Hallo Michael,

what a nice reply!
There are some websites treating these spark gap designs. I have seen one for use with 74HCMOS, connecting spark gap to input or output just by a 100 Ohm carbon mass resistor. Input and output protection circuitry of 74HCMOS devices are so fast acting and robust, that they easily withstand ESD events with this simple measure!

As a rule of thumb 1000V per milimeter is necessary to make air conducting. So, 200V breakthrough voltage as you stated underlines this.
But, what about turn-on time? It takes some time before air is conducting and ESD energy can flow to ground. Have you made any measurements? Would be highly interested in. I think, a very fast scope should be used...

Nevertheless, it seems to me, that a succeeding low pass filter is necessary in most cases.

Thanks again for your profound information,

have a nice day,

Kai