[darsunt]

We have some old electrical systems done before code required a 3rd ground wire. Is it possible to make a gfi outlet work on one of these old lines, or does it require a ground wire?

 

[Jim Port]

GFI's will work without a ground wire. A three light plug-in tester will not trip a GFI without a ground.

Also use the 'no ground' stickers in the box on the protected devices.

 

[PaliBob]

GFCI's installed in a box without a ground will still work as designed for people protection but as pointed out the test button on a three prong GFCI Test will NOT work because that type tester connects a 5mA bleed resistor between Line and Ground.

GFCI's are designed so that Ground is not required for the built in Self-Test button ON the GFCI.

This is done by connecting the Test bleed resistor back to the input side of the CT.

 

[220/221]

Is it possible to make a gfi outlet work on one of these old lines

Not only possible but preferable.

You can legally install 3 prong receps on a 3 wire system if the citcuit is GFCI protected.

The GFCI simply measures current incoming on the hot wire and compares it to the returning current on the neutral wire.

If there is more current going out than coming back, it assumes it's going to ground and opens the circuit.