The best way to ensure safety is to design the automation system with safety in mind, upfront. Start with a risk assessment. The EN1050 standard covers principles of risk assessment asking such questions as, "How frequently is a worker exposed," and "What is the danger level?" Perform the assessment, then see what equipment is needed to eliminate issues.
Machine builders have more risk than they realize, often resulting in huge losses. Performing risk assessments are a good way to show you have followed good engineering practice.
So, what is it about the new technology that makes it easier to build in saftey? Compare old and new systems:
Sensor-specific relays PLCs
Monitoring via hard wiring into PLCs Monitoring via network connections
Fixed I/O Remote I/O via safety networking
Can use PLCs with light curtains