Blue Square Electrical Control System
Starter Motor Isolation Relay (Tacho Relay)
This was fitted to Blue Square vehicles, it was a component manufactured by Smiths Instruments and supplied by B.U.T. as part of the control system. One was provided per engine.
Description
From the BUT 50000 Series Service Manual:
The relay, actuated by the engine speed indicator generator, was provided to safeguard the started motor should an attempt be made to operate it while the engine was running.
When the generator reaches a predetermined speed, the relay operates to interrupt the started motor circuit, thus preventing the started motor being operated.
To avoid possible errors in the tripping speed, the relay is mounted on the car in a vertical posistion selected to avoid excessive heat, not exceeding 50 dec. C. (122 deg. F.).
Container
The containing box seen was branded with the BUT logo and identified as a "Smiths Tacho Relay". It is seen here on the underframe of a Class 120 (on the right in this image). The front lid was hinged at the bottom, and released by a butterfly clamp at the top. Some vehicles had the two boxes, one for number one engine and one for number two engine, side by side.
Wiring
The relay had just four wires connecting it to the rest of the controls:
- 35 or 36 RPM, from the engine tacho generator
- 37 RPM Common, from the engine tacho generator
- 52 Local Negative
- 68 Engine Speed, to the Engine Relay Board
35 was number one engine, 36 number two engine. 35/36/37 also went to the "Engine Speed Indicator" (tacho) in the cab to show the engine revs.