scope:

Introduction

A requirement may arise for a relay interface to be added between an SSI-controlled signal and its controlling Signal Module. This may typically arise from:

• the repositioning of an existing SSI-controlled signal, or

• the position of the signal, typically in a tunnel or on a viaduct, causing difficulties, or

• the signal being in a fringe area.

However, relay-interfaced signals should be avoided where practical.

Note that this document does not cover the type of installation where a relay interlocking has been replaced by one or more SSIs and the external circuits from the relay interlocking to the trackside equipment have been retained. This subject is covered by SSI8507.