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If I use a plastic extension ring over a metal switch box, and if the metal switch box is correctly grounded and I use self-grounding devices with long screws screwed into the metal box, can I continue to rely on the self-grounding feature or must I add ground wires to the devices? (My assumption is the the plastic ring does not interfere with the grounding which is achieved by the screw threads screwed to the box and the back of the screw head touching the self-grounding tab on the device).

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  • Good question. The screws should give a ground path, but would the backs of the screws, be considered good enough to make a solid path. Think maybe not.
    – crip659
    Jun 16 at 18:52
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    Logic and instinct don't always help me understand why certain rules are the way they are, but FWIW, logic would suggest that if self-grounding devices are ok at all, they must not rely on the back of the plaster ears making good contact with the front of the box. Because very often they do not make any contact at all. Maybe most often. They must rely on the back of the screw head touching the device and the screw threads in the box.
    – jay613
    Jun 16 at 20:02
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    @jay613 Self-grounders don't, they have a wiper that touches the side of the mounting screws. However, if you have hard metal-metal contact between yoke and box ears, you don't need self-grounders, you can use cheap 50 cent jobs. This entire comment is about receptacles only. Switches follow liberal rules that say "mounting screws always good enough". Jun 16 at 21:09

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If the screws are going into the metal box, you should be fine. Extension rings are only to be used within reason. They also make extension boxes of various depths, and those are metal.

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    Hard to find metal extensions in small size increments under an inch. And FWIW, a three-gang 1/2 inch metal extension is available from only one maker and costs $80.
    – jay613
    Jun 16 at 20:06
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    For completeness, I am now much more apt to use oversize junction boxes and mud rings for tile and other thick walls. It's far easier to produce a flush, flat, and strong surface for the devices and wall plates that way. diy.stackexchange.com/q/248758/65210
    – jay613
    Jun 16 at 20:11

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