In this circuit, there is a warning about not having the CT be open-circuited, but I’m confused as to whether that also includes the scenario wherein the arduino is powered off but the rest of the circuit is connected as shown in the schematic on that page. I am using the recommended sct-013-000 current sensor, which I think has some protection against this but I’d still like to be sure if powering off the arduino while everything is connected is safe or not.
The CT will be happy because it has a burden R to feed, but the AVR processor might be less happy. You’ll be feeding a voltage to it while it’s unpowered. The internal protection diodes will conduct and it will effectively try to power up from that voltage source.
I don’t understand, what’s wrong with trying to power up from that source? Anything I can do to prevent this?
You’re meant to power the chip via the Vcc pin, not via an input pin. It’s been a while but I think the official current limit for those protection diodes is 1mA, anything above that and you risk damaging the diode and then the input pin. You could try a series resistor to limit the current into the ADC pin when the AVR is not powered.
Aside from the manufacturer saying “don’t do that,”
here’s some info on why it shouldn’t be done.