This application guide covers ground fault circuit intermpiem (GFCI's) which are intended to provide electrical shock hazard protection from line-to-ground fault currents on grounded circuits of 120@ 120D0@ 240 volt single phase@ 208Y/120 volt@ and 240 volt three-phase; two@ three@ and four-wire supplies.GFCi's are designated as Class A or Class B in accordance with UL Standard 943 (January 1985)@ Ground Fault Circuit Intempters.* Unless otherwise indicated@ this application guide applies to class A GFCI'S.Covered in this guide are characteristics of the various types of GFCI's@ construction details@ markhg data@ installation guidelines@ field testhg recommendations@ and field troubleshooting information.*Copies are available ?from Underwriters Laboratories Inc.@ 333 Pfingsten Road@ Northbrodt@ IL 60062?