Surge Protection for HVAC


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©.

NEC-2020 The latest release of NEC/NFPA 70 edition 2020 (NEC 2020) has placed a greater emphasis on increasing personnel safety by mandating the use of Surge Protective Devices (SPDs).

From 2014 to 2020, the number articles now requiring SPDs has increased significantly for applications ranging from Modular Data Centers, Fire Pumps, Elevators, Critical Power Systems, Dwellings (Dormatories, Homes, Hospitals, Hotels), etc. This article will focus on the protection of the HVAC unit (VFD input and output and communcation), a Critical Power Systems, defined by NFPA 70 Article 708.

What are Critical Operations Power Systems (COPS)

As stated by Article 708, Critical Operations Power Systems can be classed by municipal, state, federal, or other codes by any governmental agency having jurisdiction or by facility engineering documentation establishing the necessity for such a system. These systems include but are not limited to power systems, HVAC, fire alarm, security, communications, and signaling for designated critical operations areas.

Article 708.2 further defines how the end user can identify if the system is a COPS.

CategoryArticle 708.2 Definitions
Critical Operations Power Systems (COPS)"Power systems for facilities or parts of facilities that require continuous operation for the reasons of public safety, emergency management, national security, or business continuity.”
Designated Critical Operations Areas (DCOA)"Areas within a facility or site designated as requiring critical operations power."
Supervisory Control and Data Acquisition (SCADA)"An electronic system that provides monitoring and controls for the operation of the critical operations power system. This can include the fire alarm system, security system, control of the HVAC, the start/stop/monitoring of the power supplies and electrical distribution system, annunciation and communications equipment to emergency personnel, facility occupants, and remote operators."


Variable Freqency Drive Power Input | Variable Freqency Drive Power Output | Variable Freqency Drive Communication I/O| External PLC Control Cabinet

Variable Frequency Drive Power Input

The main AC incoming will be the primary location to protect the drive against electrical surges and overvoltage. SPDs can be installed at the main disconnect panel, external to the HVAC system, or within the HVAC system itself, typically installed by the OEM.

There can be a multitude of AC input voltages for the VFD, but the most common drives will run through a three-phase AC network without any Neutral, split into 3 voltage ranges : from 120Vac to 240Vac, and 277Vac up to 480Vac. The common surge protectors used below :

Recommended Surface Mount SPDs at Main Disconnect Panel

LocationSystemSurge Protector Model #Link
AC Input277 to 480Vac (3W+G) MS100-480D
AC Input120 to 240Vac (3W+G) MS100-240D

Recommended Din Rail mounted SPDs for VFD Input

LocationSystemSurge Protector Model #Link
VFD AC Input120 to 240Vac (3W+G) DS74US-240D DS74US-480D
VFD AC Input277 to 480Vac (3W+G) DS74US-480D DS74US-480D

Variable Frequency Drive Power Output

The AC power output of the drive to the motor, runs on the same voltage as the VFD input. Only the frequency varies. It's obvious the exposure to transients feedback from the field. However, the output of the VFD is very commonly discarded in terms of surge protection, the main reason being the presence of Temporary Over Voltages (TOV). Regular MOV-based SPDs will not provide the robustness required to handle these events. This is why hybrid solutions (such as CITEL “/G” or “VG” Technologies) are highly recommended. The common surge protectors used below:
LocationSystemSurge Protector Model #Link
VFD AC Output120 to 240Vac (3W+G) M50-240D-A M50-A
VFD AC Output277 to 480Vac (3W+G) M50-480D-A M50-A

Variable Frequency Drive Communication Ports

CategoryArticle 708.14 Wiring of HVAC, Fire Alarm,Security, Emergency Communications and Signaling systems
Critical Operations Power Systems (COPS)"A listed primary protector shall be provided on all communications circuits. Listed secondary protectors shall be provided at the terminals of the communications circuits.”

The drive output frequency shall respond to the actual need for the engine to run faster or slower. This information is provided to the VFD by PLC, which interface between the different sensors in the field on the variable speed drive. Traditionally, the communication between the VFD and the PLC was done through 3 main protocols : RS485, CAN Bus, Modbus. Nowadays the deployment and Ethernet cable with RJ45 connectors is a lot more efficient.

The common surge protectors used below:

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

PLC Control Cabinet

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.).

Please Refer to Surge Protection for PLC Cabinet

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