Hypersensing with THCAD - better way to do ohmic sensing

Attached shows how I have my ohmic sensing power supply connected. 120v AC coming in, no relay, always on. The 24V- side is tied to earth ground. My water table is connected to a star point on the table where I've tied in earth ground.

I recently went through my cabinet and tried to remove all unecessary ground connections in hopes of removing any ground loops I've created. Currently, my V- (common) on my logic 24V psu (separate from the hypersensing PSU) has continuity with earth ground. Should this be disconnected and the common rail be separated? My 48v stepper PSU common is totally separate (or so I think), but I can't quite find where the 24V common is connected at. Could the fact that the 24V common (logic PSU) connected to E.Gnd, which is also where the hypersensing PSU V- is connected to cause for it not to be isolated? I'm still not totally sure how to check for this.

Any help is greatly appreciated. Thanks!

Thanks Peter, Would grounding the plasma cutter chassis to the table help?

My powersupply is an isolation class II device with isolation resistance of 100 M ohms at 500V so I don't think isolation is an issue. See oceancontrols.com.au/files/datasheet/mw/psm-170_HDR-15-SPEC.pdf

I'll have a look later today. At the moment my water tray is empty so I'll do some tests with and without water.

What I did was open halshow and watch the ohmic volts pin for a while while probing manually with my MPG and set the low limit to be a bit above the observed voltage. I think I set it to about 22.5 volts and then set the ohmic on voltage to 23.5 volts.

With those settings, I cut one nest with over 500 pierces and I never had a probing fault.

From my testing, I don't think its unusual for water to get up in the nozzle so it may be bridging the electrodes. Earlier, with a low off threshold, I had more issues like this at higher amperages which I assumed was due to the larger hole diameter in the tip.

Anyway, the good news is we have the ability to dial it on to suit our conditions in HAL.

I would disconnect power and the THCAD and the measure the resistance from the probe to the table

Edit: disregard previous

Turned off all power to everything. Plasma cutter still plugged into wall, but off.

24v isolated psu is unplugged
Thcad-5 input and output connectors, unplugged

Wire on cap, resistance between torch shield and table: 2.3MOhms

Wire removed from cap, torch shield to table, resistance is still 2.3MOhms

Work clamp removed from table, open circuit, infinite resistance

Plasma torch to work clamp resistance is 5.6MOhms

PCW wrote: I would disconnect power and the THCAD and the measure the resistance from the probe to the table

With no water in the tray
Mains disconnected with Ohmic clip connected to the nozzle so THCAD is connected 1.15 M Ohm
With Ohmic clip disconnected from the Nozzle no connection to THCAD - Open circuit no resistance measured.

From LinuxCNC:
probe not touching: 0 volts
probe touching table 24.07 volts

Wet on/off thresholds in use the other day. 23.8V/23.6V

Uthayne wrote: Attached shows how I have my ohmic sensing power supply connected.

Well we both have the same power supply.

I will check my wiring is as I documented later today.

I'm pretty sure the issue is water entering the torch tip and creating a circuit when we don't expect one with a water table. I know I have sometimes cleared a stuck probe (with a low off threshold) by either a Plasmac torch pulse to get air flowing or flicking the airflow switch on my plasma cutter.

The behaviour on a dry table now is exactly how it was when we first put this together.