One of the first decisions control system designers must make at the onset of any design process is to decide on a system voltage their project will need. Often use of 120 V ac for control system power is based on traditional usage, simply because it has always been done that way. However, there are viable alternatives to 120-volt control systems that have evolved over the history of the control system.
Modern control systems are often very complex. Troubleshooting control systems frequently requires that they be powered up while the system is checked. The use of 24 V reduces the likelihood of injury caused by shock. A typical control system uses 24 V ac for panel and field I/O devices. This system would also use 24 V dc for analog instrumentation. And, alternately, 24 V dc could be used for panel, field I/O and analog devices. The flexibility in the choice of ac and dc or all dc or all ac covers most control situations.
Deciding what voltage level is most appropriate depends on several factors. For small systems, all the controls can be implemented using 24 V dc. For systems with motor starters, 24 V ac is available by providing a step down transformer connected to the power feeders at the starter. Systems with analog instruments and starters probably are best served by having both 24 V ac and dc available. Where 120 V ac is needed for specific applications, interposing relaying can be provided. Today, many factors are combining to help propel the adoption of 24 volts for control purposes.