I built a CNC plasma table several years ago and have been using it as/when required. It's a pretty basic setup, Mach3 on a Win XP Dell PC with an added PCI parallel port, 2 x BOB's, 4 x HaoyuTB6600 drivers, 270oz-in steppers, T5 belt drive on X & Y axis and leadscrew on Z. I have a 50mm deep water tray with steel slats in it. My water tray is sitting on a sheet of MDF and it's electrically isolated from the table frame and the electrical enclosure. Whenever I'm cutting, I always connect the plasma earth lead directly to the steel being cut and everything works well, good quality cuts, no electrical interference, etc.
Some time later, I decided to install a THC and built an open source Arduino THC from here: https://github.com/regeg/ArdunioTHC
This THC is designed for use with the Everlast Power Plasma 50 which has a CNC port that gives you full arc voltage via an internal 100K resistor. The THC has a pin you can use to monitor this voltage, and when doing the initial testing, I saw that the voltage drops quite low immediately after cutting begins. It starts at about 130v and drops down to around 30v shortly after the pierce. This causes the THC to lift the torch and obviously the arc goes out.
I took the plasma cutter off the table and used it manually to cut some steel. I used my multimeter to check the DC volts at the CNC connector while cutting. All was good, it cut perfectly and the voltage remained around 120v during the cut. This tells me there is nothing wrong with the plasma cutter.
I then installed it back on the table, but completely removed the THC. Again I used my multimeter to check the DC volts at the CNC connector while cutting, and now the voltage dropped just like it did earlier. The cuts were still great but happened at about 30v. This tells me the THC is not causing this problem.
I then discovered that if I elevate the piece of steel that I'm cutting off the slats by about 40mm, then the voltage does not drop, the THC works perfectly and controls the Z axis to +/- 1 volt. If however a cut piece only partially falls down and makes contact with the slats, the plasma voltage instantly drops.
The interesting thing is that my slats and water tray are sitting on a sheet of MDF and are electrically isolated from the table frame and the electrical enclosure. My electrical enclosure is metal, and everything inside is earthed to one spot. The power lead coming in is also earthed to the same spot (obviously not the live or neutral wires, just the earth). I have an earth cable that runs from that spot and connects to my gantry, then to my Y car and even my Z axis plate. I have tried removing that earth cable but it made no difference.
I wondered if the water tray being electrically isolated from the table frame and the electrical enclosure was somehow causing this problem, so I tried hooking up an earth strap from the slats to the earth spot inside the enclosure - this made no difference at all.
As a last resort, I drove a copper rod deep into the ground beside the table and ran a thick (car battery type cable) to the slats, and also to the electrical enclosure, but this also made no difference.
Does anyone have any suggestions?