Is it OK to connect ESD plug through an RCD?
Don't quote me but.... an RCD doesn't measure earth current directly - it infers it by measuring the "difference" current between the two AC wires. This difference current has to be an earth current whether it's thru the earth pin or thru the guy's hand who's building a big spark generator (related to a different question!!!).

This means it doesn't alter the earth impedance in any way