general question ;
what is your rule of thumb about a single resistor in a high voltage application ? at what point to you address flashover ? before you start using either a resistor designed for the voltage or start to use resistors in series ?A simple 1/4 watt carbon 1meg resistor has a voltage limit of about 150v.
+1
A very important point ... most resources don't mention this (even Atmel) and its something I overlooked mentioning. Even though the power dissipation is well below 1/4 watt, the resistors I use (@120V) are PR03 type which are 3W, 750V AC/DC max, 2000V max peak pulse voltage.
A good consideration is "will the line circuit pass a high pot test?" On the line side, the voltage used in a typical high pot test is determined by 2 x U + 1,000. So, to pass this test for a 220V circuit, 1440V would be applied for a period of time. The rules may differ depending on the safety agency.