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INFRASTRUCTURE Designing Value Into The Future 43 I INS Asia Febr
uar
y/March 2006
INFRASTRUCTURE
Designing Value Into The Future
When it comes to designing audio visual systems, thoughts
will usually be filled with equipment choices to be made,
cable selections, hardware and rack enclosures and how
the system will ultimately satisfy our clients. However, there
is more - we also need to think seriously about how we will
get the signals to the equipment locations. Not just on the
cabling itself, but the conveyance infrastructure that will
house and support the chosen cabling between equipment
locations.
The conveyance infrastructure design is one of the more
important elements in the overall design. Done correctly, the
conveyance design is the foundation to create manageable
blocks of scalable connections between equipment
locations. The value of this well-designed infrastructure
allows a smoother initial installation and adds flexibility for
maintenance, when additional equipment needs to be
added or equipment upgrades need to be done.
Audio visual system infrastructures need to be coordinated
with data / telecom networking resources and power
systems, so that a converged and coordinated infrastructure
emerges in a successful deployment for a truly usable
system to support an audio visual system.
In addition, there are emerging trends in our industry that
gives this need even more importance. For example,
InfoComm International (ICIA) has recently partnered with
Building Industry Consulting Service International (BICSI)
to create a training program that will train technical staff
in the design and installation of structured cable
conveyance, for the support of the data networks and
audio visual systems. The reasoning behind this initiative
is that we as AV designers are not alone in the need to
coordinate services between low-voltage systems such as
life safety, data and telecom networks and AV systems. In
addition, the Construction Specification Institute (CSI)
Master Format Section 27 changes in the United States;
offers the opportunity to coordinate schematic drawings
and specifications that will illustrate all low-voltage
communication resources in one section of plans. With
all of these recent changes, the outcome becomes
obvious. This is a forward direction for the AV industry as
a whole as it becomes more firmly entrenched as a part
of the construction industry. It is a wise decision for the
designers of these systems to move forward and to get
new sets of skills needed to work with these significant
changes.
What are the Standards?
In the United States, structured cable infrastructure
standards are based on the data / telecom standards
TIA / EIA 569 and in the design of power distribution in
compliance with the National Electrical Code (NEC).
There is much written about these standards and it is not
within the scope of this article to cover these in detail
here. In addition, there are electrical codes that are
regional or local that can supercede the NEC and the
TIA / EIA 569 recommended standards that must be
followed in your local areas. It is suggested that if you
wish to work with your networking engineers, electrical
engineers and contractors, that you take the time to find
out what are the details that govern your local region
and always follow your local codes. In the United States,
the National Electrical Code (NEC) and BICSI has
published best practices that follow in many instances,
the TIA / EIA 569 recommended standards. Again, local
codes will supercede the national recommended
practices.
Fortunately, there is constant review of these
recommended practices and as the building industry
evolves and new products and practices are developed,
the published recommendations will change from year to
year.
What are the kinds of conveyance and
what are the conduit sizes needed for AV
installations?
There are many types of conduit available on the market.
Each type has its place in the structured cable
infrastructure design. To keep this discussion
manageable, I will not go into all of the products
available, but will stay with ones commonly found in AV
structured designs.
Rigid Metal Conduit RMC (ferrous metal conduit)
RMC is a threaded metal conduit that comes in 10
(3 m)
sections colour-coded on the thread protectors to aid
with size identification and to keep the threads clean
and free from damage during shipping, until it is ready
for installation. RMC is available in sizes from
1
/
2
to 6
(12.7 mm to 152 mm)
. RMC is the heavy weight of
conduits, has thick walls and is sometimes coated or
galvanized to resist corrosion. This product can be used
indoors, outdoors or underground in concealed and
exposed applications.
Intermediate Metal Conduit IMC (ferrous metal conduit)
IMC is also a threaded metal conduit that comes in 10
(3 m)
sections colour-coded for size on the thread
DESIGN
By John Pfleiderer, CTS-D 44 I INS Asia Febr
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protectors. IMC is available in sizes from
1
/
2
to 4
(12.7 mm to 152 mm)
. IMC has a thinner wall
construction than the RMC products, but uses the same
couplers and fittings. This product has a zinc-based
coating and can be used interchangeably with RMC
products. The advantage is that it is a lighter product
and hence it can be used in overhead and vertical
applications where appropriate.
Electrical Metallic Tubing EMT (ferrous metal conduit)
EMT is a thinner wall steel raceway which also comes in
10
(3 m)
sections which is unthreaded. Like IMC the
coating applied to this raceway is zinc based and
provides protection from corrosion. EMT is joined
together by either set screws, indentation, or
compression fittings. EMT is a more flexible raceway and
can be bent into sweeps to enable it to be dealt with in
special circumstances. Even though this product is lighter
in weight, it will still provide substantial physical
protection and strength and can be used in most
exposed locations. This product is widely used to support
AV conveyance systems.
Polyvinyl Chloride PVC (metallic conduit with PVC
outside)
PVC conduits come in three types:
1) PVC over steel - which is similar to RMC for
environmental demands.
2) PVC over galvanized steel - for severely corrosive
environments.
3) PVC over a supplementary coating of zinc - for
extremely corrosive environments.
PVC applications need to have fittings that are also
coated in PVC to maintain environmental needs.
However, using these products requires that the installers
follow the manufacturers instructions to maintain system
integrity.
Liquid-tight Flexible Metal Conduit- LFMC
The flexible nature of the liquid-tight conduits makes them
appealing for runs that need to be snaked into place.
This conduit has a liquid tight jacket over a flexible non-
metallic core and can be used with fittings to allow it to
interface with other conduit products. The typical size
runs from
1
/
2
to 2
(12.7 mm to 50.8 mm)
diameter.
This of course is not a complete list of the kinds of
conduit product lines that are manufactured and most of
these products above can be found in metric sizes that
are similar in nature to the size ranges I have listed
above. The advice here is to do some research on the
anticipated products needed for the design before
writing any specifications.
Size is Important
When considering the size of the conduit to be used in
the system, do bear in mind that it should not only
support enough room for installing the necessary cables
planned for the run, but also allowing for future
expansion. Filling the conduits with cables on the first
pass is not a recommended practice. Many
manufacturers will specify approximately 40% fill for most
applications, but my experience in the AV world usually
indicates that 30% is a better fill leaving some room for
extra cable runs, which seem to be inevitable in our
business.
There is a rule of thumb that applies to conduit fills where
one or more cables are being pulled into the raceways.
For example, if the cable pull into the conduit is based
on three cables, the combined cross sectional area of
the three cables should not exceed 30% of the conduits
interior volume. For pulls that have only one cable, the
percentage fill can be increased slightly to
approximately 50% because of the cross section of the
cable using the space more efficiently then multiple
cables. See Figure 1.
Keep in mind that as the conduits specified for the job
has more than one cable or any number greater than
one, it will tend to form a circular shape thus filling the
conduit quicker.
If you are re-using conduits, keep in mind how many
cables are to be installed and the cross-sectional
dimensions of the new cables being pulled. Hint: There
Figure 1 - Multiple Cables in Conduit and Fill relationships
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DESIGN
are some really handy conduit fill calculators available
on the Internet that can take the math out of doing this
work. However, you will still need to find the outside
dimension of the