Tweco Grounding For Mechanized Systems User Manual

Grounding For

Mechanized Systems

In Section 3 Of Operating Manuals

NOTE
This information supersedes any
previous information supplied in
Thermal Dynamics Manuals.

Date: February 19, 2004

3.10 Ground Connections For
Mechanized Applications

A. Electromagnetic Interference (EMI)

Pilot arc initiation 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. T o 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 mm2) 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
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 it is recom­mended to use a minimum of 10 AWG (European 6
mm2) 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 mm2) or
larger wire.

4. The plasma power supply work cable (see NOTE) is

connected to the cutting table at the single point “Star”
ground.

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 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
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.

C. Creating An Earth Ground

1. To cr eate 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.
Depending 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 possible to the work table. Install a ground
wire, 1/0 A WG (European 50 mm2) or greater , between
the ground rod and the star ground point on the cut­ting table.

NOTE

Ideally, a properly installed ground r od will have a
resistance of three ohms or less.

NOTE

Do Not connect the work cable directly to the
ground rod.

T o test for a pr oper earth ground, refer to the follow­ing diagram. Ideally, the reading on the multimeter
should be as follows:

• For 115VAC: 3.0 VAC

• For 230VAC: 1.5 VAC

CAUTION

No other connections should be made at the ground
rod being tested.

This test assumes the 115 or 230 VAC source neu­tral is connected to the utility earth ground.

115VAC: 3.0 VAC
230VAC: 1.5 VAC

~

V

Meter set to
VAC setting

~

V

D. Routing Of Torch Leads

1. To minimize RF interference, position torch leads as
far as possible from any CNC components, drive mo­tors, control cables, or primary power lines. If cables
have to pass over torch leads, do so at an angle. Do
not run the plasma control or other control cables in
parallel with the torch leads in power tracts.

2. Keep torch leads clean. Dirt and metal particles bleed
off energy, which causes difficult starting and in­creased chance of RF interference.

WARNING

Use extreme caution. This

test uses live voltage.

VR COM A

_

+

Neutral

115 or 230VAC

Line (Hot)

Art # A-02971

100W

Light Bulb

Ground Rod

Ground Testing

2. Increasing the ground rod length beyond 20 - 30 ft
(6.1 – 9.1 m) does not generally increase the effective­ness of the ground rod. A larger diameter rod which
has more surface area may help. Sometimes keeping
the soil around the ground rod moist by continuously
running a small amount of water into it will work.
Adding salt to the soil by soaking it in salt water may
also reduce its resistance. When these methods are
used, periodic checking of the ground resistance is re­quired to make sure the ground is still good.