Tweco 300 Auto-Cut - Manuel (Page 34)

Manual No. 0-4817 Auto-Cut 300 3-10 INSTALLATION
3.06 Ground Connections
A. Electromagnetic Interference (EMI)
Pilot arc starting generates a certain amount of electromagnetic interference (EMI), commonly called RF noise.
This RF noise may interfere with other electronic equipment such as CNC controllers, remote controls, height
controllers, etc. To minimize RF interference, follow these grounding procedures when installing mechanized
systems:
B. Grounding
1. The preferred grounding arrangement is a single point or “Star” ground. The single point, usually on the cutting
table, is connected with 1/0 AWG (European 50 mm 2 ) or larger wire to a good earth ground (measuring leass
than 3 ohms; an ideal ground measures 1 ohm or less. Refer to paragraph ‘C’, Creating An Earth Ground. The
ground rod must be placed as close as possible to the cutting table, ideally less than 10 ft (3.0 m), but no more
than 20 ft (6.1 m) from the cutting table.
NOTE
All ground wires should be as short as possible. Long wires will have increased resistance to RF
frequencies. Smaller diameter wire has increased resistance to RF frequencies, so using a larger
diameter wire is better.
2. Grounding for components mounted on the cutting table (CNC controllers, height controllers, plasma remote
controls, etc.) should follow the manufacturer’s recommendations for wire size, type, and connection point
locations.
For Thermal Dynamics components (except Arc Starter and Gas Control Module) it is recommended to use a
minimum of 10 AWG (European 6 mm
2 ) wire or flat copper braid with cross section equal to or greater than 10
AWG connected to the cutting table frame. The Remote Arc Starter uses 1/0 earth ground wire and the Gas
Control Module should use minimum # 4 AWG wire. The connection point must be to clean bare metal; rust and
paint make poor connections. For all components, wires larger than the recommended minimum can be used
and may improve noise protection.
3. The cutting machine frame is then connected to the “Star” point using 1/0 AWG (European 50 mm 2 ) or larger
wire.
4. The plasma power supply work cable (see NOTE) is connected to the cutting table at the single point “Star” ground.
NOTE
Do Not connect the work cable directly to the ground rod.
5. Make sure work cable and ground cables are properly connected. The work cable must have a solid connection
to the cutting table. The work and ground connections must be free from rust, dirt, grease, oil and paint. If
necessary grind or sand down to bare metal. Use lock washers to keep the connections tight. Using electrical
joint compound to prevent corrosion is also recommended.
6. The plasma power supply chassis is connected to the power distribution system ground as required by electri-
cal codes. If the plasma supply is close to the cutting table (see NOTE) a second ground rod is not usually
needed, in fact it could be detrimental as it can set up ground loop currents that cause interference.
When the plasma power supply is far away from the ground rod and interference is experienced, it may help to
install a second earth ground rod next to the plasma power supply. The plasma power supply chassis would
then be connected to this ground rod.
NOTE
It is recommended that the Plasma Power Supply be within 20 - 30 ft (6.1 – 9.1 m) of the cutting table,
if possible.
7. The plasma control cable should be shielded with the shield connected only at the cutting machine end.
Connecting the shield at both ends will allow ground loop currents which may cause more interference than with
no shield at all.