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.
