The Clash of Currents: EN 50122-3 AC/DC Interaction

Introduction to EN 50122-3

In the fragmented railway map of Europe, trains often cross invisible electrical borders. A line powered by 25kV AC might run parallel to a metro line powered by 750V DC, or a high-speed train might switch from AC to DC at a station. This proximity creates complex electrical hazards. EN 50122-3, titled “Railway applications – Fixed installations – Electrical safety, earthing and the return circuit – Part 3: Mutual interaction of a.c. and d.c. traction systems,” is the safety manual for these “hybrid” zones.

While Part 1 handles general shock protection and Part 2 handles stray current (corrosion), Part 3 focuses specifically on the interference and danger that arises when two different electrical systems share the same physical space or return path (the earth).

Snippet Definition: What is EN 50122-3?

EN 50122-3 is a European standard that specifies the protective measures required when AC and DC electric traction systems interact. This interaction can occur through conductive coupling (shared tracks/ground), inductive coupling (parallel lines), or capacitive coupling. The standard defines limits for touch voltages in these mixed zones and technical solutions to prevent DC stray currents from corroding AC infrastructure (and vice versa).

Types of Interaction

The standard addresses three main scenarios where currents can “leak” or influence the wrong system:

1. Parallel Running (Inductive Coupling)

If a 25kV AC line runs next to a DC line for kilometers, the magnetic field from the AC line can induce dangerous voltages into the DC line’s cables or rails. EN 50122-3 provides calculation methods to ensure these induced voltages do not harm workers touching the “safe” DC rails.

2. Crossing / Shared Stations (Conductive Coupling)

At a border station (e.g., changing from German AC to Dutch DC), the rails are physically connected.

• Risk: DC return current might prefer to flow back through the AC system’s earthing transformers, causing core saturation and overheating.
• Solution: The standard mandates the use of DC Decoupling Devices (capacitors or voltage limiters) in the earthing connections to block DC flow while allowing AC fault currents to pass.

3. Stray Current Corrosion

DC systems are notorious for causing corrosion to buried metal. If an AC system is nearby, its earthing grid can become a victim (an anode) for the DC stray currents. EN 50122-3 sets rules to electrically isolate the AC grounding structures from the DC influence where possible.

The Separation Section (Neutral Zone)

One of the most critical physical components defined is the System Separation Section. This is the dead zone in the overhead line where the pantograph moves from AC to DC.

• Length: Must be longer than the distance between any two pantographs on a train to prevent a 25,000V short circuit into the 1,500V system.
• Earthing: The rails in this transition zone often require special bonding to manage the return currents of both systems simultaneously without creating a high touch voltage.

Comparison: EN 50122-1 vs. EN 50122-3

It is important to distinguish the general rule from the specific case.

Operational Relevance

For interoperable locomotives (multi-system), EN 50122-3 dictates the protection coordination. If a train is running on DC but undergoes a short circuit, the AC protection relays nearby must not trip incorrectly. Conversely, the DC high-speed circuit breakers must be able to handle the complex waveforms that might occur near the transition zone.