EMC Question of the Week: May 24, 2021

Car chassis/frame painted white

In order to function effectively, an EMC ground structure should be

  1. connected to the earth
  2. electrically small
  3. relatively big
  4. all of the above


The best answer is “c.” To take advantage of the EMC ground structure, large conductors throughout the system should be bonded to it. This can be difficult if the ground structure isn't already one of the biggest conductors in the system. In most cases, the EMC ground structure is the metal chassis, frame or enclosure.  

It makes no difference whether or not an EMC ground structure is connected to the earth. Connecting to the earth is a feature of safety grounds in buildings, but it has little to do with meeting EMC requirements. 

The size of the structure relative to a wavelength is also unimportant. EMC ground structures are not equipotential surfaces, they serve as local zero-volt references.  The EMC ground structure of most ground vehicles and aircraft is the frame at frequencies above 10 MHz. Yet, at those frequencies, the frame is far from being electrically small. 

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