scope:

GENERAL

Within the Solid State Interlocking system, the interlocking is directly linked to electronic trackside interface equipment by means of a baseband data highway. Information is transmitted over the baseband data highway (the data link) in the form of discrete messages of fixed format, at 10 kbaud with Manchester 2 coding. Message packets are of length 3.9ms, at intervals of 4ms to 30ms, measured from the start of the packets. The data link signal amplitude is 5v peak-peak maximum, and the cable nominal characteristic impedance is 100 Ohms.

In a.c. electrified areas, SSI communication cables running parallel to the track are subject to transversely induced voltage. The level of voltage induced is proportional to the length of parallelism and proximity to the interfering source. The C.C.I.T.T. place a maximum limit on the levels that may be induced in order to protect personnel. These limits are

a) 60 v r.m.s.under normal running conditions

b) 430 V r.m.s under traction fault conditions

In order to satisfy these limits on the data Link of an SSI scheme, it is necessary to insert isolating transformers periodically to provide a rejection of induced common mode interference.

The maximum length of a data link section without data link repeaters is 10 km, and it is anticipated such a link could require a maximum of 5 isolating transformer.

The design and construction should, where appropriate, conform to BR 924A.

The transformer will be used for isolating sections for able to BR 1932. the data communications carried by this cable should not be affected by the maximum normal level of induced voltage under traction fault conditions.

Manufactures shall ensure that the equipment complies with relevant parts of the Health and Safety at Work Act, 1974.

All units supplied to this Specification shall have a minimum service life of 25 years.