I found this post interesting in light of this thread. It was posted by someone from Hypertherm.
I can't vouch for its accuracy, but high frequency energy will be impacted by the presence or absence of a local ground rod. So if a system generates lots of HF or radio frequency energy, the grounding conductor back the the service panel likely has enough inductance to present a high impedance to that energy. YMMV
In actuality, ground rods are not generally required by the plasma manufacturer as our plasma cutters are designed to operate under extremely noisy electrical conditions. Older technology air plasma systems as well as the latest technology industrial plasma cutting systems use a high voltage, high frequency starting technology to ionize the plasma gas inside the torch, starting the plasma process.
This high frequency start pulse can easily couple through lower voltage control cables (computer cables, drive system cables, etc.), so earth grounding systems were devised in order to lead stray voltages away from components (cnc controls, drive electronics, even some height control electronics)and direct these voltages to earth through proper design of shielded cables and chassis grounding connections. On industrial high frequency start systems of older design with older technology cnc controllers the grounding schemes were complex and critical in order to keep the start pulse of the plasma from affecting sensitive electronics.
Today's industrial plasma cnc machines all use industrial hardened cnc controls and electronics (no PC's or Laptops) that have filtration, optical isolation as well as digital isolation of virtually all wiring and signals.
Fast forward to the relatively low cost "Entry Level" and "Light Industrial" plasma cnc machines that have been available for the last 12 to 15 years. These machines (for the most part) use standard office computers or laptops as the cnc control (instead of the purpose built industrial CNC's used on industrial machines). At about the same time these PC based machines were being introduced, plasma manufacturers developed a starting technique called "Blowback" that uses a short circuit arc inside the torch to ionize gases (instead of the high frequency).....which certainly minimized electrical noise at the beginning of each start cycle. This, in many cases allows the use of a PC or Laptop, sensitive electronics, a plasma cutter on systems with no complex earth grounding schemes. Many of these machines operate day to day without earth grounding without issues.
Keep in mind that there still is electrical noise generated on todays low cost cnc machines....as Tom said, the plasma cutters are now inverter based. These systems operate by producing Pulse Width Modulated outputs at fixed frequencies and varying on/off duty cycles to control torch cutting current. PWM modulated power can couple to sensitive, low voltage cables, and occasionally does have an effect on the operation of these systems. Just like the plasma, the stepper and servo drivers also produce PWM power to control the speed and torque of the drive motors.....these cables can also produce energy that can couple to sensitive cables within the electronics and PC or Laptop. In many cases, even with a Blowback start plasma there have been incidents involving electrical noise that locked up the PC (Laptop) and disrupted cutting with these machines, and often the fix is to install an earth driven ground and improve the cable layout and overall grounding scheme of the cutting machine components.
Safety? I disagree for the most part with Tom that the Earth ground is there for safety. We (Hypertherm) have over 300,000 hand held plasma cutters operating in the world. None of these units require an earth driven ground. We, of course, do not recommend holding the torch in one hand and the work cable in the other, and we certainly recommend following normal safety procedures, but earth grounding is definitely not a requirement for safety with these systems. I have used hand plasma cutters for over 35 years in virtually every possible environment, including laying on my back on wet ground under a car cutting. Never once even felt a tingle.
My opinion? Every cnc machine should be installed with an earth driven ground rod. I don't care if it is a lathe, a mill, a router or a cnc plasma cutter. Especially vulnerable are cnc machines that use a standard office PC or a laptop as the cnc control. In my experience the earth driven ground is an easy deterrent to future problems.
Jim Colt Hypertherm
