The steel strap holding a receptacle in place does electrical work as well as mechanical work. Device yoke grounding is the bond between that strap and the equipment grounding system, and it is what puts the metal cover plate screw, the strap itself, and any exposed device metal at ground potential rather than leaving them floating.
What the yoke is connected to
On a grounded device the yoke is bonded internally to the grounding screw. Connect a grounding conductor to that screw and the strap becomes part of the grounding system. In a metal box, the mounting screws can also carry that bond between strap and box, provided the connection is reliable. The visible consequence is that a metal cover plate, held by a screw threaded into the strap, is grounded. That matters because a cover plate is something people touch routinely and something a plug blade can contact if a cord cap is damaged.
Why the mounting screw alone was never trusted
The screw passes through an oversized hole in the strap, often through paint, and threads into a box ear that may itself be painted or corroded. Any of those layers can interrupt the bond. The metal-to-metal contact depends on the screw head bearing down hard on clean steel, which is not something you can verify by looking. That unreliability is why self-grounding devices with a spring clip around the mounting screw exist, and why a green pigtail from the device grounding screw to a grounding screw in the box remains the safest practice.
Adjustment ears and where they cause problems
The plaster ears on a device strap frequently need to be positioned or removed so the device sits flush with a finished surface. When the strap is held away from the box by an extender, a spacer, or a stack of washers, whatever bond existed through the mounting screws becomes questionable or disappears. This is the same situation created by installing a device over tile with a box extender. In all of those cases the grounding conductor has to reach the device grounding screw directly, and relying on the mounting hardware is not acceptable.
Nonmetallic boxes and switches without a ground
In a plastic box there is nothing for the yoke to bond to, so the grounding conductor from the cable lands on the device grounding screw and that is the entire path. Switches are worth particular attention: older switches often had no grounding screw at all, and many were installed in boxes where no grounding conductor was present. A metal switch plate on an ungrounded switch in a metal box is a combination worth investigating, because the plate screw is touchable and may be connected to nothing.
Practical habits that keep the path intact
Pigtail the grounding conductor to the device grounding screw whenever a grounding conductor is present. Add a grounding screw to a metal box where one is missing, using the correct thread. Scrape paint from the box ear where the mounting screw bears, or better, rely on the pigtail rather than the screw. Use devices listed as self-grounding where appropriate, understanding that the clip bonds to the box and is not a substitute for the box being grounded. And check switch plates in older homes for grounded metal.
What to check before you act
- Pigtail the grounding conductor to the device grounding screw rather than relying on mounting screws.
- Add a grounding screw to a metal box that lacks one.
- Treat any extender, spacer or lifted strap as removing the mounting screw bond.
- Check older switches for a grounding screw and a grounded metal cover plate.
Common questions
Does a metal cover plate need to be grounded?
It is grounded through the device strap when the device is properly grounded, which is one reason the yoke bond matters.
Can I ground a switch through the metal box alone?
Only where that box is genuinely grounded and the device is listed for it. A pigtail to a grounding screw removes the uncertainty.
The practical takeaway
Think of the yoke as a conductor that happens to hold the device. Bond it deliberately with a pigtail to the grounding screw, add a grounding screw to metal boxes that lack one, and assume any arrangement that lifts the strap off the box has interrupted whatever bond the mounting screws provided.
References and further reading
- OSHA 1910.304: grounding of equipment and enclosures
- CDC and NIOSH: why equipment grounding reduces shock hazard
- NFPA: National Electrical Code treatment of grounding and bonding
Numerical scenarios are illustrative unless identified otherwise. Follow the exact product instructions; component ratings and local installation requirements can differ.
