Issues of 120 V AC vs. 24 V DC process control systems

Today's engineers are faced with an ever-increasing number of choices in the design of industrial control systems. The selection of control voltage, while often perceived as a simple decision, has potentially the largest impact on the design and overall success of a system. This paper focuses on the comparison of application issues for two frequently used control schemes: 24 V DC and 120 V AC. Topics such as safety, device functionality, system design and wiring methods, reliability, and cost are explored to identify important considerations in the selection process.