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Residual current monitor

Overview~

A residual current monitor (RCM) detects earth-fault currents – current that leaks to protective earth instead of returning through the neutral. In an EVSE this matters because AC and, in fault conditions, DC residual currents can occur, and ordinary Type A residual current devices do not reliably trip on DC leakage.

By integrating an RCM that can sense DC residual current, a charger can provide the DC fault detection that would otherwise require an expensive Type B RCD upstream. This was one of the original design goals of the project: getting the protection an EV installation needs without the cost of a Type B RCCB. The RCM is optional and must be disabled in settings on boards that do not have one.

How it works~

The board uses an RCM module (currently supported models are RCM14-01 / RCM14-03) that exposes two signals: a trigger output that asserts when residual current exceeds the module's threshold, and a test input that forces an internal fault so the device can be verified.

  • Trip. While the RCM is enabled, the firmware watches the trigger line. If it asserts, an RCM_TRIGGERED error is raised and the charger goes to state E, opening the contactor.
  • Self-test. At the start of every charging session the firmware exercises the RCM through its test input. If the module does not respond as expected, an RCM_SELFTEST_FAULT error is raised. This confirms the protection is working before energizing the vehicle, rather than trusting it blindly.

Both RCM errors auto-clear after about 60 seconds, so a transient residual-current event causes a pause and retry rather than a permanent lockout. A genuine, persistent fault simply re-trips.

Configuration~

The RCM is declared in board.yaml:

rcm:
  gpio: 41        # residual-current trigger input
  testGpio: 26    # self-test output

gpio is the trigger line read by the firmware; testGpio drives the module's self-test. Both must be present for the RCM to be usable. With the hardware fitted, enable the RCM in Settings section of the web interface; without it, leave it disabled.

Danger

The RCM is a personal-safety device. It supplements, and does not replace, the protective devices required by your local wiring rules. Always verify the self-test and a real trip during commissioning, and follow the applicable installation standards for earth-fault and PEN protection.

CAUTION

Working with high voltage is dangerous. Always follow local laws and regulations regarding high voltage work. If you are unsure about the rules in your country, consult a licensed electrician for more information.

See also~