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I've inherited several board designs where each of the four [plated through] mounting holes are connected to the board's ground plane through a capacitor [1000pf, 1KV]. I can understand why you might want either mounting holes connected directly to signal ground, or why you might want holes that are totally isolated. But I'm confused about the benefit of a connection to ground through the cap.

While metal hardware is used to install the board, there's no guarantee that the material the board is mounted to will actually be in contact with system ground [or will even be electrically conductive].

Background: the boards are part of a piece of commercial food service equipment. All boards have microcontrollers and handle digital I/O, 24V loads. One board has a configuration where it can drive brushed DC motors.

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  • \$\begingroup\$ What's the purpose(s) of your board? \$\endgroup\$ – Nick Alexeev Nov 10 '15 at 1:20
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    \$\begingroup\$ The brushed DC motors could sting the microcontroller if the caps were not there .Also it could fail radiated EMC. \$\endgroup\$ – Autistic Nov 10 '15 at 2:48
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A capacitor provides a low impedance path to ground for high frequency noise. There may be one mounting hole that's hard-wired to ground, and others are connected through a capacitor.

(I saw a similar arrangement with cable shields.)

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  • \$\begingroup\$ Each mounting hole is connected to the board's signal ground plane through a capacitor. \$\endgroup\$ – Allen Moore Nov 10 '15 at 1:36
  • \$\begingroup\$ Is there a low-frequency path to chassis ground at all? \$\endgroup\$ – ThreePhaseEel Nov 10 '15 at 1:55
  • \$\begingroup\$ No. The board gets DC power from an external supply. The board's ground plane layer (which serves as the digital/analog/power ground) connects to the DC negative return. The power supply itself has a terminal for a chassis ground connection, which is connected to the one-point system ground. The +/-DC supply, however, is isolated from this chassis ground. \$\endgroup\$ – Allen Moore Nov 10 '15 at 18:43
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A cap to ground will provide only an AC ground to a circuit, without making a DC ground. There could be reasons like ground current control where you would not necessarily want a direct DC ground from a sensitive circuit to heavy commercial equipment.

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