scope:

This specification relates to design requirements for a miniature tractive armature slow release line relay of the plug-in type for use in ll0v signalling circuits, incorporating a rectifier feed unit, and suitable for use with S.L.34 or S.L.35 lamps to B.S. 469. It is intended to be used without an external resistor and with a lamp circuit having characteristics such that under extreme variations of filament current consumption and supply voltage the line currents will meet the operating values specified in Clause 18.2.

The design is based on the requirement that the relay shall remain in the operated position long enough to cover the contact transit time of the signal control relay.