The answer is straightforward. To ensure safety compliance in our designs, there occasionally is a breaking point where single module safety relays become more expensive and harder to use than a programmable safety controller/PLC. This is the most obvious reason based on simple economics, but there are other reasons:
Density – There are more I/O configurations within the same footprint than just stand-alone safety relays.
Flexibility – The user configures the I/O and the functionality to the design. In the picture, B&D Technologies chose a controller to integrate into existing equipment with the needed flexibility to complete all safety tasks. If we were to hard-wire this design, the cost would have been greater, but more importantly, a great deal of re-wiring would have happened based on required functionality changes.
Advanced Features – The ability to communicate with a standard PLC is available on programmable safety controls and but not safety relays.
The pictured application required interlocks to four banks of motors broken into various combinations of shut-down, from four specific zones. The zones are protected by three Banner light curtains and an ABB safety mat. When the machine was idle, the zones where muted. If the machine was running, the safety system would shut down operations of the programmed devices required only by that zone, allowing the rest of the machine (running safely) to operate.
The beauty of the Banner modular controller is that it has a very simple and intuitive programming software tool that allows users (not PLC savvy) to program and troubleshoot their system without advanced support. Other safety controllers/PLC s require advanced PLC knowledge to get the same results.
Our general rule dictates that if the application requires three or more safety relays, then we go to the safety controller. Questions? Please feel free to contact us at Automation@bdindustrial.com
C. Tolbert DOE