Covers ground fault circuit interrupters (GFCI) that provide electrical shock hazard protection from line-to-ground fault currents on grounded circuit of 120, 120/240, and 208Y/120 volt two-, three-, and four-wire supplies. Covered in this guide are characteristics of the various types of GFCIs, construction details, marking data, installation guidelines, field testing recommendations, and field troubleshooting information.