The engineering controls don't independently make systems safe, they make things more reliable and cost-effective, and hopefully reduce the number of times the

The engineering controls don't independently make systems safe, they make things more reliable and cost-effective, and hopefully reduce the number of times the process controls kick in. The process controls do however independently make things safe. The reason for this is that there are 'unknown unknowns'—we accept that our knowledge and skills are imperfect, and there may be failures that occur which could have been eliminated with the proper engineering controls, but we, as imperfect beings and organisations, did not implement the engineering controls because we did not identify this possible failure mode. There are also known errors, where the cost of implementing engineering controls may simply outweigh the benefits when adequate process controls are in place.