Hi!
1. If ohmic sensor sense dross it will rise (or stop cutting - you have that option too) to avoid collision.
2. On small circles is recommended to be cut with 60% to 100% of cutting speed depending on circle diameter and material thickness. If speed is reduced then metal is removed at a smaller speed so arc voltage value will increase. Torch will lower trying to adjust the arc voltage in search for material. Then collision will issue.
For avoiding that Automatic Voltage Control - AVC can be disabled (is your option) in some cases (small circles, corners with specific angles, lead ins and leadouts, etc). If ohmic sensor will sense contact with the sheet metal while AVC is disabled then it will temporary (until next IHS) set Z0 value and retract very fast at cutting height (usually 1.5 mm).
Hope this helps.