4-20mA and RS-485 communications in automatic control systems, as well as temperature, pressure, flow, and level sensors, require SPDs (surge protection devices).

Reason for necessity

• Transient voltages (surges) caused by lightning, power instability, external noise, etc. can have a fatal impact on sensors and communication lines, so surge protection is essential to ensure system reliability and prevent equipment damage.

• Because surges can cause data signal distortion, malfunction, equipment damage, and safety accidents, SPD application is required in industrial sites.

• RS-485 and sensor circuits are often exposed to external environments with long field wiring lengths, making surge protection a high risk.

Applicable specifications and standards

• IEC 61643-21 (International Standard for SPD for Low-Voltage Signal and Communication Circuits) applies to 4-20mA sensor and signal circuits and RS-485 communication lines.

• For sensor protection, it is recommended to select and install a dedicated SPD product according to each sensor type and wiring structure.

• Surge immunity and safety standards include IEC 61643-21 (surge immunity test) and IEC 61010-2-030 (safety and insulation requirements).

Practical application

• In practice, SPDs are installed in field transmitters and signal circuits for temperature, pressure, level, flow rate, etc., as well as in outdoor facilities and long cable sections.

• Communication equipment and sensors must be suitable products that meet international certification standards (TUV, ISO9001, etc.).

In this way, the installation of SPDs in various sensors and communication circuits of automatic control systems is currently required by international and practical standards.

Surge protection devices (SPDs) must be installed on the signal lines (including sensor lines) of Siemens PLCs. The practical application methods and precautions are as follows.

Basis for application

• PLC signal lines (sensor input/output, analog/digital communication) are directly exposed to lightning, power surges, and noise from adjacent equipment, which can lead to malfunctions or system damage.

• Especially when there is a lot of external wiring (sensors, remote communication, I/O modules, etc.) or the wiring is long distances, the possibility of excessive voltage inflow is high, so SPD installation is required on the signal line.

• Signal line SPDs are applied as a field standard to improve the reliability and lifespan of important equipment such as PLCs and sensors.

How to install

• For signal lines (4-20mA sensor circuits, RS-485 communication lines, etc.), use dedicated signal SPDs according to IEC 61643-21.

• It is effective to insert the SPD near the signal line input during installation or in the wiring box (terminal box) where the sensor and PLC are connected.

• The wiring length must be kept as short and straight as possible to ensure proper performance of the SPD.

• Proper grounding is essential for SPD operation. Poor/defective grounding or insufficient signal line grounding are major causes of SPD performance degradation and malfunction.

• Depending on the installation location, you can choose between DIN rail-mounted SPD and panel-mounted SPD for each PLC product.

Practical Tips

• Before installing an SPD, check the SPD rating (V) and type that are suitable for the device/circuit.

• Applying power SPDs and signal SPDs in parallel to the distribution board/panel around the PLC will improve the overall protection effect.

• Regularly check the SPD operating status (indicator lights, alarms, test functions, etc.) and immediately replace any damaged or expired SPD.

When operating Siemens PLCs, the application of SPDs to signal lines is a field standard for system stability and equipment protection.

Practical data and related standards

• Empirical data: Data from global manufacturers such as Raycap, Siemens, Hakel, ABB, M-System, and Surgefree on automatic control signal circuits and communication lines.

The need for SPD is repeatedly stated