[quote user="Jens-Michael Gross"]ztener diode parallel to the 100Ohms resistor.[/quote]
If this is to be used in various types of installations which are not installed/controlled by hardware vendor then I suggest to use surge protection devices. Copper Ethernet surge protector like TVS3V3L4U (but not limited to) can do the job. Series resistor between 100 ohm and ADC input is highly recommended because high current surge events can cause clamping voltage jump above VCC of the uC. You don't want 4V & 20A spike into clamping diode of ADC pin, right?