EMC Question of the Week: February 28, 2022

Three-terminal electrical outlets

For the standard electrical outlets shown in the figure, the terminals that electrically connect to the earth outside the building are labeled 

  1. HOT
  3. GROUND 
  4. Both NEUTRAL and GROUND


The best answer is “d.” Both the neutral and ground terminals are attached to wires leading directly to a copper bus in the building's service panel. That bus is electrically connected directly to the earth outside the building. By convention, the neutral wire is white (in the U.S.) or blue (in Europe or China). The ground wire is green (in the U.S.) or green with a yellow stripe (in Europe or China).

Although both the neutral and ground terminals are connected to the same place, they serve VERY different functions. The neutral wire carries power return current, while the ground wire carries no nominal current (except briefly under fault conditions). The ground connection is essentially the reliable zero-volt reference for the power distribution system.

Ground and current-return are essentially incompatible functions. If the ground and neutral terminals shared the same wire connection back to the service entrance, the outlet would still be functional, but products using the outlet could become a safety hazard. The terminal labeled "ground" in an electrical outlet is a safety ground. It would be confusing (and a code violation) to label the neutral connection "ground." 

EMC engineers often encounter traces, planes and wires labeled "ground." Sometimes these are non-current carrying connections to an EMC ground structure and sometimes they carry intentional power or signal currents. Making good EMC design decisions often relies on knowing the difference between a "ground" conductor and a "current return" conductor (even when the current-return conductor is labeled "ground").  

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