Tweco 1000 Merlin(Dec2004) - Manual (Page 29)

Manual 0-2708 3-11 INSTALLATION
3.10 Ground Connections For
Mechanized Applications
A. Electromagnetic Interference (EMI)
Pilot arc initiation generates a certain amount of electro-
magnetic interference (EMI), commonly called RF noise.
This RF noise may interfere with other electronic equip-
ment such as CNC controllers, remote controls, height con-
trollers, etc. To minimize RF interference, follow these
grounding procedures when installing mechanized sys-
tems:
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 (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).
NOTE
All ground wires should be as short as possible.
Long wires will have increased resistance to RF fre-
quencies. Smaller diameter wire has increased re-
sistance to RF frequencies, so using a larger diam-
eter wire is better.
2. Grounding for components mounted on the cutting
table (CNC controllers, height controllers, plasma re-
mote controls, etc.) should follow the manufacturer’s
recommendations for wire size, type, and connection
point locations.
For Thermal Dynamics components 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
connection point must be 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 con-
nected. 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 pre-
vent corrosion is also recommended.
6. The plasma power supply chassis is connected to the power
distribution system ground as required by electrical 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 chas-
sis 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.
C. Creating An Earth Ground
1. To create a solid, low resistance, earth ground, drive a
1/2 in (12 mm) or greater diameter copper clad ground
rod at least 6 - 8 ft (1.8 - 2.4 m) into the earth so that the
rod contacts moist soil over most of its length. Depend-
ing on location, a greater depth may be required to
obtain a low resistance ground (see NOTE). Ground
rods, typically 10 ft (3.0 m) long, may be welded end to
end for greater lengths. Locate the rod as close as pos-
sible to the work table. Install a ground wire, 1/0
AWG (European 50 mm
2 ) or greater, between the
ground rod and the star ground point on the cutting
table.
NOTE
Ideally, a properly installed ground rod will have
a resistance of three ohms or less.