Rice Lake SURVIVOR PT Series - Manual (Page 21)

Cable Installation 17
5.6 Electrical Ground Connections
Improper grounding systems on outdoor truck scales often cause corrupted data as a result from ground-loop
current flows. Improper grounding systems may also invite possible lightning strikes and cause damage to
costly electronic systems. Always attempt to establish a single-point grounding system.
When establishing a ground system for the scale installation, do not install more than one ground rod for each scale
location. Separate earth grounds do not share the same zero reference as the existing earth ground for the AC power
system. The difference in electrical potential invites ground-loop current flow between the separate grounds, often
corrupting serial data, such as 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 to
the scale ground rod, damaging load cells on its way.
• A 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. A lightning surge takes the easiest path to the lower-potential
ground: through the scale wiring and back to the scale house ground, therefore possibly damaging the indicator
on its way.
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 ground rod system at the
scale house or other building where the indicator is located. This 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
to measure resistance. A reading of 3 3/4" 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 assure an adequate electrical ground. Do not
connect the scale to the AC power supply until the grounding system is adequate.
Each load cell grounding strap is securely fastened to the top and bottom plates of each load cell mount. The
grounding strap is designed to channel power surges on the deck around—rather than through—the load cell to
ground. All ground connections must be properly torqued and rechecked at regular service intervals. After
connecting to bare metal apply a thick coating of anti-oxidant grease 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. As a minimum requirement for
ground wire installation, the conductor must be at least a 10-gauge unsheathed copper wire. 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 of the junction box. Use a short piece of 10-gauge wire
to ground the scale frame by running a ground wire from the junction box ground lug to a junction box mounting
bolt on the scale frame.
5.7 Install Transient Protection
An five-year lightning protection package comes standard on all SURVIVOR
®
920i truck scales. This package is
designed to protect the AC and DC portions of the system. The package includes:
• A DC transient protection board mounted within the junction box. This DC transient protector can
accommodate up to eight load cells and will help protect serial communication lines. Scales with more than
eight load cells require a DC transient protection board in each junction box.
• A self-contained DC transient protection unit in the homerun cable at the indicator.
• A 10-gauge bare ground conductor cable. This conductor cable, to be buried underground, runs from the scale
frame to the junction box DC transient board, to the indicator DC transient board, and finally to the AC power
ground lug.
• A 120 VAC uninterruptable power supply/surge protector in the AC line before the indicator.
Transient protection for additional peripheral devices (except some boards) is not included in the standard
transient protection package. Call the Rice Lake Weighing Systems Sales Department at (715) 234-9171 for
additional transient protection for these devices.
Note