14
CABLING
"Factory terminated cable assemblies, therefore,
provide clear speed of installation and
implementation advantages coupled with
reduced on-site installation costs when
compared to field terminated systems"
be RJ45-based, unless it’s old, and a category cabling system
will be backward compatible. For fibre, the cable needed
could be multimode or singlemode with the many different
fibre connector variants that have been produced over the
years and could already be in place in existing patch panels
meaning that the pre-term will need the same connector
type. When installing a fibre pre-term, the cable type needs
to be clearly defined beforehand particularly when adding
to an existing site. Over the years I have lost count of the
number of occasions a fibre connector or pre-term cable
assembly has been bought only for the engineers to get to
the site and find something different.
The application for pre-terms should also be understood
as armoured cable pre-terms are recommended when rodent
protection is an issue. Ruggedised assemblies which provide
greater crush resistance are more suitable for long line cable
lengths or cabinet to cabinet applications.
MPO pre-terminated assemblies
MPO fibre pre-terms have been an issue for many over the
years. The polarity on MPO assemblies (key up or key down)
needs to be correct, but it doesn’t end with that. There has
been much confusion in the market surrounding the correct
wiring for 24 position MPO Trunk Pre-Term Assemblies
because there was no industry standard.
The previous TIA-568-C3 standard only gave the wiring
for 12 position MPO’s, so when 24 position versions
appeared cable assembly manufacturers came up with their
own specifications for what they thought the additional
wiring should be and gave them non-industry standard
designations such as straight or cross etc. This resulted in
24-way pre-term assemblies from different sources not being
compatible and causing a great deal of confusion.
The TIA 568.3-D standard has since clarified the
wiring for MPO/MTP assemblies ensuring that assemblies
manufactured to this specification are compatible. However,
we do still see installations that are not to the standard
which can cause issues when additional pre-terminated
cables are installed.
Constructors Product Regulations (CPR) should also be
considered when installing pre-terminated systems, whether
it’s a leading manufacturer’s standard assembly or an
individual pre-term manufactured by an assembly house.
CPR specifies how quickly a cable reacts to fire in terms of
how quickly fire spreads, heat is released, smoke production,
acid gas production and flaming droplets, putting it into
cable classes.
There’s still an alarming lack of knowledge on this subject,
particularly among some smaller installers and distributors/
wholesalers who are not specialists in the networking
market. But the simple fact of the matter is that BS6701
states that as a minimum CPR cable class Cca should be
used in permanent link applications due to high flame
resistance. It will be argued by some that long line pre-terms
or other pre-terms are not a permanent link because they
can be disconnected. However, they are typically long cable
lengths running through a building just like a permanent
link so whatever your view on that, it makes good and
safer sense to specify CPR Cca cable as a minimum in pre-
terminated assemblies.
Factory terminated cable assemblies, therefore, provide
clear speed of installation and implementation advantages
coupled with reduced on-site installation costs when
compared to field terminated systems. However, as can
be seen from the above, there’s still much that should be
considered to take advantage of the benefits they bring. n
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