Cabling FAQs

What is the Minimum Bending Radius for a Cable?

According to EIA SP-2840A (a draft version of EIA-568-x) the minimum bend radius for UTP is 4 x cable outside diameter, about one inch. For multipair cables the minimum bending radius is 10 x outside diameter. SP-2840A gives minimum bend radii for Type 1A Shielded Twisted Pair (100 Mb/s STP) of 7.5 cm (3-in) for non-plenum cable, 15 cm (6-in) for the stiffer plenum-rated kind.

For fiber optic cables not in tension, the minimum bend radius is 10 x diameter; cables loaded in tension may not be bent at less than 20 x diameter. SP-2840A states that no f/o cable will be bent on a radius less than 3.0 cm (1.18-in). The ISO DIS 11801 standard, Section 7.1 General specs for 100 ohm and 120 ohm balanced cable lists three different minimum bend radii. Minimum for pulling during installation is 8x cable diameter, min installed radius is 6x for riser cable, 4x for horizontal.

For fiber optic cables not in tension, the minimum bend radius is 10 x diameter; cables loaded in tension may not be bent at less than 20 x diameter. SP-2840A states that no f/o cable will be bent on a radius less than 3.0 cm (1.18-in).

Some manufacturers recommendations differ from the above, so it is worth checking the spec sheet for the cable you plan to use.




Cable Test Equipment  

DVM = Digital Volt Meter (measures volts)

DMM = Digital Multi Meter (measures volts, ohm, capacitance, and some measure frequency)

TDR = Time Domain Reflectometer (measures cable lengths, locates impedance mismatches).  

Tone Generator and Inductive Amplifier = Used to trace cable pairs, follow cables hidden in walls or ceiling. The tone generator will typically put a 2 kHz audio tone on the cable under test, the inductive amp detects and plays this through a built-in speaker.

Wiremap tester: checks a cable for open or short circuits, reversed pairs, crossed pairs and split pairs.   A least-cost wiremap type tester that detects split pairs correctly (using a NEXT test) is the Fluke 610, at $400. MOD-TAP and UNICOM make a similar device.   Noise tests, 10Base-T: the standard sets limits for how often noise events can occur, and their size, in several frequency ranges. Various handheld cable testers are able to perform these tests.  

Butt-in set: a telephone handset that when placed in series with a battery (such as the one in a tone generator), allows voice communication over a copper cable pair. Can be used for temporary phone service in a wiring closet.  

Fiber Testing - for fiber optic test equipment.


Fiber Optic Cable   Multimode (MM) Fiber Step index or graded index fiber. In North America the most common size is 62.5/125; in Europe, 50/125 is often used. These numbers represent the diameter of the core (62.5) and diameter of the cladding (125) in microns. Multimode fiber is typically used in applications such as local area networks, at distances less than 2 km.  

Single Mode (SM) Fiber Single mode fiber has a very small core. Typical values are 5-10 microns. Single mode fiber has a much higher capacity and allows longer distances than multimode fiber. Typically used for wide area networks such as telephone company switch to switch connections and cable TV (CATV).

Loose Buffer The fiber is contained in a plastic tube for protection. To give better waterproofing protection to the fiber, the space between the tubes is sometimes gel-filled. Typical applications are outside installations. One drawback of loose buffer construction is a larger bending radius. Gel-filled cable requires the installer to spend time cleaning and drying the individual cables, and cleaning up the site afterwards.  

Tight Buffer Buffer layers of plastic and yarn material are applied over the fiber. Results in a smaller cable diameter with a smaller bending radius. Typical applications are patch cords and local area network connections. At least one mfr. produces this type of cable for inside/outside use.  

Ribbon Cable Typically 12 coated fibers are bonded together to form a ribbon. There are higher density ribbons (x100) which have the advantage of being mass-terminated into array connectors. A disadvantage is that they are often harder, and require special tools to terminate and splice.  

Fiber Connectors There are a lot of different types of connectors, but the ones commonly found in LAN/MAN/WAN installations are:  

FSD - Fixed Shroud Device, such as the FDDI MIC dual-fiber connector.

SC - A push-pull connector. The international standard. The SC connectors are recommended in SP-2840A. The SC connector has the advantage (over ST) of being duplexed into a single connector clip with both transmit/receive fibers.

SMA - Threaded connector, not much used anymore because of losses that change with each disconnection and reconnection.

ST - Keyed, bayonet-style connector, very commonly used.    

Fiber Optic Test Equipment  Continuity tester: used to identify a fiber, and detect a break. One type resembles a f/o connector attached to a flashlight.

Fault locator: used to determine exact location of a break. Works by shining a very bright visible light into the strand. At the break, this light is visible through the cable jacket.

Tone Generator and Tracer: used to identify a cable midspan or to locate a strand at its far end. Similar in purpose to the tone testers used on copper cable. The tone generator imposes a steady or warbling audio tone on light passing down the cable. The tracer detects and recovers the tone from light lost through the cable jacket as a result of bending the cable slightly.

Optical Source and Power Meter: used to measure the end-to-end loss through a f/o strand, or system of cable, connectors and patch cables. Measurements are more accurate than an OTDR.

Optical Time Domain Reflectometer (OTDR): used to measure the length of a cable, and detect any flaws in it. Can also be used to measure end-to-end loss, although less accurately than a power meter.

Fiber Talk set: allows using a pair of f/o strands as a telephone line.   Fiber Optic Testing, standards: see EIA-455-171 (FOTP-171), EIA 526-14.