Ground Faulty

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A circuit to detect ground faults


  • "one shot" check on startup or continuous checking
  • self test by turning on both test faults at the same time
  • alarm when and while fault detected or continuous alarm after fault first detected
  • 60s or 1s delay between checks


Source code & schematic available via git at

I've prototyped most of the design, but haven't got the whole thing working yet. There are likely some changes needed:


Sense Resistor Selection

The H11AA4-M opto-isolator has a 60mA maximum current, so we need to select sense resistors to remain below this current. The sense resistor is in series with the fault resistance.

Enter system voltage and desired ground fault test current to obtain test resistor specifications (note this doesn't work yet):

Volts Amps =

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