ERICO Practical Guide to Electrical Grounding - Manual (Page 82)

Practical Guide to Electrical Grounding 74
(1). Where possible, cluster the interconnected
electronic equipment into an area that is served by a
single signal reference grid (SRG). If the intercon-
nected equipment is located in separate, but
adjacent rooms, then a common signal reference
grid should serve all the rooms.
(2) Effectively bond each frame/enclosure of the
interconnected equipment to the SRG. In this way,
the SRG acts like a uniformly shared ground
reference that maintains a usefully low impedance
over a very broad range of frequency. Typically,
from dc to several tens of MHz, for example.
(3) Where a work area exists and its PC is connected to
a network, keep all of the work area’s equipment
(e.g., CPU, monitor, printer, external modem, etc.)
closely clustered and powered by a work area
dedicated branch circuit. If it is required to use more
than one branch circuit for the work area’s power,
be sure that both are powered from the same
panelboard. Avoid connecting any other equipment
to the branch circuit(s) used by the work area’s
equipment.
(4) Use fiber optical paths for data circuits. The best,
but also the most expensive solution is to use fiber
optical cables for all data circuits since there can be
no ground loops with these kinds of circuits (or
surge current problems). However, due to increased
initial cost and added complexity, the use of fiber
optic cable circuits is usually (and unfortunately)
viewed as a last resort. Instead, it should be viewed
as an important first strategy that avoids problems
that may ultimately cost more to resolve.
(5) Use opto-isolators which can provide several kV of
isolation for the data path that they are used on.
These are available as add-on data transmission
protocol converters for most popular forms of data
circuits. This is a very useful retrofit option for data
circuits being affected by surges and ground loops.
Surge protection devices (SPD) are also
recommended to be applied to these circuits if
protection from the higher voltages associated with
larger currents is needed.
(6) Other forms of protocol converters can be applied to
standard forms of signal circuits to make them less
susceptible to common-mode noise on grounding
conductors associated with the signal path. For
example, a conversion from RS-232 to RS-422 or
RS-485, etc. should be considered in especially
noisy environments.
(7) Improve the shielding provided for the data signal
cables. Place the cables into well and frequently
grounded metal conduits or similar raceways.
(8) Follow the recommendations for installing signal
cables in IEEE Std. 1100, Recommended Practice
for Powering and Grounding Sensitive Electronic
Equipment (e.g., the Emerald Book).
Equipment interconnected by data signal cables and
located on different floors or that is widely separated in a
building, may not be able to effectively use some or all of
the above solutions, except those involving optical isolation
and certain of the protocol conversion techniques. This
occurs since the terminating equipment for the signal cables
is likely to be powered from different branch circuits,
panelboards, and even separately derived ac systems.
Therefore, the associated equipment ground references are
likely to be at different potential at least some of the time.
While the best solution to the above situation involves
either fiber optic or opto-isolation techniques, it is often
possible to achieve good performance by providing each of
the separate locations with an SRG, and then intercon-
necting the SRGs with widely spaced apart and multiple
grounding/bonding conductors, solid-bottom metal cable
trays, wireways, or conduits containing the data signal
cables.
An example of using widely spaced grounding/bonding
conductors to interconnect two SRG areas is when there is
structural building steel available and when it can be used
in this role. Since structural steel columns are installed on
standard spacings in a given building, these columns can
typically be used for the purpose. Wide spacing is necessary
since the conductors involved are inductors and the mutual
inductance between such conductors that are not widely
spaced, is quite high. This makes several closely spaced
conductors appear as a single inductor and not as paralleled
inductances, which exhibit lower overall reactance between
the items they are being used to interconnect.
Also, each of the above separated equipment areas
containing SRGs should be ac powered from a locally
installed and SRG referenced isolation transformer as
opposed to them being powered from panelboards and
feeders from some remotely located power source.
Finally, since separated areas in a building are subject to
large potential differences due to lightning discharge
currents and some forms of ac system ground faults, the
ends of the signal cables should always be equipped with
surge protection devices (SPDs).
Grounding Book 4/14/99 10/5/99 6:02 PM Page 74 (Black plate)