EMC Question of the Week: April 6, 2020

Earth ground symbol on the door of an electronics cabinet

According to IEC 60417, which of the following is NOT a ground symbol?
a is pitchfork shape, b is horizontal lines forming an inverted triangle c is inverted triangle d is similar to b with half circle over the triangle


The correct answer is "c". This inverted triangle symbol is commonly used to represent a local circuit reference on a printed circuit board. It generally represents a connection to a conductor whose primary function is returning signal or power currents to their source. The standard does not call this symbol a ground. The authors of the standard recognized that "ground" and "current return" are not the same thing. In fact, the function of a ground and the function of a current-return conductor are generally incompatible.

Proper grounding and proper current-return are both essential aspects of a good EMC design. Applying the rules for grounding to current-returns (or vice versa) can result in substantial electromagnetic interference problems. Forty years ago, engineers generally understood grounding, but often neglected current returns. In today's high-speed digital designs, engineers are more likely to get the current-returns correct, but they often fail to address important grounding issues. If you're having trouble distinguishing between current-returns and grounds, we'd like to suggest that review our EMC Tutorial article on Grounding at https://learnemc.com/grounding.   

BTW: In the problems above, (a) is a chassis or frame ground, (b) is an earth ground, and (d) is a "noiseless earth ground." 

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