No need to earth the back box as the accessory is plastic and even if it wasn't, the screws through the accessory faceplate contact the lugs of the back box via the earth strap that runs across the back of it, providing earth continuity between all items.
No need to earth the back box as the accessory is plastic and even if it wasn't, the screws through the accessory faceplate contact the lugs of the back box via the earth strap that runs across the back of it, providing earth continuity between all items.
Why are there two earth terminals?