Engineers are used to making financial decisions based on factual inputs and realistic assumptions. Determining whether to invest in a project using the return on investment (ROI) accounting method has long been a safe & consistent decision making tool. However, when it comes to evaluating redundancy in industrial control systems, this method doesn’t hold up.
ROI is defined as: Net benefit of a project (over a set period of time) / amount invested.
A control system without redundancy will keep running smoothly under normal operating conditions. Since the extra investment in duplication does not increase the output, the ROI of redundancy is a negative value (extra output ($0) – cost($X) ) / cost ($X) = -1.
The true value of redundancy in a control system is shown when a critical failure occurs and there is no loss in production, damaged to equipment or injury/death of humans.