Yellow Diamond Electrical Control System
Train Switch
One of these was fitted in each cab and was basically a master control switch which needed to be turned on for the cab controls to work.
On the Derby Lightweights they were behind the driver's seat, on Met-Camm Lightweights they were fitted to the desk.
On blue square vehicles these were replaced by a regular key (FG1) with the switch normally on the desk in front of the driver.
Description
From the BUT A Series Maintenance Manual:
The train switch, mounted in the driver's cab at the rear of the driver's seat or on the front panel of the control table, is of the double pole type and is provided with a detachable key.
This key can only be removed when it is in the "OFF" position.
The function of the train switch is to connect a battery to the train wiring.
Schematic
This shows the connections to the switch, taken from the BUT A Series Maintenance Manual.
Closing of the switch connects Local + and Local - circuits to the Train + and Train - circuits.
The Battery Isolating Switch - the switch on the left in the diagram - needs to be closed first to power the Local circuits.