Choosing A Fiber Cable - Tight Buffer Cables Part 1

Light Link

Glass optical fibers used for fiber optics communications are very stable and reliable. However
they are also very small and, in some sense, delicate. Thus for any practical fiber optic installation
the fibers use to carry the light signal are protected by a cable structure which prevents the fibers
from being damaged. There are a number of different cable designs for fiber optics, but they can
be categorized into two general groups: loose buffer cables and tight buffer cables. In this tip, we
are going to look at one type of tight buffer fiber optical cable design.

In virtually every tight buffer fiber optic cable, the first step in manufacturing the cable is to apply
an extra layer of plastic jacket to protect the fiber. In most tight buffer cables, this jacket is 900
microns in diameter (about the thickness of a dime.) The 900 micron plastic jacket adds some
stiffness to the fiber and makes it a little more robust when the fibers are routed inside a splice
tray, splice enclosure, or patch panel. After the 900 micron jacket, the tight buffer cables sub-
divide into two categories, one of which is called the break out or fan out cable. (The other, the
riser cable, will be discussed next time.)  In the break out or fan out cable a layer of Kevlar is
applied around the 900 micron buffered fiber.
 
 

(Diagram #1)   

Kevlar is a very high strength synthetic yarn which provides tensile strength to prevent the fiber
from being stretched if there is tension on the cable. In the break out or fan out cable, a plastic
sheath of about three millimeters in diameter is then applied over the layer of Kevlar. At this
point each fiber is individually protected and strengthened. This type of construction with a
single fiber is commonly used for jumper cables and patch cords. Since each fiber is individually
strengthened with the Kevlar, a connector can be attached to the fiber such that the Kevlar will
take any strain between the connector and the fiber cable.

Aside from the obvious application as patch cords, the break out or fan out style cable is also
commonly constructed as a dual fiber zip cord design or a multi fiber cable with an overall sheath.
 
 

These cables are shown in diagram #2.   

The advantage of easy connectorization makes these cables very useful for short runs.
Since the multi-fiber break out or fan out cables have extra strength members and an
overall jacket they are very rugged and can be placed in cable trays or tie wrapped
to supports and run in commercial or industrial settings. The disadvantage of the breakout
or fan out cable is that it is relatively bulky, heavy, and expensive for fiber counts above six.
 
The money saving tip for fiber system owners: Often break out or fan out cables can save you
money, especially for short runs and in situations where you otherwise would have to run conduit.
The multiple levels of protection result in a more reliable, rugged cable than riser cables.

Revenue generating tips for installers: By choosing the proper cable type, it is possible to
reduce your time and materials costs for installations, increasing your competitiveness and
ultimately your business.

Next time: Tight buffer cables, part 2.
 

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