Instrumentation and Control Surge Protection 120 V AC I/O digital signal protection using the PT PLUGTRAB TRABTECH Application Note 2556A Description The type of signal protection for 120 V AC digital input/output (I/O) signals is different if the signal source neutral line is shared or isolated. Each solution requires protection to ensure the PLC/controller is not damaged by surges. This document explains why a product is recommended, how it’s wired, and how to remotely monitor the status. Both types of signal protection offer remote monitoring with local indication of a failure. Plugs are hot-swappable without interruption of the signal. In a shared source application, follow-currents are possible. Follow-currents can pass through systems with gas tube type protection, possibly causing downstream fuses to trip that would require manual resetting. In this application, a PT 2X1 VA is used (see Figure 1) and only MOVs are connected between the signal lines and ground. For isolated 120 V AC circuits, the MAINS-PLUGTRAB PT2-PE/S-120AC is recommended (see Figure 2). This product includes a gas discharge tube, but is not used independently in a common mode surge event, eliminating the follow-current issue. In this application, the neutral/ return for the 120 V AC source could be at a different potential so it needs to be isolated from the ground with a gas tube. This assures optimum personnel and equipment safety while providing effective surge protection for each circuit. 5 11 12 1 3 Unprotected IN Figure 2. Circuit diagram with PT 2-PE/S-120AC Figure 1. Circuit diagram with PT 2X1 VA 6 Remote monitoring Normally closed 1 © PHOENIX CONTACT - 04/2009 2 4 Protected OUT Instrumentation and Control Surge Protection PLC/Controller PLC/Controller 11 12 11 12 Protected OUT Protected OUT Unprotected IN Unprotected IN N N N L L L Figure 3. Shared 120 V AC input connections Figure 5. Isolated 120 V AC input connections PLC/Controller PLC/Controller Protected OUT 11 12 11 Protected OUT 12 Unprotected IN Unprotected IN N N N L L L Figure 4. Shared 120 V AC output connections 2 Figure 6. Isolated 120 V AC output connections Phoenix Contact 2556A Instrumentation and Control Surge Protection Monitor Monitor PLC/Controller Protected OUT Unprotected IN Figure 7. Remote monitoring connections Monitoring can be daisy-chained so only one alert is sent to the PLC/Controller. An LED on the plug identifies which plug needs replaced. 12 10 Field Devices 11 9 5 7 3 6 4 8 2 OUT 11 9 Signal 1 IN PT BE/FM base 12 10 5 3 7 1 6 8 4 2 OUT IN Control Panel PT BE/FM base Figure 8. Base element grounding Only one base element is used for protection of 120 V AC signals. The grounding and remote monitoring are the same for both types of signals. 2556A Phoenix Contact GmbH & Co KG • 32823 Blomberg • Germany • Phone: +49-(0) 5235-3-00 Phoenix Contact • P.O. Box 4100 • Harrisburg, PA 17111-0100 • USA • Phone: +1-717-944-1300 www.phoenixcontact.com 3