Try this.
This is what I use.
G00 X? Y? ( to first cut position )
G38.2 Z - ( Z-, the amount of travel needed to activate switch )
G92 Z0.0 ( temp zero )
M667 F ( THC start plus F for nominated Z axis speed when in THC mode )
M03 ( Plasma on / relay )
M08 ( if another relay is needed )
G01 X? Y? F ( cutting speed )
M666 ( THC off )
M05 ( Plasma off )
M08 ( relay off )
G00 Z ( safe height )
G00 X0 Y0 ( return to start position )
Cheers.