Understanding test winding in core balance current transformers
If you take a look at a typical UL943-compliant GFCI interrupter circuit, based on this Onsemi chip: 



The IC produces a test signal for the UL943 mandatory periodic self-test operation. The test current of 6-8mA is 1/2 of a mains cycle and is below the trip threshold for an actual fault, but high enough that it can be detected so that the circuit does not trip out on test failure.