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.
The purchase price is declining, and safety PLC options are even more attractive at installation because the long cable home runs get decreased. Instead of having all safety device cables return to the central location where the safety relays reside, the devices can land locally at distributed Ethernet IO locations which communicate via regular Ethernet cable. The Ethernet IO cable already must be pulled back to the central PLC; this gives a significant cost savings for cabling and installation time.
The drawback to SafetyPLC option is the complexity for post-acceptance troubleshooting. The skill level required to maintain and troubleshoot such a system is much higher. Trouble-shooting a system using regular safety relays can be much simpler.
Interesting piece, TJ. With safety becoming more an integral part of the integrated architecture/product line-ups for many automation control vendors, it will be interesting to see how the pricing trends play out over the next few years. Would expect that the premium will be heading downward but it's impossible to know how fast.
Beth, I have to write a correction here. I realize I neglected to SUBTRACT the cost of the standard PLC from my safety PLC figure (the safety PLC performs both the safety function and the regular PLC function in one unit and thus replaces the standard PLC).
This would adjust the total safety PLC cost down to around $7000. The safety PLC decision becomes even more attractive for the next system.
I love this real-world example of how one simple design decision and component change can have real impact on the cost of the BOM and the ultimate performance of the product. This is a classic example of what happens every day in design iteration. It really shines a spotlight on how a more holistic view and big-picture thinking can really have impact on design results not to mention, costs. Thanks for sharing, TJ.
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