Wow, the logic of safety is getting expensive, isn’t it? Never mind the rework charge for taking out the single-function master relay to put in these modular relays -- this is just for the logic hardware.
Then came an unwelcome surprise. I wrote previously here about the ethics of selling safety components with known defects; I could not let this problem remain for the end user. Thus arose the third physical iteration. All the modular units had to be replaced with a new bill of materials comprising the manufacturer’s next-generation devices. The BOM included:
- 7x Single Input/Single Dual-Channel Output Relays
- 7x Dual Input/Single Dual-Channel Output Relays
- 2x 4-Channel Expansion Relays
The cost of this third BOM of 16 modules was $4,574.93.
This seems like a much better deal than iteration No. 2, doesn’t it? I wish they’d had the next-generation hardware last year.
All of this configuration changing got me thinking: What if I’d gone with a safety PLC in the first place? The bill of materials becomes much simpler; all the logic necessary for the zones, and the prime-contractor zones, becomes quite easy to implement.
The BOM for this final iteration would look like:
- 1x Logic + Safety PLC Processor
- 4x 8-point Safety Input Modules
- 4x 8-point Safety Output Modules
And the cost of this would be $10,736.30.