Since a GFCI only monitors current on the hot leg compared to the neutral, the GFCI can be used to upgrade older two-prong (non-grounded) outlets to three-prong (grounded/earthed) outlets without installing any new wire. A circuit with a GFCI device without a ground is far safer than a two-prong outlet without a ground. A GFCI installed this way must be labeled "No Equipment Ground." 