The Driver's Safety Device is often abreviated to 'DSD' and as also know commonly referred to as the 'deadman's'.
When running, depression of the throttle controller closes a contact so that current is passed to the solenoid, which is then energised. The lower valve is lifted on its seat so isolating the control valve from atmosphere. With the exhausters running air is extracted from the timing chamber and the underside of the emergency valve diaphragm. There is then vacuum above and below the emergency valve diaphragm and the emergency valve is held on its seat by the spring.
In the case of a DSD application, the solenoid is de-energised and atmospheric pressure above the diaphragm forces the lower valve off its seat. Air is admitted through the gimp filter into the timing chamber, and the lower half of the emergency valve. Air cannot pass directly into the train-pipe because the upper seat of the control valve is covered.
As air pressure builds up under the emergency valve diaphragm, spring pressure is overcome and air is admitted direct into the train pipe through the emergency valve. When the train-pipe vacuum has been destroyed, the emergency valve diaphragm is in balance, with atmospheric pressure above and below. Therefore the valve is closed under the action of the spring so that vacuum can be recreated in the train-pipe. There will be approximately 3" of vacuum showing on the train-pipe gauge due to the action of the spring.
The delay period before the brakes are applied is 5-7 seconds after release of the throttle, and this represents the time taken to build up pressure in the timing chamber and emergency valves.
Should the valve become defective, it can be isolated. This is done by moving the isolating handle to the position indicated.
When the DSD operates the gearboxes return to neutral and the engines to idling speed by de-energising the appropriate EP valves.