Surge Protection for
Programmable Logic Controller (PLC)



Today’s increased reliance on sensitive electronics makes surge protection an important topic for most industries. The Insurance Institute for Business & Home Safety study found that $26 billion dollars was lost due to non-lightning power surges. In addition, there are about 25 million lightning strikes in the US each year that cause between $650M to $1B in losses according to the Insurance Information Institute, State Farm©.

A Programmable Logic Controller, commonly referred as a PLC, seats at the center of any control panel of modern equipment. This industrial computer, replacing the prior relay logic systems has been optimized to provide full autonomy to most systems, ranging from manufacturing process on assembly lines, robotics, to irrigation pump panels. Acting as main brain of a system, the PLC in commonly installed inside a UL508A panel and interacts with a multitude of equipment inside an installation. Usually powered through a traditional single phase AC networks, the latest generation can now be power through POE (Power Over Ethernet) as well. Thanks for the information (data I/O) received from the different sensors in the machine of dispatched on the field (pressure sensor or transducer, temperature sensor, speed sensors, etc.), the PLC will transmit that information to the VFD (Variable Frequency Drive), to adjust the speed of an engine for example (pumping more or less water, adjusting the speed of a conveyer belt, rotating a robot arm, etc.). Today’s world being fully interconnected to the internet, the PLC will often also transmit information to a SCADA System (through ethernet, RS485 or other communication protocol) That central position makes the PLC one of the most critical part of a system. Its failure will force the complete shutdown of the equipment. In the meantime, the electrical interconnections across the installation makes the PLC extremely prone to electrical transients. Based on solid state components, the Programmable Logic Controller surge withstand capability is very limited, hence the need for a global surge mitigation solution avoiding costly downtime.

This section will identify the surge protectors to efficiently protect the multiple inputs and outputs of a PLC from power input (UL 1449 5th Edition) to signal and communication lines (UL 497B).


PLC Power Input | PLC I/O Communication | PLC Network Connection

PLC Power Input

As being one of the primary surge entry point, this input is very often well identified and protected by end users, when the PLC is power through an AC Network. However, when power is run through a power supply or POE, this critical input tends to be forgotten.

Here would be the most commom type of SPD used based on PLC designs today :

LocationSystemSurge Protector Model #Link
PLC AC Input120 to 240Vac (2W+G) DS72US-240T/NN DS72US-240TNN
PLC DC Input24Vdc (2W+G) DS220S-24DC DS220S-24DC
VFD POE Input8pins+G MJ8-POE-A MJ8-POE-A

PLC I/O Communication

The common surge protectors used below:

LocationSystemSurge Protector Model #Link
PLC I/O portModbus (2W+Sh+G) DLA-12D3 DLA-24D3
PLC I/O portProfibus (2W+Sh+G) DLC-12D3 DLC-24D3
PLC I/O port4-20mA (2W+Sh+G) DLA-24D3 DLA-24D3

PLC Network Connection

LocationSystemSurge Protector Model #Link
PLC Twisted Pair WireRS485, CAN Bus, ModbusDLA-12D3 DLA-24D3
PLC Ethernet PortRJ45 up to CAT6AMJ8-C6A

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