Protection of PLCs Against Overvoltage

PLC systems are the heart of an automation system. From factories to airports, PLCs perform critical tasks across a wide range of applications. Similarly, RTU and SCADA systems are fundamental components of an automation system. As Yılkomer, we have witnessed production halting in many factories when a PLC or RTU goes offline. These highly sensitive systems must be protected starting with main panel protection and finishing with a precise protection placed directly in front of the PLC.

You may also be interested in: AG Surge Protectors

Concept of Up

When protecting sensitive systems like PLC, RTU, and SCADA, it is essential to understand the concept of Up protection level. The Up level is the voltage that the surge protector can reduce the incoming surge to. The values differ for products used in main and sub panels. The lower the Up value of a surge protector, the more sensitive the protection. However, as In and Imax values increase, the Up level also rises. Therefore, the main panel, sub panels, and finally D-class surge protectors reduce the surge to a level that will not damage the target device. Protecting only the main or sub panels is not sufficient to achieve the desired Up level.

Protection of Analog or Digital Input/Output (IO)

These systems usually operate at 24 V with currents below 250 mA. Protection is applied using a D-class surge protector connected in series. Systems operating between 5V–110 V can be safely protected.

PLC 24 V protection

Systems up to 6 Amps

For systems with currents higher than 250 mA, different surge protectors are used. The connection scheme is the same as previously described.

4-20 mA Protection

Another common system in factories is the 4-20 mA loop. The loop is protected with a surge protector, and the LED status indicates both loop and protector status.

RS232-RS485 and RS-422 Overvoltage Protection

RS232, RS485, and RS-422 are high-speed IO lines. The selected surge protector must be compatible with the required frequency range. Profibus and Modbus lines are protected similarly.

RS232 Protection

RS232 line protection can be seen in the following diagram:

RS485 Protection

Point-to-point RS-485 line protection is shown in the diagram below:

PLC systems can also be protected during installation by using terminal block surge protectors. Preference for terminal block surge protectors is common in many applications. The images show examples of terminal block type protectors. PLC systems can be protected during installation, or if not previously protected, a site inspection can determine the correct products for retrofitting. For protection against lightning and sudden overvoltage surges, you can contact Yılkomer.

Terminal block surge protectors used for protecting sensitive systems