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Truck Scale Grounding and Lightning Protection
As a general rule, always strive for a single-point grounding system. Do not drive ground rods at the scale
location and establish a separate earth ground for the scale. This separate earth ground may 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 two grounds, often corrupting data communication
like RS-232 which depends on a stable zero reference.
In addition, a separate earth ground system at the scale can actually invite lightning damage!
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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, possibly damaging load cells en route.
•
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 affected less. That lightning surge will now
take the easiest path to the lower-potential ground—through the scale wiring and back to the scale
house ground, possibly damaging the indicator en route.
Therefore, we believe 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 will be connected to
an existing earth-grounded rod at the service panel of the scale house or other building where the indica-
tor is located. This earth ground must 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 ground tester
that measures zero resistance. If that test determines that the grounding system is inadequate, the utility
company can suggest methods to improve the system. It's crucial that the scale owner authorize and make
the recommended improvements to assure an adequate electrical ground.
An integral part of the load cell lightning protection system is a
grounding strap securely connected to the top plate and bottom plate
of each load cell mount. This strap is designed to channel power
surges on the deck around—rather than through—the load cell to
ground.
A separate grounding system conductor must extend uninterrupted from the main service panel to the
scale to protect load cells and scale wiring from lightning and other transient damage. This ground wire
conductor must be #10 bare copper wire or larger. Run the ground wire conductor unsheathed and intact
from the scale house power earth ground connection underground in a shallow trench to the transient
protection device and the grounding lug within the junction box.
All ground wire connections must be torqued tightly and retightened at regular service intervals. A thick
coating of anti-oxidant grease should be maintained on all ground wire conections for further insurance.
Even the most carefully designed lightning protection system is useless if connections become loose.
Corroded or loose ground wire connections could cause thousands of dollars of load cell damage from a
nearby lightning strike in a thunderstorm.
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