The Allen-Bradley 849-ZAB321 Pneumatic Timing Relay is a 600 Volts AC rated 849 Series time-delay switch that uses a pneumatic bellows system to delay a device’s contact activation until a user-defined time interval has been completed. This time delay is achieved by a set of mechanical links that join a magnetically activated shaft and the pneumatic bellows to the contacts in the relay, in a way that extends the time it takes for the contact to change status after an electrical coil is activated. When the electrical signal reaches the coil in the Allen-Bradley 849-ZAB321 Pneumatic Timing Relay, it will magnetically attract a metal shaft, and this sets a valve in motion to release the air contained inside the pneumatic bellows. As the bellows empty out, the electrical contacts get closer to each other until their status changes from open to closed, and they start conducting an electrical current.

The Allen-Bradley 849-ZAB321 Pneumatic Timing Relay from the 849 Series has a user-configurable amount of time that it takes to engage the electrical contacts. Its minimum delay time can be set to 0.05 seconds and its maximum delay time can be set to 180 seconds. The electrical contacts of the Allen-Bradley 849-ZAB321 Pneumatic Timing Relay are normally open and they have a spring return system that brings them back to their default position after the electrical coil is de-energized. The delay-type is the on-delay type, meaning that the electrical contacts in the device will not change status immediately after the coil receives an activation signal, but they will wait until all of the air inside the pneumatic bellows is displaced before closing the circuits.