Along the railway line, between substations and signal boxes, there are hundreds of cabinets and containers that operate without local supervision: automatic block signalling huts beside signals, automatic level-crossing warning-system containers, transformer points of the non-traction auxiliary power line, disconnector-drive and remote-control boxes, and point-heating supply cabinets. They are supplied from substations by the 15, 20 or 6 kV non-traction auxiliary power line, stepped down at transformer points to 0.4 kV, while SBL automatic block and SSP level-crossing equipment operates at 230 V.
A common feature of these facilities is the requirement for fail-safe behaviour during faults and loss of power: a track circuit indicates occupancy whenever an abnormal condition occurs, a de-energized track relay means “occupied”, level-crossing barrier drives close automatically after a power failure, and opening an SSP cabinet is reported to a staffed control post. These installations must also cope with an outdoor environment without air conditioning, proximity to the 3 kV DC overhead contact system with its overvoltages and stray currents, and track circuits carrying signalling voltages of approximately 2–12 V alongside power circuits.
Equipment used in this area includes: power supplies and DC/DC converters, uninterruptible power supplies and buffer modules, surge protection devices for power and signalling circuits, DIN-rail circuit breakers with tripping characteristics independent of ambient temperature, interface and signalling relays, solid-state relays for point heaters, terminal blocks, fibre-optic boxes, wall-mounted and pole-mounted enclosures, and outdoor cabinets.
Selection criteria for this area:
From the non-traction auxiliary line (LPN) at 15, 20 or 6 kV, which runs along the route from a traction substation; at transformer points it is stepped down to 0.4 kV. The LPN supplies stations, stops, signals, automatic block cabinets, level-crossing barriers and point heating. It has overcurrent, undervoltage and earth-fault protection, plus backup supply from the neighbouring substation.
A transformer in the cabinet reduces 230 V to about 10 V, and a track transformer further reduces it to about 2 V applied to the rails of an insulated section; the signal returns through the rails to a relay transformer (about 12 V) and the track relay. A vehicle axle shunts the rails, the relay drops out and indicates occupancy. Any abnormal condition - an open circuit, short circuit or loss of power - produces the same “occupied” state, so a failure cannot result in a permissive signal.
The warning lasts at least 30 s (two half-barriers or category C) or 46 s (four half-barriers); barriers must not start lowering earlier than 13 s after the lights are activated, must close within 10 s, and at least 7 s must elapse between full closure and the train's arrival. Total closure time should not exceed 2 minutes.
Drives equipped with an automatic release mechanism close the barriers automatically after loss of power; the barriers can also be lowered manually after the drive is released. The “closed” state is the safe state, so the control system must not require energy to reach it.