The All-Island Strategic Rail Review (AISRR) has been jointly commissioned by the Department of Transport in Ireland and the Department for Infrastructure in Northern Ireland. Once completed, this will inform the development of a National Railway system across Ireland by 2050.
Implementation of this strategy is in line with the commitment to net-zero emissions in both jurisdictions.
Ireland previously published its own railway standards, particularly relating to safety measures.
LPI’s Engineering Manager, Hadi Beik Daraei outlines the design considerations attributed to earthing and bonding for railway systems. This technical blog will help readers to understand the fundamental safety concepts attributed to electrical shock hazards from an earthing and bonding expert.
In this blog, Mr Hadi outlines:
- Main System Components
- Technical Concerns for Earthing and Bonding
- Earthing and Bonding Requirements
- Outlines Key takeaways.
General Overview
Overhead Wiring Structure
The Overhead Circuit of the traction system could be energised by different nominal voltages AC or DC as stated in the below table.
Table 1: Nominal Voltages and their permissible limits in values and duration
Auxiliary Power supply
For all types of traction systems, there are sets of auxiliary circuits and equipment which provide different services such as lighting and HVAC, etc.
Generally, for both DC and AC Traction systems, the Alternating Current (AC) voltage is provided to supply the auxiliary system.
Figure 1 illustrates the typical schematic which shows the overhead wiring structure along with the auxiliary circuits. For both Direct Current (DC) and AC Traction systems the AC voltage has been used to supply the auxiliary circuit.
Figure 1: Typical diagram of power distribution for auxiliary system in AC railways
Figure 2: Typical diagram of power distribution for Auxiliary system in a DC railway
Technical Concerns Relating to Earthing and Bonding
AC Traction system
- Protection against electrical shocks in indirect contact situations
- Electromagnetic interferences
DC Traction System
- Protection against electrical shocks in indirect contact situations
- Protection against corrosion (Stray Current control)
- Electromagnetic interferences in certain circumstances
Important Note: If corrosion provisions affect electrical safety, protective provisions against electric shock shall take precedence over provisions against the effects of stray currents.
2. Earthing and Bonding Design Considerations
2.1 AC Traction System
For Auxiliary Circuit:
Protection against indirect contact for equipment or installations which are not protected as Class II shall be realised by automatic disconnection of supply as described in IEC 60364-4-41. Therefore, exposed conductive parts shall be connected to a protective conductor.
In this case, the earthing configuration only provides supplemental protection before the faulted circuit is de-energised. For example, the Circuit Protective Conductor (CPC) or Protective Earthing Conductor (PEC), will conduct a short circuit current through the low impedance paths back to the power supply to help disconnect the circuit breaker to function for less than a certain time (e.g., 0.1, 0.4, 0.5, 1 second, etc.)
If the “main protection measure” changes, the earthing and bonding practices will change accordingly.
Figure 3: Different types of earthing connections based on the type of protection against indirect contact.
2.1.2 For AC Overhead Wiring Structure
Based on the different levels of voltages used in a traction system, the following should be given attention:
- Touch and step voltage value due to the fault at the overhead wiring
- Touch voltages due to different metal parts that could be energised and are located in the vicinity of the overhead wiring structures.
- The longer the presence time of people at a certain location, such as platforms, corridors, etc., the more possibility of experiencing transferred voltage which originated from earthing electrode system.
Therefore, standard BS EN 50122-1, the standard used by the rail and tram industries, requires the following conditions to be considered as a conservative aspect for the calculation of the tolerable touch (contact) voltage:
- Large surface areas of contact, dry conditions, and body impedances not exceeded by 50% of the population.
- Additional resistance for short-term conditions (less than 0.7 seconds), and no additional resistance for long-term conditions (longer than 0.7 seconds).
- Gloves will not be worn.
To achieve protection against electrical shock, different types of regulations might be considered simultaneously with the BS EN 50122-1 standard, depending on the situation and hazards that could occur.
In the following sections of the technical blog, assume the accepted protection measures to protect users of a railway system against electrical shock have been introduced.
2.2 DC Traction System
2.2.1 For Auxiliary Circuits:
As the voltage for auxiliary circuits and equipment is generally ‘’AC ‘’ the same regulation specified in 2.1 is applicable for this case.
2.2.2 For DC Overhead Wiring Structure
2.2.2.1 Electric Shock Concerns
- Electric shock risk due to DC high-voltage touch and step potential rise under fault conditions, regarding remote earth.
- The longer people are present at a certain location, the more possibility of experiencing the transferred voltage which originated from the earthing electrode system.
Therefore, standard BS EN 50122-1, requires the below conditions to be considered as a conservative aspect for the calculation of the tolerable touch (contact) voltage:
- Large surface area of contact, dry conditions, and body impedances not exceeded by 50% of the population.
- Additional resistance for short-term conditions (less than 0.7 seconds), and no additional resistance for long-term conditions (longer than 0.7 seconds).
- Gloves will not be worn.
2.2.2.2 Stray Current Control
Stray currents originating from direct current systems may cause severe material damage. This can be in the form of corrosion like stray current corrosion on buried or immersed metal structures, particularly in long-buried horizontal structures, like pipelines and metal-sheathed cables.
Since corrosion damage can appear after only a short time of exposure to the stray current, it is important to make provisions for protective measures at an early stage and to check the effect of these measures regularly. That is why the corrosion issue also needs to be assessed even in the case of a fault lasting for a short duration.
How the Stray Current is Created
At a typical DC traction system, the voltage called’’ URE’’ (voltage of return circuit) will create a stray current as shown in Figure 4. The “URE’’ itself is established by flowing DC Current in the return circuit.
Figure 4: Principle of interference due to D.C. operated railways
Rail resistance and conductance per length (G’RE) of the return circuit to earth are two main parameters that affect the URE and stray currents.
How the Stray Current causes Corrosion
Where DC stray currents leave the metallic structure through the soil, which is in contact with the structure, an anodic interference occurs. This incident happens because of a positive shift in potential on the structure located in the soil where the DC current leaves the structure and returns to its Power supply. Figure 4 above shows the area exposed to corrosion.
In the case of tunnels with metal-reinforced concrete structures or other conductive structures, it is possible that stray currents can flow into such structures and from there, cause influence on other conductive structures outside the tunnel.
Additionally, if the reinforcement is not longitudinally interconnected, the stray currents flow into the earth via the outer reinforcement of the structure. In areas where the conductance is not homogeneous, concentrated leakage of stray current can arise and lead to corrosion on the outer reinforcement. Figure 5 illustrates this issue.
In this case, the effect of such influence shall be reduced by means of equipotential bonding in the lower part of the individual tunnel sections or other conductive structures to achieve the voltage requirements according to the criteria mentioned in the standard (BS EN 50162:2004).
Figure 5: Demonstration of possible corrosion points of the reinforced rebars
How the Stray Current Value can be Determined
There are two different methods to obtain the amount of stray current as follows:
With measurement and manual calculation
- Measurement of rail resistance according to the method explained in ‘Annex A’, ‘Standard BSEN50122-2′
- Measurement of conductance per length G’RE according to ‘Annex A’, ‘Standard BSEN50122-2’
- Calculation of URE by the formula stated in standard ‘Annex C’, ‘Standard BSEN50122-2’
- Calculating the stray current ‘I’ s‘’ by using the following formula:
With Measurement and Software Calculation
- Measurement of Rail resistance according to the method explained in ‘Annex A’ ‘Standard BSEN50122-2′
- Measurement of conductance per length G’RE according to ‘Annex A’ ‘Standard BSEN50122-2’
- Measurement of the soil resistivity value
- Simulating the return current circuit into adequate software, for example, CDEGS.
- Review the outcome from the software.
What are the Criteria for the Allowable Amount of Stray Currents?
Experience demonstrates that there is no damage in the tracks over a period of 25 years if the average stray current per unit length does not exceed the following value: I’max = 2,5 mA/m (average stray current per length of a single-track line).
What are the Methods to Control the Damage?
1. Control The disturbances from the Interference Source.
To minimise stray current caused by a DC traction system, the traction return current shall be confined to the intended return circuit as far as possible. For this purpose, two main measures are generally taken:
- Insulation of the DC return from the earth
Note: As emphasised, protective provisions against electric shock shall take precedence over provisions against the effects of stray currents. Figure 3 and Figure 6 show a sample installation of the voltage-limiting device in the DC traction system.
This device maintains the isolation between the railway and the earth in normal operation, but in the case of exceeded voltage between the railway and earth, they will function in a way to maintain the contact voltage within an allowable limit.
Figure 6: Sample of Voltage Limiting Device Application.
- Reducing the rail resistance value by using a return conductor to conduct the main part of the return current back to the power supply
2. Control the Effects on the Interfered Structure
Generally, the main method to avoid anodic interference is to limit the area in contact with the soil where the stray current is going to leave the structure. Essentially, this will be achieved by establishing other attractive paths for flowing stray currents. This could be achieved by either a bonding system or using an earth electrode to minimise the circulating stray currents to a confined area.
Figures 7 and 8 illustrate two different bonding practices used to reduce the corrosion effect caused by stray current on the inferred structure.
Figure 7: Mitigation of interference using a unidirectional drainage bond
Figure 8: Mitigation of Interference using a Forced Drainage Bond
3. Electromagnetic Effects
Circulating AC current or changing the level of DC Current will create an electromagnetic field that could affect the system. The below figure demonstrates the electromagnetic field that can affect the circuits.
Respecting the specific wiring regulation will avoid adverse effects of electromagnetic propagation through the power and data circuits connected to the equipment.
To find about the requirements for EMC co-ordination refer to Irish Railway Standard IRS-203-A.
Figure 9: Demonstration of the Electromagnetic Effects
4. Key Takeaways
- Due to different levels of voltages and the different possible accessible points by service users and railway working personnel, the interface between various types of earthing and bonding practices shall be addressed and studied at the early stages of design.
- In the DC Traction system, the return circuit (running railways) is isolated from the earth and does not contribute to protection measures against indirect contact. The insulation between the return circuit and soil is to control the stray current.
- In the AC Traction system, the running rails of A.C. railways are connected to the structure earth consisting of mast foundations, slab track reinforcement and foundations of other wayside structures like tunnels and viaducts. Running rails and the main equipotential busbar of the auxiliary power supply system shall be connected.
- The protective provisions against electric shock shall take precedence over provisions against the effects of stray currents.
- To Achieve protection against direct and indirect contact, different protective measures may need to be considered simultaneously. The following points represent these protection measures.
- Protection by clearance (against direct contact)
- Protection from obstacles (against direct contact)
- Double or reinforced insulation (against direct and indirect contact)
- Protection by automatic disconnection (against indirect contact)
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