Calculation of relay isolation
A better model of the &quot;isolation&quot; is to view the air_gap as part of voltage_divider.
The other part of the voltage divider is a parallel RC: the total capacitance on the node susceptible to interference, PLUS the resistance of that node.
The resistance defines the corner frequency of the builtin HIgh Pass filter, which causes ZERO RESPONSE AT DC.
At high frequencies, above the HPF corner frequency, your isolation is accurately computed as the ratio of the 2 capacitances.