scope:

Purpose

The purpose of this specification is to define the performance requirements for position light signals used either independently, as Limit of Shunt signals, or as subsidiary signals to main signals.

Background

Earlier specifications for signals laid down requirements for components which, when assembled, were deemed to comprise an adequate position light signal. These specifications laid down no performance requirements and were specifically related to the use of filament lamps.

This is a performance specification which does not prescribe the source of illumination or the means by which the light from that source is presented to the driver.

Overview of Requirement

Note that the functional elements which are identified need not be physically separate entities.

The signal must produce an unambiguous output to the driver.

The signal output to the driver must not indicate a less restrictive aspect than required by the control system.

The principal functional elements of the signal are as follows:

• A means of converting electrical energy to optical energy.

• A means of focusing and filtering the optical energy so that it is of the colour and concentrated into a beam appropriate to the type of signal.

• A means of masking so that the shape of the light aperture is appropriate to the type of signal.

• Means of terminating and securing the cable connecting the signal to the controlling devices.

• A housing for the components described above to provide environmental protection and maintain the correct spatial relationship between them.

• A means of mounting the signal head on a support structure; the support structure itself not being the subject of this Requirement Specification.

Other Specifications to be Considered

The Group Standard for Lineside Signals and Indicators ^[1]0, which relates to the existing signal, apply to the signal specified in this document. A list of referenced documents is given at Section 14.

Safety Requirements

Throughout this document, Functional Safety Requirements are identified by an (S) at the end of the requirement statement.