Tweco 400 Ultra-Cut(October 2014) - Manual (Page 35)

ULTRA-CUT 100 XT/200 XT/300 XT/400 XT
Manual 0-5264 INSTALLATION 3-9
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 possible
to the work table. Install a ground wire, 1/0 AWG (European 50 mm2) 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.
D. Low Cost Ground Rod Tester
1. A key component of reduced EMI is a good low resistance earth ground rod. There are several very expensive
instruments to measure the ground but cost from several hundred to a few thousand dollars. Below is a low
cost alternative which can be constructed by qualified personnel familiar with established electrical construction
and safety practices. Previously suggested method using an incandescent light bulb will not work with GFCI
outlets which are increasingly being used and the bulbs are becoming obsolete.
2. This method, as well as the light bulb method and some of the expensive instruments, assumes the utility ground
is perfect, Zero ohms. It connects the rod being tested in series with the utility ground and measures the resis-
tance of both in series. If the utility ground is not zero ohms, no matter how good your rod is, you won’t get a
low reading due the higher resistance of the utility ground. Fortunately this is rare. Also if your rod is right next
to another earth grounded structure you may get a false lower reading of only resistance between that structure
and your rod rather than to gnd.
NOTE
In the United States most standard AC outlets are 120 VAC 60 Hz. Elsewhere most outlets are 220
VAC 50Hz.
3. Obtain a transformer rated for at least 25 VA with primary voltage and frequency matching your standard outlets.
The transformer should have an isolated secondary of either 220 VAC (220 -240 is OK) or 120 VAC (110-120 is
OK) and be rated for at least 100 ma. The transformer could also have dual 115VAC primaries wired in series
for 220V or in parallel for 120 VAC. An example is Triad N-68X, shown below, rated 50VA, 50/60 Hz.
Obtain a power resistor of either 1200 (1.2K) ohms, 15-25W min, if using a 120V secondary or 2200 (2.2K) ohms,
25 -30W for a 220V secondary.
4. Assemble the transformer and power resistor in a metal box. Connect a 3 wire (w/gnd) power cord with ground
wire attached to the metal box for safety. If a plastic box is used instead, connect the transformer core and the
resistor mounts to the power cord ground wire. There should be a fuse ¼ - ½ A, in series with the transformer
primary. From the transformer secondary connect one wire to the utility safety ground. This could be the cutting
table frame, the ground terminal of the 120 or 220 VAC outlet or the test box if grounded as indicated.
An excellent ground measures 1 ohm or less. Up to 3 ohms is often acceptable, higher reduces the effectiveness
of the EMI suppression.
0.1 VAC = 1 OHM,
0.3 VAC = 3 OHM,
etc.
Triad N-68X
Utility (building) GND
115
VA
C
220 VAC
GND
Triad N-68X
F
F
115
VA
C
115
VA
C
115
VA
C
115
VA
C
GND
120 VAC
R = 1.2K, 15W
(2.2K, 25W for 220 VAC)
Ground Rod with other
connections removed