Ludgate Cellars PLC ‘Hot Standby’ Suite System – Thameslink – L. C. Switchgear

Project Completed: 16 Apr 2024
Customer: Network Rail

The PLC system has been developed to comply with the Network Rail operations diagram that detailed four suites. The specific requirements are given in N244 Procurement Specification 22. Ludgate Cellars Contactor Control system Functional Requirements version 2.0 The key requirements are

  • That each suite has a PLC.
  • That if as a result of a PLC failure it shall be possible to changeover to off line and standby suite shall continue normal operation.
  • The control of each track Northbound and Southbound shall be fully independent.
  • The system be 100% redundancy referred to as bomb proof or as near as possible.
  • Each suite shall have a local display and shall be capable of indicating the status of the contactor suite in the other half of the substation.
This uses a total of 4 PLCs. Two for each direction of traffic. (A and B section of the substation) Each PLC has its own HMI (screen), inputs and outputs. Each pair of PLCs is connected using the various data cables shown. These are fibre optic in some cases which reduce the risk of interference or noise from unclean earth connections. The main interconnecting optic cables are routed differently within the substation to minimise the risk of damage to both. Each pair of PLCs is synchronized so that they act as one master and standby and give full redundancy in the event of a single failure. This gives the ability for them to change from master to standby very quickly. The PLC takes track circuit signals to determine which circuit is made and communicate with the SCADA system which controls the main power devices in the system.


  • Steel enclosure painted RAL 7035 grey
  • Environmental rating IP54
  • Cable entry arranged through the top


  • Master / hot stand-by PLC
  • 19" colour touch screen
  • Digital input and output modules
  • SCADA interface
  • SCADA controlled latching relay
  • 48V to 24V DC - DC power supply

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