Emerson Process Management ControlWave EFM 3808 - Manual (Page 287)

Section 5 - Wiring Techniques Page 5-3 S1400CW
5.2.3 Grounding of Cable Shields
DO NOT connect the cable shield to more than one ground point; it should only be grounded
at one end. Cable shields that are grounded at more than one point or at both ends may
have a tendency to induce circulating currents or sneak circuits that raise havoc with I/O
signals. This will occur when the ground systems associated with multipoint connections to
a cable shield have a high resistance or impedance between them and a ground induced
voltage is developed (for what ever reason, i.e., man made error or nature produced
phenomena).
5.2.4 Use of Known Good Earth Grounds
ControlWave units should only have one connection to earth ground. For ControlWave
and ControlWave MICRO Process Automation Controllers, ControlWave MICRO,
ControlWave EFM Electronic Flow Meters, ControlWave GFC/XFC Gas Flow Computers
and ControlWave I/O Expansion Racks, this connection is provided via the Ground Lug
that is situated on the bottom of the unit. ControlWaveLPs require the installation of a
ground lug, ground bus or ground plate/panel. Since ControlWave units are DC-based
systems, grounding does not take into account AC power grounding considerations. Earth
grounding the unit is absolutely necessary when the unit is equipped with a radio or
modem. Additionally these units should be connected to earth ground when they are
installed in areas that have frequent lightning strikes or are located near or used in
conjunction with equipment that is likely to be struck by lightning or if struck by lightning
may cause equipment or associated system failure. Earth Grounds must be tested and must
be known to be good before connecting the ControlWave. Earth grounds must be
periodically tested and maintained (see Section 4).
5.2.5 Earth Ground Wires
Earth connections must utilize smoothly dressed large wire. Use AWG 3 or 4 stranded
copper wire with as short a length as possible. Exercise care when trimming the insulation
from the wire ends. Twists the strands tightly, trim off any frizzes and tin the ends with
solder. The earth ground wire should be clamped or brazed to the Ground Bed Conductor
(that is typically a standard AWG 0000 copper cable. The earth ground wire should be run
such that any routing bend in the cable is a minimum 8-inch radius above ground or a
minimum 12-inch radius below ground.
5.2.6 Working Neatly & Professionally
Take pride in your work and observe all site and maintenance safety precautions. After
properly trimming the stranded pair wire ends, twist them in the same direction as their
manufacturer did and then tin them with solder. Install the tinned wire end into it’s
connector and then secure the associated connector’s clamping screw. Remember to check
these connections for tightness from time to time. If solid copper wire is used (in
conjunction with the DC Power System or for Earth Ground) make sure that the conductor
is not nicked when trimming off the insulation. Nicked conductors are potential disasters
waiting to happen. Neatly trim shields and whenever possible, coat them to protect them
and prevent shorts and water entry.