Grounded in Safety: Decoding EN 50122-1 and Railway Earthing

What is EN 50122-1?

EN 50122-1 is the European Standard titled “Railway applications – Fixed installations – Electrical safety, earthing and the return circuit – Part 1: Protective provisions against electric shock.” It is the primary regulation governing how railway infrastructure (stations, bridges, masts, and fences) must be grounded to prevent electrocution.

In electric traction systems, the rails act as the return conductor for the current. This causes the “Rail Potential” to rise relative to the earth. EN 50122-1 defines the limits for Touch Voltages—the voltage difference a person can bridge between a conductive structure and the ground—to ensure that passengers and staff are not harmed during normal operations or short-circuit conditions.

Permissible Touch Voltage ($U_{tp}$)

The standard does not set a single safe voltage limit (like 50V) but rather uses a time-dependent curve. The human body can withstand higher voltages for very short durations but only low voltages for longer periods.

• Long Duration (> 300s): The permissible touch voltage is typically limited to 60V DC or 60V AC.
• Short Duration (< 0.1s): During a short-circuit fault, higher voltages (e.g., hundreds of volts) are permissible because the protective breakers will trip almost instantly, limiting the exposure time.

The Conflict: Safety vs. Stray Current

A major challenge in DC railways is balancing safety (EN 50122-1) with corrosion protection (EN 50122-2).

For Safety (Part 1): You want to connect everything to the rails/earth to prevent shock.
For Corrosion (Part 2): You want to isolate the rails from the earth to prevent stray currents from eating away at buried pipes and rebar.

To solve this, EN 50122-1 introduces the use of Voltage Limiting Devices (VLDs). These devices keep the structure isolated (Open Traction Earth) under normal conditions but short-circuit to the ground/rail (Closed Traction Earth) if the voltage rises to dangerous levels, instantly bonding the system for safety.

Comparison: Earthing Concepts