Rice Lake AX 3040 - Manual (Page 13)

J-Box Connections 9
3.3 Electrical Ground Connections
Important
Improper grounding systems on axle scales often cause corrupted data from ground-loop current flows
and costly lightning damage to electronics.
Always strive for a single-point grounding system. Do not drive ground rods at the scale location to establish
separate earth grounds for the scale. These separate earth grounds do not share the same zero reference as the
existing earth ground for the AC power system. This difference in electrical potential invites ground-loop current
flow between the separate grounds, often corrupting serial data like RS-232 which depends on a stable zero
reference.
In addition, a separate earth ground system at the scale can actually invite lightning or power surge damage:
• A minor powerline surge in the scale house electrical supply should immediately be shunted to ground. If a
separate ground system exists at the scale with a lower potential than the main ground, the surge may travel
out to the scale ground rod, damaging load cells on its way.
• A nearby lightning ground strike may instantly raise the zero potential of a ground rod at the scale location,
while leaving the scale house ground rod unaffected less. That lightning surge now takes the easiest path to
the lower-potential ground—through the scale wiring and back to the scale house ground, possibly
damaging the indicator on its way.
Therefore, the best grounding system for the scale is the same grounding system used for the incoming AC power
system. The 120 VAC power source used to power the indicator is connected to an existing earth grounded rod
system at the scale house or other building where the indicator is located. This should consist of a double ground
rod system of two 5/8" x 8' copper rods driven 8' deep at the service entrance where the local utility company
brings their lines into the building.
The local utility company can test the resistance of the existing ground rods with a clamp-on megohmeter that
measures zero resistance. A reading of 3¾ or less is acceptable as a ground. If the test determines that the
grounding system is inadequate, the utility company can suggest methods to improve the system. It is crucial that
the scale owner authorize and make the recommended improvements to ensure an adequate electrical ground. Do
not connect the scale to the AC power supply until the grounding system is adequate.
Be certain each load cell grounding strap is securely connected to the top plate and bottom plate of each load cell
mount. Some models have ground straps included to install between modules if more than one segment is used.
These inter-module straps ensure that the entire scale is connected to the same single point ground. There should be
metal-to-metal contact with no presence of paint or grout. This strap is designed to channel power surges on the
deck around—rather than through—the load cell to ground. These, and all, ground connections must be torqued to
a specified value and retorqued at regular service intervals. A thick coating of anti-oxidant grease should be
maintained on all ground connections to prevent corrosion.
A separate grounding system conductor must extend uninterrupted from the main service panel ground to the scale
to protect load cells and scale wiring from lightning and other transient damage. This ground wire conductor must
be an unsheathed #10 copper wire or larger. Run the bare ground wire conductor intact from the AC power ground
rod to the scale in a separate trench. Bring the wire up from the trench near the J-box and attach it to the ground lug
located in the J-box pocket. This grounds the scale frame to the same single-point ground as the AC power for the
indicator.
A ground wire is included to attach the J-box ground lug to the ground lug located in the J-box pocket. Grounding
of the J-box is essential for operation of the DC transient protection incorporated into the J-box.