The Solid-State Interlocking (SSI) Data Link Isolating Transfer (DLIT) is used to isolate sections of the SSI data link, in order to limit levels of induced common mode interference.
Electrified railways are well known to present various issues and problems regarding Electro Magnetic Interference (EMI). In 25 kV AC railway electrified areas, it has been best practice to use the traction return earths rather than separate earth systems. The effect of 50 Hz interference can degrade the components of the surge protection units (SPUs) installed to protect the SSI data link, due to constant stress, thus rendering them less effective as protection devices, and cause potential problems with data corruption.
The current practice is to install DLIT’s to restrict common-mode voltages and reduce the propagation of transients. Where older installations do not have these devices, retrospective upgrades should be considered. A word of warning is to check the current operation of each data link against the original design and ensure the installation of each DLIT does not exacerbate any existing design or maintenance defects. It should also be noted that in longer base band data links where repeaters are used, each DLIT added reduces the maximum permitted length of the data link.
The DLIT must be used in accordance with Network Rail SSI8503, Issue 04, SSI Applications Manual, Earthing and Bonding of Solid-State Interlocking Equipment.
The Network Rail Certificate of Acceptance reference for the DLIT is PA05/02963.
The Network Rail catalogue number for the DLIT is 0086/069367 and for the DLIT Housing, 0086/069368.
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