Murphy Three Point Speed Relay

The 3ST provides three independent speed relays in one compact unit. It is designed to work with an engine mounted speed transducer, typically a magnetic pickup.The 3ST measures the transducer signal frequency (which is proportional to engine speed) and compares this with three user-set trip levels.
The three non-latching relays then activate or deactivate as appropriate.Each relay circuit has a front facia LED that lights when the relay is energised. On standard units, the relay functions are designated S1 (crank), S2 (underspeed) and S3 (overspeed): all three relays are energised, and all LEDs lit, when the engine is running at normal speed - see diagram overleaf for relay/LED operation. Nominal speed calibration and relay trip levels are set via four multi-turn potentiometers - see ‘calibration’ overleaf. The 3ST also features a ‘meter’ output, which may be used for calibration and/or engine speed indication. The 3ST has a robust, polycarbonate case, designed for DIN rail or surface mounting. Electrical connection is by 12 screw terminals, suitable for stripped panel wires or narrow blade crimps.