The Safety Loop in Railway Vehicles
An old string of fairy lights, bulbs wired in series. One bulb fails, and the whole string goes dark. The same principle sits behind a core safety concept in rail vehicles: the safety loop, known in German as the Grünschleife (green loop). Before a train pulls away, several safety-relevant functions have to report their status reliably. Are all doors locked? Are the brakes in the correct state? Only when every signal is right may the train move. The safety loop brings those signals together.
What the safety loop is and how it works
The safety loop connects the safety-relevant subsystems of a train into a single series circuit. Each subsystem, a door lock or a brake function for example, feeds its status signal directly, or via a downstream system, into the loop.
As long as every subsystem reports the safe state, the loop is continuous. The signal is present, the train receives traction release. If a subsystem leaves that state, a door fails to lock, a brake reports a fault, the loop breaks at that point. The signal drops out, and so does the release.
The loop works on the closed current principle: an interruption always forces the safe state. That simplicity is the point. The less logic sits between subsystem status and traction release, the less can fail unnoticed.
Why forcibly guided contacts to IEC 61810-3
The weak point of any series circuit is a contact that lies. A welded normally open contact reports "closed" even though it is mechanically stuck. In a safety loop, that would mean an unsafe state, an open door or a faulty brake signal, is reported as safe. The train moves.
Relays with forcibly guided contacts according to IEC 61810-3 prevent precisely this case. The normally open and normally closed contacts are mechanically linked and can never be closed at the same time, not even with a welded contact. The actual contact position can therefore be read back and evaluated safely. A stuck contact is detected before it becomes a hazard. This is the basis for a high diagnostic coverage in the safety loop.
EN 50155 requirements in daily railway service
Components in rail vehicles operate under conditions rarely seen on a factory floor. EN 50155 defines them. Relays for the safety loop have to meet them reliably:
- temperature from -40 °C to +85 °C
- supply voltage from -30% to +25%
- high resistance to shock and vibration
- up to 95% relative humidity
There is also the time axis. Rail vehicles run for up to 30 years. A relay in the safety loop should survive that service life and remain available as a spare part for decades.
ELESTA relays in the safety loop
ELESTA develops and manufactures relays with forcibly guided contacts in Switzerland for exactly these demands. The SIF, SIR and SIS series comply with the requirements of EN 50155. The SIR series switches loads up to 10 A and is available in 4- to 10-pole variants, configurable to customer specification.
Across the portfolio the relays cover a broad field: 2 to 10 contacts, switching currents from 3 mA to 16 A, coil voltages from 3 VDC to 220 VDC. They serve as components in the safety control systems of railway technology, from door locking through the brake function to interlocking logic.
Read more about how our relays for railway technology perform in service, and about choosing the right series for your application. For specific project questions, our specialists are available directly.