I want to find out why, in professional PCB's, they separate between the AC (220 VAC) points on the PCB with a hole in the PCB:
Is it for more isolation between the AC lines?
Is leaving a distance between the two AC input points on the PCB not sufficient?
I'm making a board that needs to stay connected to AC permanently, and I'm using a Varistor for more protection from AC spikes, is this hole urgently required for me to make?
Here are images for the hole I mean:
Yes, it's for a sort of clearance.
Depending on the voltage and the environment (eg. dust, humidity) for safety (and safety agency approvals) you may need clearance across surfaces that is in the 8mm or even greater range. That's inconvenient in a 2-dimensional design like a PCB (on a barrier strip or switch, you can have a 3D barrier between terminals that increases the clearance across the surface).
The name for the linear clearance distance on the surface is "creepage" distance.
This is just addition to the other answers.
Generally these things will be used in power meter designs between phase to neutral or phase to phase etc, which is useful during Surge, ESD events.
This diagram also gives explanation of creepage and clearance. creepage generally used in case of opto-couplers also.
Generally we will use air gaps in PCB's if isolation provided by PCB gap is not sufficient. We have used rough calculations, like at room temp and humidity let's say 1mm of air gap in PCB will sustain 1-1.5KV of voltage (I missed that reference which lists a graph which gives rough calculations about PCB gaps and air gaps), then based on IEC standard which are we going to adhere for testing, we will set the clearance. We set them approximately.
