Q: Why do I need to connect the GREEN wire to earth ground?

A: The R-3W flashes whenever there is a voltage differential between any two of the four leads. The NFPA 70E defines a hazardous voltage as 50 volts or more (referenced to earth ground). The R-3W needs to be referenced to the same safe potential as the personnel. Any potential other than earth ground, may set-up a potential shock hazard.