What is the main purpose of EN 50122-1?

The main purpose of EN 50122-1 is to specify the requirements for protecting people against the risk of electric shock from railway fixed installations. It covers both AC and DC systems and details provisions for earthing, bonding, and limiting touch voltages to safe levels during normal and fault conditions.

What is 'touch voltage' in the context of the EN 50122-1 standard?

Touch voltage is the electrical potential a person might be exposed to when simultaneously touching two conductive parts or one conductive part and the ground. EN 50122-1 sets strict limits on permissible touch voltage, with different values for short-term exposure (during a fault) and long-term exposure (normal operation).

Does EN 50122-1 apply to both AC and DC railway systems?

Yes, the standard is applicable to all types of electric railway systems, including both AC (Alternating Current) systems, such as high-speed lines, and DC (Direct Current) systems, commonly used for metros, trams, and third-rail networks.

Why is the return circuit (running rails) so important in this standard?

The return circuit, typically the running rails, is critical because it serves a dual purpose. It is the primary path for the traction current to return to the substation, and it also acts as the main protective conductor. All earthing and bonding for safety are referenced to the return circuit to ensure fault currents have a low-impedance path, allowing protective devices to operate quickly and limit dangerous touch voltages.

Europe: EN 50122-1 Standardizes Rail Electrical Safety

Unlock EN 50122-1: The crucial standard for railway electrical safety. Protect against electric shock with expert insights on earthing and touch voltage limits.

December 15, 2024 2:02 am

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What is EN 50122-1? A Guide to Electrical Safety in Railways

EN 50122-1 is a fundamental European standard that specifies the protective provisions relating to electrical safety in fixed installations for railway applications. Its primary objective is to protect people from the dangers of electric shock, whether arising from normal operation or from fault conditions within the traction system and other associated electrical installations.

This standard is a critical component of railway engineering, providing a framework for designing, constructing, and maintaining safe electrified railway lines. It addresses the complex interactions between high-voltage traction power, the return circuit, earthing systems, and the surrounding environment, ensuring the safety of passengers, railway personnel, and the general public.

Key Principles and Scope of EN 50122-1

The standard establishes a set of harmonized requirements for all types of electric railway systems. Its principles are based on established electrical safety engineering practices, adapted specifically for the unique environment of railway fixed installations.

Scope of Application

EN 50122-1 applies to a broad range of railway systems, including:

New electrified railway lines (AC and DC).
Modifications, renewals, and upgrades of existing electrified lines.
Tramways, metros, and light rail systems.
Both overhead line (catenary) and third-rail conductor systems.

The standard covers all fixed installations associated with the railway, such as substations, tracks, signalling equipment structures, and any conductive parts that could become energized.

Fundamental Safety Objectives

The core of EN 50122-1 is built around two primary levels of protection against electric shock, consistent with international electrical safety standards:

Basic Protection (Protection against direct contact): This involves measures to prevent contact with parts that are intentionally live during normal operation. This is typically achieved through insulation of live parts, or by placing them out of reach using barriers, enclosures, or safe distances.
Fault Protection (Protection against indirect contact): This addresses the danger that arises when an accessible conductive part, which is not normally live, becomes energized due to an insulation failure (a fault). The primary method for fault protection is protective equipotential bonding and automatic disconnection of supply.

In the railway context, this means ensuring that all accessible conductive structures (e.g., station platforms, metal fences, bridges, signal posts) are effectively earthed and bonded to the return circuit to prevent dangerous touch voltages from appearing during a fault.

Technical Provisions for Protection Against Electric Shock

EN 50122-1 details specific technical requirements and limits to achieve its safety objectives. The most critical of these is the management of touch voltage.

Touch Voltage Limits

Touch voltage is the voltage that may appear between two accessible conductive parts, or between a conductive part and the earth, when a person touches them simultaneously. The standard defines acceptable limits for touch voltage, which vary depending on the system type (AC or DC) and the duration of exposure.

The core principle is that the human body can withstand higher voltages for very short periods. Therefore, the standard sets different limits for long-term (permanent state) and short-term (fault conditions) exposure.

Comparison of Permissible Touch Voltage Limits

The following table summarizes the generally accepted permissible touch voltage limits as defined within the standard for different conditions. These values are crucial for system designers when calculating fault currents and designing protective earthing systems.

System Type	Exposure Condition	Duration	Permissible Touch Voltage Limit
AC Systems (16.7 Hz & 50/60 Hz)	Normal Operation / Long Term	> 300 s	60 V AC
AC Systems (16.7 Hz & 50/60 Hz)	Fault Condition / Short Term	< 0.2 s	630 V AC (Typical value, can vary)
DC Systems	Normal Operation / Long Term	> 300 s	120 V DC
DC Systems	Fault Condition / Short Term	< 0.1 s	620 V DC (Typical value, can vary)

Note: The short-term limits are derived from curves in the standard and depend on the precise fault clearance time. The values above are representative examples.

Earthing and the Return Circuit

A unique aspect of railway electrical systems is the dual role of the running rails. EN 50122-1 places significant emphasis on the design and management of the return circuit.

As a Power Conductor: The primary function of the rails is to act as the return conductor for the traction current, completing the circuit from the train back to the traction substation.
As a Protective Conductor: The rails also form the backbone of the protective earthing system. In a fault scenario (e.g., a catenary wire falling and touching a metal structure), the rails provide a low-impedance path for the fault current to flow back to the substation, which causes protective devices (circuit breakers) to operate quickly and disconnect the power.

All conductive parts within a defined zone around the track must be bonded to this return circuit to create an equipotential zone. This ensures that even during a fault, there is no significant voltage difference between any two parts a person could touch, thus keeping touch voltages within the safe limits defined by the standard.

Conclusion: The Importance of EN 50122-1

EN 50122-1 is more than just a regulatory document; it is a comprehensive safety framework that underpins the design of all modern electrified railway systems in Europe and beyond. By providing clear, technically robust guidelines on earthing, bonding, and protection against electric shock, it ensures a high and consistent level of safety for everyone interacting with the railway environment. Adherence to this standard is non-negotiable for achieving a reliable, interoperable, and, most importantly, a safe railway network.

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