Do GFCI units need to be tested with an external tester?
No. The test button, which should be used daily, actually puts an imbalance between the hot and neutral circuits and self-tests the unit. If the test button fails to trip the GFCI, the unit should not be used. If an inspector does test a GFCI with an external tester, the GFCI should trip between 4 and 6 milliamperes, according to UL standards.