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IEC 62128: Railway Installations — Electrical Safety, Earthing and Return Circuit
IEC 62128Railway EarthingStray CurrentTraction Safety
IEC 62128 is the leading international standard for electrical safety, earthing, and return circuits in fixed railway installations. Published in three parts — protective provisions against electric shock (Part 1), stray current protection (Part 2), and interaction between AC and DC systems (Part 3) — this standard provides the complete engineering framework for ensuring the safety of passengers, staff, and the public in the vicinity of electrified railways, while also protecting the railway infrastructure and adjacent third-party assets from electrolytic corrosion.
Engineering Insight: The most challenging design aspect addressed by IEC 62128 is the inherent conflict between traction return current requirements (low-impedance path to minimize voltage drops) and stray current mitigation (high-impedance path to prevent leakage into earth). Resolving this conflict requires careful optimization of rail-to-earth resistance — typically targeting 2-10 Ω·km for DC systems — combined with impedance bonds that pass traction current while blocking signalling track circuit frequencies.
1. Protective Provisions Against Electric Shock (Part 1)
IEC 62128-1, updated in 2023, defines the requirements for protecting people from electric shock in both AC and DC traction systems. It establishes permissible touch voltage and step voltage limits based on fault duration and sets out the earching architecture for the entire railway installation.
1.1 Touch and Step Voltage Limits
The standard defines maximum permissible touch voltages (UTp) and step voltages (USp) as a function of fault clearing time, following the IEC 60479-1 curves for ventricular fibrillation. For AC systems at 50/60 Hz with fault clearance within 0.1 s, the permissible touch voltage is 650 V, while for DC systems it is 700 V. For longer fault durations (up to several seconds), these limits drop significantly to 75 V AC and 90 V DC, reflecting the increased physiological risk with prolonged exposure.
1.2 Earthing Architectures
The standard defines four earthing principles for railway installations: direct earthing (TN-like), uninsulated return, diode earthing, and unearthing (IT-like). The choice depends on the traction system type (AC vs. DC), the signalling system compatibility, and the stray current sensitivity of the surrounding environment. For modern DC metro systems with concrete-embedded track slabs, diode earthing with a controlled rail-to-earth voltage threshold of typically +50 V/-100 V is the most common configuration.
2. Stray Current Protection (Part 2)
| Protection Measure | Technical Implementation | Typical Design Target | Verification Method |
|---|---|---|---|
| Rail-to-earth resistance | Insulated rail fasteners, resilient pads, ballast mat | > 2 Ω·km (DC), > 0.5 Ω·km (AC) | DC insulated resistance measurement |
| Stray current collection | Structural reinforcement bonded collector mesh | < 5% of return current leakage | Current mapping with polarization cells |
| Return conductor cross-bonding | Periodic cross-bonds every 300-500 m | Rail potential < 100 V at any point | Longitudinal voltage drop measurement |
| Negative return cables | Copper cables paralleling the running rails | ≥ 50% of rail cross-section equivalent | Current sharing ratio measurement |
| Polarization drain | Diode-grounded devices at 50-100 V threshold | Threshold ±50 V to ±100 V | Conduction voltage verification |
| Corrosion monitoring | Permanently installed reference electrodes | Structure-to-electrolyte potential > -850 mV (Cu/CuSO4) | Half-cell potential survey every 6 months |
Critical Design Parameter: Stray current corrosion follows Faraday’s law: 1 A of DC leakage current flowing for one year electrolytically dissolves approximately 9.1 kg of steel or 10.4 kg of copper. A typical metro system with 5 A/km of stray current leakage will dissolve 45 kg of steel per km of track per year — potentially compromising buried utility pipes and structural reinforcement within a decade if unmitigated.
3. AC-DC System Interaction (Part 3)
IEC 62128-3 addresses the complex interactions that occur when AC and DC traction systems share corridors or earthing infrastructure — increasingly common in multi-system railway stations and depots where 25 kV AC mainline trains and 750 V/1.5 kV DC metro trains operate in proximity.
3.1 Harmonic and Transient Coupling
The standard identifies three primary coupling mechanisms: conductive coupling through shared earthing conductors, inductive coupling through parallel track alignment, and capacitive coupling in station environments. For each mechanism, IEC 62128-3 specifies maximum permissible interference levels and mitigation requirements including separation distances, screening conductor placement, and harmonic filter design for traction substations.
3.2 Hybrid Earthing System Design
A key contribution of Part 3 is its guidance on hybrid earthing systems that must simultaneously satisfy the requirements of both AC and DC installations. This includes the specification of DC-blocking devices (such as E-Boosters or polarization cells) that prevent DC stray current from flowing through AC earthing conductors while still providing a low-impedance path for AC fault currents. The standard mandates that such devices be rated for the full prospective fault current and include monitoring for bypass conduction.
Innovative Approach: Modern “E-Booster” systems — voltage-controlled switching devices that actively manage rail potential — are gaining adoption as an alternative to traditional diode earthing. By dynamically adjusting the rail-to-earth connection impedance, they can maintain rail potential within ±30 V under normal operation while providing a low-impedance path during faults, greatly reducing stray current leakage compared to fixed-threshold diode schemes.
4. Engineering Applications and Compliance
IEC 62128 is mandatory for all new railway electrification projects in countries that adopt IEC standards and is widely referenced in procurement specifications for metro, light rail, and mainline railway projects worldwide:
- Urban metro systems (600 V DC, 750 V DC, 1.5 kV DC)
- Light rail and streetcar networks (600-750 V DC)
- Mainline railways (25 kV AC, 15 kV AC, 3 kV DC)
- Mixed-traffic corridors with both AC and DC traction
- Railway stations and depots with multiple traction systems
Compliance Note: IEC 62128:2023 (Part 1) introduced more stringent touch voltage limits for maintenance staff working in wet conditions — reducing the acceptable touch voltage from 75 V to 50 V for fault durations exceeding 0.2 s. Existing installations have a 5-year transition period for retrofitting protective bonding and insulation measures.
5. Frequently Asked Questions
Q: What is the difference between IEC 62128 and EN 50122?
A: EN 50122 is the European standard that is technically identical to IEC 62128. The two standards are maintained in parallel through the IEC-CENELEC Frankfurt Agreement. Since 2023, EN 50122 refers directly to IEC 62128 without additional national deviations for most European countries.
A: EN 50122 is the European standard that is technically identical to IEC 62128. The two standards are maintained in parallel through the IEC-CENELEC Frankfurt Agreement. Since 2023, EN 50122 refers directly to IEC 62128 without additional national deviations for most European countries.
Q: How is the rail-to-earth resistance measured in practice?
A: The standard specifies the DC insulation resistance measurement method using a high-voltage insulation tester (500 V or 1000 V) applied between the running rail and a reference earth electrode. Measurements are taken under dry conditions and corrected to a standard temperature of 20°C. For continuous monitoring, some systems employ permanently installed rail potential monitoring devices that calculate leakage resistance from measured voltage and current.
A: The standard specifies the DC insulation resistance measurement method using a high-voltage insulation tester (500 V or 1000 V) applied between the running rail and a reference earth electrode. Measurements are taken under dry conditions and corrected to a standard temperature of 20°C. For continuous monitoring, some systems employ permanently installed rail potential monitoring devices that calculate leakage resistance from measured voltage and current.
Q: What is a polarization cell and how does it work?
A: A polarization cell (also called a drainage cell or spark gap) is a device consisting of stacked zinc or copper plates immersed in an electrolyte, connected between the return circuit and earth. It presents a high impedance to small DC voltages (preventing stray current flow) but breaks down at higher voltages (typically 50-100 V) to provide a low-impedance path for fault currents and lightning surges. Modern units use solid-state thyristor equivalents.
A: A polarization cell (also called a drainage cell or spark gap) is a device consisting of stacked zinc or copper plates immersed in an electrolyte, connected between the return circuit and earth. It presents a high impedance to small DC voltages (preventing stray current flow) but breaks down at higher voltages (typically 50-100 V) to provide a low-impedance path for fault currents and lightning surges. Modern units use solid-state thyristor equivalents.
Q: Can IEC 62128 be applied to non-railway DC systems?
A: While specifically developed for railways, the stray current protection principles in Part 2 are applicable to any DC power distribution system with earth return — including DC microgrids, electrolysis plants, and cathodic protection systems for pipelines. The touch voltage requirements in Part 1 are derived from generic IEC 60479-1 shock physiology data and can be referenced for any low-voltage DC installation where human access is possible.
A: While specifically developed for railways, the stray current protection principles in Part 2 are applicable to any DC power distribution system with earth return — including DC microgrids, electrolysis plants, and cathodic protection systems for pipelines. The touch voltage requirements in Part 1 are derived from generic IEC 60479-1 shock physiology data and can be referenced for any low-voltage DC installation where human access is possible.
