GFCI’s operate by measuring and monitoring the current imbalance between the grounded and ungrounded conductors. This is done by a current transformer built into the device, either a receptacle or circuit breaker. At .5 mA the circuit protection device opens. However, the equipment ground conductor serves no functional purpose with regard to the ground fault protection circuit. Why then will my GFCI tester not trip a GFCI receptacle device if the receptacle is ungrounded? I have experienced this during inspections. I have also bench tested this to prove the theory. Additionally, the NEC permits replacement of non grounded two pole receptacles with GFCI receptacles provided the GFCI receptacle is marked with “No Equipment Ground”. All downstream receptacles protected by this device (4 max) must be marked “Protected by GFCI” & “No equipment Ground”. If a non grounded GFCI receptacle with not trip using a GFCI tester how is it protecting ungrounded downstream devices?
I’m less familiar with AFCI’s. Do they operate on the same principle as the GFCI. That is a current transformer measuring and monitoring the current imbalance between the grounded and ungrounded conductors. I would assume this device is looking for current spikes. Anyone understand?