It is difficult to detect the fault in a relay circuit. Relay provides a less flexible control system. Essentially, it's physical hardware-based logic, arranged into circuits that control equipment based on specific conditions. Instead. However, they had several drawbacks: 1. High Maintenance Costs: Repairing and replacing relay circuits is. We compared PLCs vs relay-based control systems to help you make the right choice for your industrial applications. The global PLC market is growing, and by 2026, the market size should amount to over 15. A quick look at these key differences between PLC control systems and. At its core, a relay is an electrically operated switch.