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FR Cable: The Inside Scoop

by Bobby Salerno on March 21, 2019

FR Cable: The Inside Scoop

When considering fiber optic outside plant cable, there’s certainly an array of environmental challenges to face. From protecting against extreme climate fluctuations to ensuring a squirrel can’t destroy a cable with one little nibble, there’s much to be considered.

But what types of considerations do you have when cabling moves indoors? And what kind of cable can you use? That’s where flame-retardant (FR) cable comes in.   

Indoor Cable Design and Construction

Indoor cables, commonly known as flame retardant (or FR), are designed to protect optical fibers from the rigors of installation in a cable tray, duct, or sub-floor. NEC regulated fire ratings also call for two different cable designations, plenum and riser.

Plenum cables are designed specifically for use in spaces that facilitate circulation for air conditioning systems (i.e. space between the structural ceiling and the dropped ceiling or under a raised floor.

Riser cables run between floors in non-plenum areas. The fire requirements on riser cable are not as strict. REMEMBER: plenum cable can ALWAYS replace riser cable, but riser cable cannot replace plenum cable in plenum spaces.

Basic construction elements include colored PVC cable jacketing designed to remain flexible but protect against the rigors of indoor applications. Indoor cables can be either all-dielectric (no metallic components) or armored to improve overall cable protection.  Armoring for indoor cables can either be metallic or dielectric.  If armored, cables must be grounded appropriately.

When considering flavors of inside plant cable, there are two types of constructions: tight buffered and ribbon. See the table below introducing Corning's MIC cable.

Cable Corning Brand Name





FR (flame retardant)

Fire rating

Riser, Plenum, LSZH

Fiber Type

Single mode, multimode

Cable construction

Tight buffered, ribbon

Cable Jacket Color

Orange (OM1), Aqua (OM3, OM4), Yellow (OS2)

Fiber counts

2-144 (tight buffered); up to 432 (ribbon)

Tight-buffered Construction – Optical fibers are coated with a robust 900 micron “tight buffer” meant to allow for easier field termination. Optical fibers will be one of twelve colors, per industry standard, with maximum fiber counts up to 144 fibers. These types of cables are typically terminated with field-installable connectors.

Ribbon Cable Construction – Typically 12 or 24 optical fibers are matrixed side-by-side in a “ribbon” pattern and permit much higher fiber counts.  Cables of this construction can reach 432 total optical fibers.  Unlike tight-buffered cables, these types of cables are terminated via fusion splicing but require specialized ribbon fusion splicers. 

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