ARLINGTON, Virginia — The Air-Conditioning, Heating, and Refrigeration Institute (AHRI) released its Phase II report examining compatibility between variable-speed HVAC equipment and Ground Fault Circuit Interrupter (GFCI) breakers, concluding that nuisance tripping is primarily caused by the interaction between inverter-driven equipment and the frequency response of individual GFCI devices rather than a universal equipment defect.
According to the report, researchers moved beyond the observational findings from Phase I to identify the electrical mechanisms responsible for nuisance trips required under National Electrical Code (NEC) Section 210.8(F). The study combined frequency-dependent testing of multiple GFCI breaker models with laboratory testing of two inverter-based HVAC systems operating under controlled startup and normal operating conditions.
The report found that GFCI performance varies significantly among breaker models because each device responds differently to higher-frequency leakage currents generated during normal operation of variable-speed HVAC equipment. As a result, one breaker may repeatedly trip on a specific system while another breaker does not, even under similar installation conditions.
Researchers also determined that the greatest likelihood of nuisance tripping occurs during equipment startup rather than during steady-state operation. Extended off-time energization, compressor startup sequences, switching events, and electrical transients were identified as the highest-risk operating conditions.
Testing also revealed that one HVAC system exhibited an additional 60-hertz leakage mechanism during startup related to equipment topology. According to the report, an approved component modification eliminated the condition, allowing researchers to continue evaluating high-frequency leakage characteristics.
The report recommends that AHRI continue working with standards organizations to develop frequency-aware testing methods and interoperability requirements that better reflect the operating characteristics of modern inverter-based HVAC equipment. Researchers also recommend standardized reporting of leakage current characteristics and additional evaluation of transient startup conditions.
For manufacturers, the report recommends characterizing GFCI performance across a broader range of frequencies, improving immunity to non-hazardous high-frequency leakage currents, and expanding pre-market interoperability testing. HVAC equipment manufacturers are encouraged to measure and reduce leakage currents at dominant switching frequencies while refining inverter and filtering designs to minimize common-mode leakage.
According to AHRI, the recommendations are intended to preserve the safety objectives of NEC’s GFCI requirements while improving compatibility with today’s variable-speed HVAC systems.
The current NEC GFCI exemption for outdoor HVAC equipment is set to expire Sept. 1, unless a Tentative Interim Amendment (TIA) request to extend the current 210.8(F) exception to Sept. 1, 2028, is approved.
To view AHRI’s full report, click here.
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