Surge Protection for HVAC
Overview
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©.
The latest release of NEC/NFPA 70 edition 2026 (NEC 2026) has placed a greater emphasis on increasing personnel safety by mandating the use of Surge Protective Devices (SPDs).
From 2014 to 2026, the number articles now requiring SPDs have increased significantly for applications ranging from Modular Data Centers, Fire Pumps, Elevators, Critical Power Systems, Dwellings (Dormitories, Homes, Hospitals, Hotels), etc. This article will focus on the protection of HVAC Systems (VFD input and output and communication), 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 with 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 alarms, 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:
Category | Article 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 Frequency Drive Power Input
The main incoming AC Input is the primary location to protect the drive against electrical surges and overvoltage. SPDs can be installed on 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 120 V ac to 240 V ac, and 277 V ac up to 480 V ac.
Recommended Surface Mount SPDs at Main Disconnect Panel
Location | System | Surge Protector Model # | Link |
|---|---|---|---|
AC Input | 277 to 480 V ac (3W+G) |
| |
AC Input | 120 to 240 V ac (3W+G) |
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Recommended DIN Rail mounted SPDs for VFD Input
Location | System | Surge Protector Model # | Link |
|---|---|---|---|
VFD AC Input | 120 to 240 V ac (3W+G) |
| |
VFD AC Input | 277 to 480 V ac (3W+G) |
|
Variable Frequency Drive Power Output
The AC power output of the drive to the motor runs on the same voltage as the VFD input. This area is where the risk of transient surge feedback from the field is typically observed. However, the output of the VFD, which is commonly ignored in terms of surge protection, is the primary reason for 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.
Location | System | Surge Protector Model # | Link |
|---|---|---|---|
VFD AC Output | 120 to 240 V ac (3W+G) |
| |
VFD AC Output | 277 to 480 V ac (3W+G) |
|
Variable Frequency Drive Communication Ports
Category | Article 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 interfaces between the different sensors in the field on the variable speed drive. Traditionally, communication between the VFD and the PLC are done through 3 main protocols: RS485, CAN Bus, Modbus.
Location | System | Surge Protector Model # | Link |
|---|---|---|---|
VFD Twisted Pair Wire | RS485, CAN Bus, Modbus |
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VFD Ethernet Port | RJ45 up to CAT6A |
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PLC Control Cabinet
Acting as the main brain of the system, the PLC is commonly installed inside a UL 508A control panel and interacts with a multitude of systems inside an installation. Usually powered through a traditional single phase AC connection, newer PLCs can now be power through POE (Power Over Ethernet) as well. Using data received from the different sensors placed throughout vital systems (pressure sensor or transducer, temperature sensor, speed sensors, etc.), the PLC will transmit that information to the VFD (Variable Frequency Drive) and adjust the speed of the engine (pumping more or less water, adjusting the speed of a conveyer belt, rotating a robot arm, etc.).
