Rice Lake SURVIVOR PL Rail Scale - Manual (Page 13)

Junction Box and Grounding 9
3.2 Junction Box Connections
Each junction box is large enough to hold the summing board, transient protection devices, packaged desiccant,
and extra load cell cable coiled inside the enclosure. An industrial corrosion inhibitor and desiccant such as the
RLWS Industrial Corrosion Inhibitor (PN 16037) should be added to the junction box enclosure before final
closure.
A summing card mounted within the junction box is used to make all cable terminal connections. All terminal pins
are clearly marked as to function.
3.3 Junction Box to Indicator
Sixty feet of six-wire homerun cable is supplied for wiring the junction box to the indicator. It is run in 3/4"
galvanized metal conduit from the junction box to the indicator. Conduit for this purpose is to be obtained locally.
A 30" flexible conduit section and conduit connector is provided where this cable exits the junction box. Do not run
any other electrical cables in or near the conduit to the indicator.
3.4 Indicator to Peripherals
All 3/4" conduit for cabling from the indicator to remote displays and other peripheral devices is to be obtained
locally. Conduit runs may be buried in a trench or secured above ground. Use separate conduit runs for AC power
and DC data lines to avoid interference. As a general guideline, run AC and DC cables in separate trenches if
possible. When DC data cables must run in the same trench as AC power lines, separate cables as much as possible
(preferably more than 34" apart).
3.5 Single-Point Ground Conductor
A bare 10 gauge solid wire is to be run from the scale frame to the grounding lug on the junction box then
underground to the main AC power earth ground. If the DC transient protection board is installed, the ground
conductor should also be connected to the transient protection board’s ground lug.
3.6 Electrical Ground Connections
Important
Improper grounding systems are often a cause of 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 which
establishes separate earth grounds for the scale. These separate earth grounds will 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 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. 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 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 will be 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’s 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.