The in-service reliability performance of the SSI Long Distance Terminal (LDT) design has never met expectations, with the module failures often manifesting themselves as shutdowns of the LDT causing it to cease communication. The LDT is not fitted with a fuse (as is the case for the SSI trackside modules) and thus can be reset by the maintenance technician by powering on. This combined with the fact that the LDT forms part of a redundant network, i.e. duplicated for availability, means that most railway operators have tolerated the problem.
The addition of an external filter designed and manufactured by Park Signalling, significantly improves performance of the system. The LDT LF01 Filter fits in series with the SSI LDT and its associated external wiring.
The Mean Time Between Failure (MTBF), calculated using MIL-HDBK-217, is 31.5 Years. The module is of robust construction and is suitable for attachment to its associated LDT Module housed either in a Network Rail standard pattern SSI Equipment location case or SSI Interlocking Cubicle.
The LDT LF01 Filter has been designed to meet the requirements of BR 967 Category D (the same requirements as the SSI LDT Module). All electrical connections are made via 75-way CANNON Trident connectors. Pin allocation for the connector is defined in Appendix One of specification BR1906 (September 1989). Note that the N12/B12 battery connections are not through connected in the LDT LF01 Filter.
The LDT LF02 Filter has been developed in response to a problem found at some installations where fitting the LDT LF01 Filter caused missing telegrams on the SSI data link.
· The LDT LF02 Filter is for use with the SSI Long Distance Terminal Type SS 2501 only and for installations using current loop 1.
· The LDT LF02 Filter shall be used for all new installations requiring only current loop 1.
· The LDT LF01 Filter shall be used for all new installations that require more than one current loop.
To watch a video on the LDTF02, click here.
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