Vdc takes on Vac [system control voltage selection]

Selecting a system control voltage should be made based on careful consideration of several factors. The two common voltages being utilized in North American process-control applications include 120 Vac and 24 Vdc. The use of 120 Vac control is currently more common in industrial applications in North America. Extreme care should be taken to assure safety from potential electrical shock. Although this potential is presumed a low risk by most electricians that routinely work on equipment at much higher voltages, the fact remains that exposure to a live and poorly grounded 120 Vac circuit is a relatively frequent occurrence that can be fatal. The use of 24 Vdc control is growing, particularly in process-control applications. It is inherently safe and is aligned with the new codes and standards established for enhanced safety. It allows for a reduction in the times where lock-out/tag-out must be performed, and, often, control panels can be serviced while still energized. The devices are becoming extremely reliable, are often less expensive, and have better response times. 24 Vdc power supplies provide signal isolation and protection against shorted outputs and power quality issues. 24 Vdc allows direct interfacing with process-control equipment, reducing the need for interposing relays. Renewed emphasis on personal safety and improved performance gives a strong edge to 24 Vdc control for new applications. As new product designs become available, and costs continue to decrease, 24 Vdc will inevitably become the control system voltage of choice for industrial applications.