I have quite a few GFCI outlets around the house, barn and shop and I asked one of my electronics kids how they functioned and he produced this video on Youtube: Inside a GFCI Outlet - YouTube
I manufacture 220 volt single phase distillery controllers. Our largest requires a 120 amp circuit and powers four 5,500 watt immersion elements, good for roughly 100 gallons. Because the elements are in direct contact with liquids we ALWAYS require the electrician to protect the circuit by means of a GFCI breaker. Better safe than sorry IMHO.