In plasma, you don't set the Z switch as a probe. You should only have one switch that trips as the floating head moves up, after the torch comes in contact with the work.
Set the switch input as the Z Home.
When the Z Home switch trips, the action causes the Z axis to reverse direction, and move UP until the Z Home switch opens.
At this point, the Z axis is set to Z0.
The distance from the initial point of contact when the torch touched the work surface, and the distance the Z axis moved to set the "Z" Zero is the "OFFSET".
You can determine what this offset dimension is manually.
Go to the Mach3 diagnostic page and jog the Z until the torch tip is close to the work surface. Set the jog steps to .001, and begin jogging down until the Z home indicator LED turns on.
Set the Z axis to 0.000
Now begin jogging UP until the torch head is just barely clear of the work surface. You can determine this using a piece of paper.
Write down the distance the Z axis moved.
This is the "Offset" distance for your floating head. This offset is entered into the Plasma cutting program, so the machine "knows" how far to move vertically, to set your programmed cutting height, and the pierce height.
John