I'm designing a circuit to detect presence of AC line voltage. It's actually a very simple device with AC side made up of a 200Kohm 1W resistor and an optocoupler and a protection diode. There will be sufficient gap between AC line side and output of the optocoupler. Entire board will then be covered with resin or silicone compound to permanently secure it inside a case. I am having hard time to interpret clearance distance in this case. Do we consider ONLY air gap when determining clearance distance? Would it be reduced if we have filler like in this case?