The Brain of Braking: EN 15355 Distributor Valves

Introduction to EN 15355

In a pneumatic railway brake system, the driver does not send air directly to the brake cylinders of every wagon—that would be too slow. Instead, the driver lowers the pressure in the main Brake Pipe (BP). On every single wagon, a complex device called the Distributor Valve detects this drop and creates a proportional flow of air from a local tank to the brake pads.

EN 15355, titled “Railway applications – Braking – Distributor valves and distributor-isolating devices,” is the standard that governs this critical component. It ensures that the valve is smart enough to distinguish between a dangerous leak (brake!) and a minor pressure fluctuation (ignore), and that it applies the brakes smoothly across a 700-meter long train.

Snippet Definition: What is EN 15355?

EN 15355 is a European standard specifying the functional and performance requirements for Distributor Valves and their isolating devices. It defines the valve’s response to pressure changes (sensitivity/insensitivity), the maximum brake cylinder pressure, the graduation of application and release, and the required filling times for reservoirs. It is the modern successor to the classic UIC 540 leaflet.

The Logic of the Distributor

EN 15355 dictates how the valve must manage four key pressures to maintain safety.

1. The Three-Pressure Principle

The valve operates by comparing pressures:

• Brake Pipe (BP): The command signal from the driver (running at 5.0 bar).
• Control Reservoir (CR): The reference pressure (stored memory of 5.0 bar).
• Brake Cylinder (BC): The output pressure pushing the brake blocks.

When BP drops below CR, the valve opens to send air to BC. EN 15355 defines exactly how much BC pressure must be generated for every 0.1 bar drop in BP.

2. Sensitivity and Insensitivity

The standard is extremely strict on “Insensitivity.” The valve must not apply the brakes if the Brake Pipe pressure drops slowly (e.g., due to natural leakage of less than 0.3 bar in 60 seconds). However, it must react instantly (“Sensitivity”) if the pressure drops sharply (indicating a brake command). This prevents “stuck brakes” caused by minor air leaks.

Operational Modes (P and G)

Most freight distributors typically have a changeover lever for “Passenger” (P) and “Goods” (G) modes. EN 15355 defines the timing for these:

• P Mode (Passenger): Fast application and release. Used for shorter, faster trains.
• G Mode (Goods): Slow application and release. Used for long heavy freight trains to ensure the brakes apply evenly from the front to the rear, preventing the wagons from crashing into each other (buffing shocks).

Comparison: EN 15355 vs. UIC 540

While they cover the same device, the EN standard introduces modern rigors.

The Isolating Device

Sometimes a brake jams or fails. EN 15355 specifies the requirements for the “Isolating Cock” (usually a red handle on the side of the wagon).

• Function: When turned, it must cut the connection between the Brake Pipe and the Distributor, AND vent the Brake Cylinder to release the brakes on that specific wagon.
• Safety: It must be visually clear whether the brake is active (handle vertical) or isolated (handle horizontal/angled).