Blue Square Electrical Control System
Engine Isolating Switch
One was provided per engine located on the underframe. The outside was normally painted red.
Description
From the BUT 50000 Series Service Manual:
This switch is one of the tumbler type and is operated by a standard railway carriage key when for any reason it becomes necessary to isolate and engine.
Each engine is provided with a switch, which is waterproofed and requires no maintenance.
Schematic
This shows the connections to the switch, taken from the BUT 50000 Series Service Manual.
It is a simple on/off switch in the complicated path of connections that leads to the operation of the starter for the associated engine, and is located between the "Engine Control Relay Panel" and the "Fire Alarm Control Panel".
The two wires for number one engine (illustrated) are 62 and 101, for the number two engine they are 63 and 104.