Yellow Diamond Electrical Control System
Battery Isolating Switch
One was provided per power car located on the underframe, fitted inside the control unit box.
The images below show them inside the early (first image) and (second) later styles of boxes.
Description
From the BUT A Series Maintenance Manual:
This is a two-pole switch that, when open, isolates the battery from the remainder of the vehicle electrical equipment.
Note — This is NOT a circuit-breaker, therefore all other switches on the car should be opened before this is operated.
Schematic
This shows the connections to the switch, taken from the BUT A Series Maintenance Manual. The BIS is highlighted in red.
Closing of the switch connects the battery to the Local + and Local - circuits. Some substantial items such as the starter motors are fed direct from the these.
Most other items are fed via the Train + and Train - circuits. These are activated through the other switch on the right which is the train switch.