Solid State Block Proving By Axle Counter (Digital)


SSBPAC is a solid state system used for controlling the coordinated movement of the train in the block section working on absolute block working principle. The system has been developed as per RDSO/SPN/175/2005 both for single line working and double line working.

Salient Features:

The SSBPAC (D) is of 2 out of 3 hardware architecture with implementations of the fail safety principles. It complies with the CENELEC standards and conforming to Safety Integrity Level - 4 (SIL-4).

  • The system incorporates auto features and eliminates manual co-operations from other end for Line Clear and Line Close operations.
  • Emergency cancellation at the receiving station.
  • Automatic Train On Line.
  • Automatic Line Close after sequence proved at receiving end and section is free.
  • Simplified Block Back / Block Forward Operations.
  • Cooperative Cancellation.
  • Line Clear Block Key (LCB Key) to prevent automatic granting of Line Clear.
  • Station Master Key (SM Key) to prevent unauthorized panel operations.
  • Station Master Key (SM Key) to prevent unauthorized panel operations.
  • Single Line Working with provisions for shunting with shunt key EKT
  • Event logging operations and data storage in built in the system.
  • Data logger network interface.
  • Communication media: (1 pair cable in quad or on voice channel in OFC.)
  • Block bell and telephone on separate pair of quad or voice channel of OFC.
  • Two repeater relays to repeat input status at the other end.

Safety Features :

  • Safe shutdown if the output is less restrictive.
  • Mine field in the non-program area for trapping the CPU.
  • Inputs and Outputs are exchanged and validated among 3 CPUs in Inter Processor
  • communication before delivering to independent voter module.
  • Continuous monitoring of system cycle time. In case of deviation safe shutdown of system.
  • 2 Bit bi-state control for all the vital output relays.
  • Continuous monitoring of output for every 50ms.
  • Online Hardware diagnosis and self checking.
  • Fault detection and negation failsafe measures.


  • Less maintenance and requires no periodical over-hauling in consideration with the electromechanical systems.
  • It considers signal overlap distance for granting line clear, which is not available in some of the present systems.
  • It has inbuilt data logging capability which logs all the panel operations, events, relays status and other information.
  • All the panel indications are with LEDs.
  • No need for next train on PLC after push back operation.

Solid State Block Proving By Axle Counter Brochure

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