mitigation. Typically, on a project each solution is further developed based on contract requirements, applicable standards, procurement practices, design specification, quality control and assurance plans, method statements for installation, testing requirements, and other criteria.
Achieving the desired control of stray current in modern railway systems requires a comprehensive approach, combining improved track design, interface between subsystems, regular maintenance, and advanced monitoring systems.
