EMC Question of the Week: July 19, 2021

Chassis-to-circuit-ground connection through SMT component

In 12-V (or 42-V) automotive components, the circuit board's "ground" planes should normally be tied to any existing nearby chassis ground through

  1. capacitors
  2. capacitors in parallel with a resistance
  3. capacitors in series with a resistance
  4. low-impedance bonds


The best answer is “d.” In most situations, the signal and power returns of low-voltage automotive components should not be DC-isolated from the enclosure or chassis. Like most EMC design "rules," there can be exceptions; however components with isolated power returns present significant EMC and product reliability challenges.

Medical devices and many aerospace components have legitimate safety reasons for controlling the amount of DC current that flows to a chassis ground. The same is true for high-voltage automotive components. Low-voltage automotive components do not have these same constraints. In fact, many automotive sensors and actuators rely on the vehicle chassis as an intentional power and signal return path. Attempts to DC-isolate low-voltage automotive components from the chassis generally provide no quantifiable benefit, but they add to the cost and tend to decrease the reliability of automotive systems. 

Note: Chassis grounds can be unreliable over time and many component-level EMC tests require that the components and harnesses be elevated above their reference ground to ensure they are not relying on a low-impedance connection to that ground. Well-designed automotive systems do not have a safety-critical reliance on a DC connection to chassis ground, but they don't intentionally avoid it unless they operate at unsafe voltages.

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