Ineffective Calibration Testing of Residential Branch Circuit Breakers

Previous test results demonstrated that some presently-marketed residential branch-circuit breaker brands have a double-digit defect rate. The defective breakers do not trip as required at 135% of rated current. The unique test results reported in this present paper clearly show that these calibration defects cannot be reliably detected by testing at 200% or 300% of rated current. One hundred defective breakers of three brands from the previous tests are retested. They all fail to trip at 135% of rated current, confirming that they are defective. The defective breakers then all pass the UL489 calibration test at 200% of rated current (must trip within 120 seconds), and they all also pass the NEMA AB4 calibration test at 300% of rated current (must trip within 50 seconds). These higher-current tests have a 100% error rate for detection of these improperly calibrated breakers. This explains how defective breakers come to be shipped from some factories. For some residential branch circuit breaker brands, ineffective testing - relying only on high-current calibration testing - has allowed defective breakers into the supply chain for many decades, increasing the risk of fire and injury for occupants of the buildings in which they are installed.