As you can see, this is by far the costliest system. And yet, with foresight, I might have saved a lot in change orders. We incurred two days of design time and three days of wiring time for the first contractor-requested change. The second manufacturer-fault-driven change cost us two days design time and two days of wiring time. The cost of this change time could easily run up to $8,000. If I’d used a safety PLC and safety I/O in the first place, a contractor-requested change might need one day of extra programming -- at about $1,000.
What’s the final score? I spent more than $6,350 in hardware and $8,000 in change time with dedicated, modular safety relays. I could have spent $11,700 for safety PLC and change time. I assume you're thinking that the manufacturer defect could not have been predicted, so the change time should not be counted against the modular relay route. That's a fair assumption, so we’ll price out the modular relay approach at $10,350 versus the $11,700 for the safety PLC.
One more change to the safety design, and the safety PLC looks rather attractive, doesn’t it? I plan to push harder for the safety PLC on the next industrial automation project. I might lose the gamble, because no changes will be necessary, but it is rather nice to be able to make changes in software rather than hardware. This compares quite well with the argument for hand drafting versus computer-aided drafting. The initial drafting work takes about the same amount of time regardless of tool, but as soon as a change is required, software wins every time.
Is the cost of this safety worth it? Tell us what you think in the comments section below.