What is the HF note?
As the NEC now requires Ground Fault protection for a larger scope of applications & higher rated circuits in the recent Code cycles, high frequency during start up & cool down phases of equipment involving power inverters & variable frequency drives (VFDs) has created issues with inadvertent fault conditions. This is because the High Frequency “chatter” in the circuit is seen as electrical leakage by the GFCI. High Frequency (HF) rated Ground Fault Circuit Interrupters have been created to address this issue, following an inverse time curve (depicted in the chart above) that de-rates the value of leakage current corresponding to the frequency within in the circuit, which will allow safe operation while avoiding nuisance tripping.

This HF measure was codified to Underwriters Laboratory (UL) 943 (the UL standard for Ground Fault Circuit Interrupters) in August of 2025, in preparation for the 2026 NEC Code cycle. Also note, HF-rated GFCIs are meant to work in conjunction with equipment that is Listed to UL 101 standards (the UL standard for appliances) to ensure optimal operating conditions. The leakage limit for equipment that meets UL 101 can be seen as the bottom orange line in the above graph. Additionally, NSS created the chart below for you to be able to view the difference between what equipment can leak up to per UL 101, and where an HF rated GFCI will trip at certain frequency thresholds. Note, the GFCI threshold shown is the lowest possible trip point (x. At 60 Hz it will trip between 4.01mA to 6mA, hence the 5mA +/-1).

What does this Code mean?
The 2026 NEC Code cycle added some key changes to Ground Fault protection standards for a wide scope of applications and appliances. It still requires Class A (5mA +/- 1 trip level) GFCI protection for a majority of its scope, while mentioning that Class C (18mA +/- 2 trip level) SPGFCI protection can be used for HVAC applications, as long as the HVAC unit & the SPGFCI are Agency Listed. It also addressed concern for larger circuits that have encountered nuisance tripping from high frequency introduced from certain equipment, such as Variable Frequency Drives (VFDs), now allowing Listed HF-rated GFCIs to accommodate the demands of those applications.

North Shore Safety
North Shore Safety offers a wide variety of GFCI, ELCI, and EVSE products in our LineGard electrical protection series. The UL Listed Class A & C GFCI products, as well as our CSA Listed ELCI products, provide protection for 15 to 60 Amp single, split, and 3 phase applications. This allows you to stay protected in hazardous environments while adhering to NEC guidelines.

Tecmark®
Tecmark® designs, develops and manufactures pressure switches, vacuum switches, air switches and other electrical components, including level sensors, thermostats, timers, spa side controls and more. For years, our specialty in air switch and pressure switch controls has helped us deliver hundreds of product variations to market segments worldwide. We provide options for OEMs with interchangeable components allowing for design flexibility.

Quality control at every step of the process, with an emphasis on rigorous product testing. Tecmark is certified to ISO 9001: 2015 requirements.