VFD is blowing my RCD

[Doddy]

If you are using GRBL v0.9 then D11 should put out a 0-5v analogue signal
https://github.com/grbl/grbl/wiki/Connecting-Grbl. I have the feeling that describing D11 output as PWM is in error.

No. The MEGA328/2560 based arduinos do not have a DAC, or any other form of analogue output. They provide a number of PWM-enabled discrete outputs (those prefixed with a tilde on the board silk-screening) with the signal value switched between GND and VCC - which if used for e.g. LED drives allows a direct control over the LED luminance, similarly for a DC motor drive; but the output remains a digital PWM signal (they do, however, support multiple ADC inputs)