brucechase
Member
Almost NEVER use them.
Like other, I have over 125 Ab SLC PLC systems installed here since the late 1990s with thousands of I/O. The only time I've used relays in on drives as the start signal.
All motor starters on 120VAC card (going to MCCs). All other outputs are 24VDC going to solenoid valves, lights, and alarms. I've replaced very few cards in these systems (most being input cards).
Using a relay on every output adds wiring, wasted space, additional failure points, and a substatial cost.
Like other, I have over 125 Ab SLC PLC systems installed here since the late 1990s with thousands of I/O. The only time I've used relays in on drives as the start signal.
All motor starters on 120VAC card (going to MCCs). All other outputs are 24VDC going to solenoid valves, lights, and alarms. I've replaced very few cards in these systems (most being input cards).
Using a relay on every output adds wiring, wasted space, additional failure points, and a substatial cost.