ERICO Practical Guide to Electrical Grounding - Manual (Page 83)

75 Chapter 4: Transients & Other High Frequency “Grounding” & Bonding
ELECTRONIC GROUNDING DETAIL
When a metallic mesh is embedded in the structural
concrete subfloor, it may be used for an electronic signal
reference grid (SRG). When this is done the problem
becomes how to make connections to the SRG. The typical
approach is to embed a ground plate at each intersection of
the SRG’s conductors, but on a spacing of around 2x2 or
4x4 feet square. This conforms to the standard practices for
SRGs such as those used in conjunction with a computer
room’s cellular raised floor. In other cases where
connection points on 2-foot centers are not needed, a
ground connection plate per Figure 4-3 may be installed
wherever necessary. The exposed surface of the embedded
stud or tie-plate is then used to make connections to and
from the SRG that is below the surface of the concrete.
Note that for electrical equipment and mechanical
equipment rooms, spacings of 2x2 feet are often closer than
is needed— especially if the floorplan is known in advance.
In these cases the concrete embedded SRG studs or ground
plates are installed to place them close to the equipment that
is planned to be permanently installed in the room.
Spacings of around 4 to 6 feet square are common in these
kinds of cases.
The concrete floor embedded SRG is often combined with
the steel reinforcing bar system that is installed in the
poured concrete. In some cases where the reinforcing steel
system is welded together, it can serve as the actual SRG,
otherwise the reinforcing steel is simply periodically
welded to the SRG at those points where the two structures
have nearby or intersecting elements.
SOME FURTHER THOUGHTS ON
NETWORKED WORKSTATION
EQUIPMENT
Workstations that are part of a network and use Local Area
Network (LAN) interface plug-in cards or modems, or are
connected to servers, printers, or similar peripheral devices
that are not located at the workstation, typically need
special attention to be paid to how they are grounded so that
common-mode noise will not be a significant problem with
their operation. Accordingly, here are some suggestions:
(1) Provide an externally applied supplementary
equipment grounding conductor network that is
connected to each item of the workstation and to the
“greenwire” of the branch circuit(s) serving the
workstation.
(2) If there is any excess length in the ac power line
cords or data signal cables used to connect the
workstation’s equipment to the branch circuit or
network’s signal circuits, loop the excess into a
small coil whose loops are secured by tie-wraps or
plastic electrical tape. This creates a “choke” effect
that can reduce the higher frequency common-mode
noise currents in the path to which the technique is
applied, and without affecting the power or signal
transport process. Observe bending radius limits of
conductors to avoid overstressing the insulation or
causing excessive heat rise.
(3) Electromagnetic Interference (EMI) in the form of
coupled radio waves into signal cables, is not a
common problem in most installations. However, it
is not an unknown problem either, especially if the
source of the EMI is located close to the affected
cable and its served circuits. Where interference
with low-level signal processes is encountered and
if traced to EMI at radio frequencies such as from a
radio transmitter or some other industrial process
occurring at radio frequencies, additional signal
cable shielding and in extreme cases signal filtering
at the cable’s ends, may need to be provided on the
affected circuits. The application of such filters may
need to be carried out inside of the related
equipment, so close involvement of the equipment’s
original manufacturer (OEM) is very important.
To Equipment
Ground
ERICO
Ground
Bar
ERICO
Mesh
CADWELD
CADWELD
CADWELD
Lugs
CADWELD
Ground
Plate
Assembly
For applications of mesh used as signal reference grid (SRG) embedded in the concrete
floor, a CADWELD Cast Ground Plate is mounted flush with the finished floor and
connected to the mesh. Future equipment is then connected to the ground plate.
Fig. 4-3
Grounding Book 4/14/99 10/5/99 6:02 PM Page 75 (Black plate)