Grounding a CNC plasma system is a necessary precaution you need to take. This will not only prevent shocks, but more importantly, it will reduce or eliminate any EMI (Electromagnetic Interference) noise and RFI (Radio Frequency Interference).
Read on as I walk you through some practices on how to ground a CNC plasma table.
How to Ground a CNC Plasma Table
The EMI and RFI noise interferences can happen due to the information and power passing through the wires or the plasma cutting machine.
You may find yourself using the electrical grounding that came with the system and working just fine, but this isn’t always the case. This isn’t very common since it’s a delicate, case-by-case issue.
The types of noise you can come across are:
- High frequency from HF starts machines
- The noise produced by the plasma power unit
- The noise produced by the Arc itself.
Please note: This is common sense, but it bears repeating, you must disconnect the electrical power cables from the power sockets before grounding the CNC plasma table.
There are three types of grounding:
- Safety or service ground
- Direct current power or cutting the ground
- RFI and EMI shielding and grounding.
Safety or service ground
This system works on the incoming line voltage. It takes care of the service ground coming into the plasma power supply, the CNC controller, motor drivers, and the supplemental ground rod connected to the plasma table.
This way, it keeps a person using the work table or other equipment from any electrical shock hazard.
Direct current power or cutting ground
This method requires the slats to make good contact with the table and the workpiece and a properly sized cable for the positive electric lead from the power supply to be connected to the work table ground bus.
This way, the path of the cutting current from the torch is completed back to the power supply.
RFI and EMI shielding and grounding
This method helps reduce the amount of electrical noise interference produced by the plasma and motor drive systems and the amount of noise received by the CNC and the control measurement circuits.
CNC Plasma Table Grounding and Shielding Practices
Please note: The practices I will list below have been proven to provide good results when it comes to RFI/EMI noise reduction issues and ensuring your machinery lasts longer.
However, as I mentioned above, there are case-by-case deviations. I still recommend implementing these practices during the installation process. Our CNC plasma table is consistent with these practices.
Some practices on how to ground a CNC plasma table:
Use a welding cable
An AWG #6 (16 mm 2) welding cable is the preferred cable you should use for the EMI ground cables unless stated otherwise.
The table should have threaded studs
The cutting table serves as the common, or star, EMI ground point and should have threaded studs set on a copper bus bar that are welded to the table. As near to the driving motor(s) as is practical, a separate bus bar should be installed on the gantry.
A separate EMI ground wire should be run from the far drive motor to the gantry bus bar if there are driver motors at each end of the gantry. A separate heavy EMI ground wire should connect the table bus bar to the gantry bus bar.
The table must have a ground rod
A ground rod in accordance with local and national electrical codes must be installed within 20 feet (6m) of the table. This is a protective earth ground, so an AWG #6 green/yellow grounding wire or an equivalent should be used to connect it to the ground bus on the cutting table.
Preferred cables for best possible shielding
Use CNC interface cables for I/O signals , serial communication signals, multi-drop connections between power supplies, and connectors between all components of the system for the best possible shielding.
Hardware parts for the grounding system
Brass or copper hardware parts must be used for the ground system. The one exemption is that steel studs can be used to install the ground bus on the table after being welded to it. Stay clear of aluminum or steel hardware parts.
Electrical code laws
Local and national electrical codes apply to the AC power, protective earth, and service grounds connected to the equipment.
Arc leads placement
You need to keep the positive, negative, and pilot arc leads as close together as possible. Only if there is a minimum separation of 6″ (150mm) between the torch lead, work lead, and pilot arc (nozzle) leads can they be run parallel to other wires or cables. Run the power and signal lines in separate cable tracks if possible.
A separate ground wire to the bus bar on the cutting table is required for the ignition console, which should be located as close to the torch as possible.
Ground cable connection for each component
Each component needs its own ground cable connected to the common (star) point on the table. The same goes for the ignition console, whether it’s bolted to the power supply or the cutting machine.
Torch connection and protection
The torch must be securely attached to both the ignition console and the metal braided shield on the torch leads. It must be electrically shielded from any metals, contact with the ground, and the structure itself.
Torch, lifter, gantry, and valve assembly connections
A copper braided wire at least 1/2″ (12.7mm) wide must be used to link the torch holder and torch break-away mechanism—the one mounted to the lifter—to the stationary part of the lifter.
The bus bar on the gantry must be connected to the lifter by a separate wire. A separate wire connection to the gantry bus bar is also required for the valve assembly.
When the gantry supporting rails aren’t welded to the table
A ground wire needs to be run from each end of both rails to the table if the gantry is supported by rails that aren’t welded to the table. The shortest route to the table could be used instead of having to go to the common (star) point.
Equipment installed by the manufacturer
The OEM-installed voltage divider board should be placed as near as possible to the arc voltage for use in the control system that samples the arc voltage. A good location for this is inside the plasma power supply.
A twisted, insulated cable should be used to transmit the processed signal. Instead of a foil shield, the cable must have a braided shield. When connecting the shield, make sure to leave the other end disconnected and connect it to the power supply’s chassis.
Connectors for other signals
The analog, digital, serial, and encoder signals should be run in a shielded cable as twisted pairs. These cables should have metal-housing connections, and the shield—not the drain—should be attached to the connector’s metal housing at each cable’s connecting point. Never connect any of the pins with the shield or drain running through the connector.
Source: shutterstock.com / Photo Contributor: Zoran Zeremski
I hope you have found this article helpful and learned how to ground a CNC plasma table.
Grounding and shielding a CNC plasma table is an exhausting process. It takes a lot of planning and preparation. I would advise you to take the time.
Don’t rush through the process to get to know the subject matter a bit better. Then make a plan on how you’ll tackle the issue. These practices are only as good as you implement them.