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
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.
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