Light fittings installed on home circuits become a part of a circuit loop wired in parallel that is itself grounded. As for the light fittings or fixtures themselves, they may or may not be grounded depending on the type of box used, whether plastic or metal. As a general rule, when doing home wiring projects, get in the habit of pig-tailing ground wires together and connecting one end to each fixture. Regardless of whether it is an outlet, light or switch, grounding each fixture is a sensible, although sometimes unnecessary way to maintain the utmost safety.
Grounding Light Fittings
A light is wired to a circuit by feeding the necessary wires through a light box. This box can be made of either plastic or metal and is attached to the stud or joist and concealed from final view with a faceplate. Traditional boxes are metal, while the most common used today are plastic. Metal light boxes, without question, must be grounded at the light fitting. Even though the circuit wiring itself includes a ground that is connected to the circuit panel, metal boxes must have the ground wire attached to the grounding screw provided on the box. Should the hot wire dislodge, it could charge the metal box and by extension the entire light fixture.
Plastic light boxes do not conduct electricity, so they are not required to be grounded. They provide a grounding screw anyway, though, so it is a good habit to get into when wiring.