Information Technology Reference
In-Depth Information
Eventually, the periodic nature of the interference caused the researcher to suspect
that it might be an HVAC system that was the culprit.
We routinely use LAN equipment that generates and is susceptible to interfer-
ence to/from power lines. The closer we allow the wires to be, and the longer we
allow them to run in proximity, the greater the potential for interference. Thus, the
basic guideline here is to limit both the run length and the separation of these types
of cables. In a divided power/communications cable tray, it is very easy to run the
two types of cables beside each other for hundreds of feet. This increases the chance
for interference to couple between the cables, even if the separation is moderate.
A good rule of thumb is to place power and communications cables in separate
runs, to avoid long parallel runs. The cables can generally cross at right angles or
briefly run together with little or no coupling. Fortunately, the tight twisting and
controlled geometry of modern LAN wiring minimizes the coupling to and from
outside conductors.
The safety issue is a little more ominous. Electrical codes, such as the
NEC
, for-
bid placing power conductors and communications cables in the same raceway.
Power conductors are considered Class 1 circuits, and are expected to have haz-
ardous voltages and the ability to deliver dangerous levels of current.
Communications cables, on the other hand, are Class 2 circuits, as defined by
NEC
,
Article 800 Communications Circuits.
The
NEC
also specifies a 2-in (50.8-mm) separation from electric light, power,
and other Class 1 circuits, except when they are in a raceway or metallic sheath, or
where they are separated permanently. For example, a LAN cable and a power con-
ductor can be contained in the same power pole, or modular furniture pathway, if
they are run in totally separate raceway sections.
The main idea to get from this discussion of power/data separation is that you
need to keep power and data cables widely separated, except in that last run
through the modular furniture fixtures, where the cables must at least be placed in
separate raceways.
Modular furniture presents challenges for LAN wiring, but it is an office tech-
nology that is here to stay. As more manufacturers respond to the special needs of
data and communications cabling, the process of running the needed cabling in this
type of furniture will become routine.
