On 2009-07-24, blueman wrote:

But how does the GFI test button work? Does it really just test if
the breaker works or does it have a way of testing that the current
monitoring part works?

The GFI uses a current transformer to detect a downstream imbalance
between hot and neutral current. The test button has access to the
hot and neutral both before and after this current transformer. So it
shunts a small amount of current from the hot after the transformer to
the neutral before the transformer (or vice versa, I don't know),
which creates an imbalance in the current transformer.

Cheers, Wayne