Você não precisa anexar a cauda verde ao fio de aterramento na caixa. Ao conectar corretamente este dispositivo a uma caixa metálica adequadamente aterrada, você está, inerentemente, fornecendo um caminho de aterramento adequado.
NEC 2011
404.9 Provisions for General-Use Snap Switches
(B) Grounding.
Snap switches, including dimmer and similar control switches, shall be
connected to an equipment grounding conductor and shall provide a
means to connect metal faceplates to the equipment grounding
conductor, whether or not a metal faceplate is installed. Snap
switches shall be considered to be part of an effective ground-fault
current path if either of the following conditions is met:
(1) The
switch is mounted with metal screws to a metal box or metal cover that
is connected to an equipment grounding conductor or to a nonmetallic
box with integral means for connecting to an equipment grounding
conductor.
(2) An equipment grounding conductor or equipment bonding
jumper is connected to an equipment grounding termination of the snap
switch.