Rice Lake SURVIVOR OTR Series, Руководство по установке

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SURVIVOR OTR Flattop Truck Scale
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7.4 Electrical Ground Connections
Improper grounding systems on outdoor truck scales often cause corrupted data from ground-loop current
flows and costly lightning damage to electronics.
Always strive for a single-point grounding system ( Figure 7-3 on page 23 ). 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 power-line 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 megohmmeter 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.
Ensure 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 ( Figure 7-2 ). 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 re-torqued at regular service intervals. A thick coating of
antioxidant grease should be maintained on all ground connection.
Figure 7-2. Inter-Module Ground Strap
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 junction box and attach it to the ground lug located in the junction 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 junction box ground lug to the ground lug located in the junction box pocket.
Grounding of the junction box is essential for operation of the DC transient protection incorporated into the junction box.
IMPORTANT