Install Cabling in Conduit
13
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 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 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 two-year lightning protection package comes standard on all SURVIVOR
®
PT Pit-Type
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.
NOTE:
Transient protection for additional peripheral devices (except some boards) is not included in the standard transient
protection package. Call the Rice Lake Weighing
S
ystems
S
ales Department at (715) 234-9171 for additional transient protection
for these devices.
