Ah, but what I mean is this...
Set volts to say 90v because thats where you reckon it should be...
Start the cut and the THC reads 98v so tells Mach "Send the torch down.."
Mach drops the torch but as it so close to the surface the volts only reach say 94v,
The THC shouts "I want more, drop the torch again..."
Mach says whatever and this loop continues with the Z dropping but the volts not changing anymore because they can't.
Sounds spurious but on thin material with the 30A consumables its very easy to do.
Mach or the THC should see the repeated DOWN calls AND the fact that volts is NOT changing and say "I'm not allowing that anymore", maybe even lift the Z a mm or so. 
Found it, it does exist, its in the Mp3000 settings screen and is what CandCNC calls Tip-Saver. It (should) work if you set it to a value that keeps the torch just off the plate, so if normal volts is 90, the tip-saver is set for 2% this means once it reaches 88.2v it will stop further downward movement. Of course you have to know what value will keep the torch off the plate but it sounds good.
I have no idea what my one is set for but I know for a fact that its not as low as 2% so will be playing with that I think.
There is also max and min volts limit which i think I have set at 200 & 50 but no idea what the actual max and min cut voltages are for the PM45??