Tweco 100 Auto-Cut - Manuel (Page 33)

Manual 0-4764 3-9 INSTALLATION
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.
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 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.
To test for a proper earth ground, refer to the following diagram. Ideally, the reading on the multimeter should
be 3 VAC for 115VAC line, or 1.5 VAC for 230 VAC line.
CAUTION
Remove all other connections from the ground rod during this test.
This test assumes the 115 or 230 VAC source neutral is connected to the utility earth ground.
Art # A-07252
WARNING
Use extreme caution. This
test uses live voltage.
NEUTRAL
Earth Grounded
HOT
On 115V AC Line:
3VAC = 3 ohms
1VAC = 1 ohm
On 230V AC Line:
1.5VAC = 3 ohms
0.5VAC = 1 ohm
Ground Rod
with other
connections removed
100W Incandescent
light bulb*
* Can replace light bulb with
a 100w resistor.
Use 100 ohm for 115VAC.
Use 500 ohm for 230VAC
NEUTRAL
PROBE
VAC
HOT
PROBE
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.
D. Routing Of Torch Leads
1. To minimize RF interference, position torch leads as far as possible from any CNC components, drive mo-
tors, 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.