The Importance of Field-based Cable Length Measurement for Cable Installers

For premises and outside plant cable installations, the ability to quickly and confidently measure cable length is of critical importance.

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By Sean O'Flaherty

For premises and outside plant cable installations, the ability to quickly and confidently measure cable length is of critical importance. A quick and accurate determination of cable length brings value to cable installers in terms of workplace efficiency and quantitative confirmation of adherence to cabling standards.

A need to measure cable length can arise when an installer wants to know the amount of cable remaining on an open spool or in a box to ensure that a sufficient amount of cable is on-hand before starting a job. A quantitative measure of cable length for a freshly installed run can be used to ensure compliance with internationally recognized standards such as TIA/EIA-568, which specifies maximum recommended cable length runs. A quick length measurement can identify or prevent cabling runs that are out of specification. This helps prevent callbacks for troubleshooting of poor performing networks or reinstallation requests because of non-compliance with cabling standards. A cable length measurement can also identify cable faults and damage by measuring the length of a cable to an open or short fault that may be required when a cable is severed or damaged. Field-based cable length measurements on freshly installed cable runs provide immediate, accurate billing information, reducing time required to issue invoices, thereby improving cash-flow. Finally, the experience gained through measuring cable lengths in the field may improve the installers' ability to estimate future jobs more accurately.

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In general, hand-held cable length measurement instruments use capacitance or time domain reflectometry (TDR) technology. Field instruments using these technologies typically require paired-conductor cables to measure length. This means the cable must have two similar conductors running along the same path in close proximity to one another for these measurement technologies to work. Either method is suitable for measuring the length of category data, twisted pair and coaxial cables.

Capacitance based measurements estimate length based on the ability of two conductors (separated by a dielectric) to store a charge. This technology is used by most opening price point and mid-range cable length measurement instruments. Capacitance is measured in Farads (F). The Mutual Capacitance Coefficient for a paired-conductor cable is typically specified by the cable manufacturer and is usually expressed as pF/ft. (pico-Farads per foot) or pF/m (pico-Farads per meter). A CAT5e cable, for example, may have a mutual capacitance coefficient in the region of ~4.6 pF/m or approximately 4.6pF per each meter of cable. When operating, an instrument measures the total capacitance in the cable and then uses the cable's Mutual Capacitance Coefficient to calculate the cable's length. Capacitance based measurement instruments can typically measure the length accurately to some 4-5 percent. If the Mutual Capacitance Coefficient is not readily available from the cable manufacturer, it can be measured by calibrating the measuring device using a cable of known length.

TDR uses a measure of the time it takes an electrical pulse to travel along a cable to estimate the cable's length. A TDR instrument transmits a short electrical pulse along the conductor. Because the conductor is of almost uniform impedance, any macroscopic impedance changes or discontinuities will cause some of the incident pulse to be reflected back towards the source. By measuring the time it takes for an electrical pulse to be sent and received, the cable length can be estimated. The speed electrical pulses travel along cables is almost constant for any uniform cable, and the travel time can be interpreted as a function of cable length. In real-world applications, instruments use the Nominal Velocity of Propagation (NVP) of the cable to measure length. The NVP is typically specified by the cable manufacturer and is expressed as a percentage. A CAT6 cable, for example, may have an NPV of 70 percent, which implies an electrical pulse travels along this cable at 70 percent of the speed of light in a vacuum. TDR-based measurement instruments can typically measure cable length with an accuracy of some 1-3 percent.

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One of the advantages TDRs offer over capacitance-based instruments is that they can be accurate to some 1-3 percent, dependent on the instrument. This is a two-fold to five-fold accuracy improvement over capacitance-based instruments. In addition, another major advantage that TDR measurements offer over capacitance measurement is that they can accurately measure cable length to open and short faults-while capacitance based instruments can only measure cable length to an open termination or open fault. This TDR feature can be an advantage when trying to measure length to locate a short on a cable that is buried underground or embedded behind finished walls. In summary, both methods can measure cable length to an open termination or open fault-but, only TDRs can measure to a short fault. In addition, TDRs are typically two to five times more accurate than capacitance-based instruments.

Field suitable, hand-held cable length measurement tools are affordable, easy-to-use and are readily available. The units can be set up using cable manufacturers' specifications or calibrated on-site using a cable of known length. Quick, accurate determination of cable length and line continuity testing using hand-held cable length instruments brings value to the cable installer, the contractor and the customer during installation, trouble-shooting and routine maintenance cabling jobs.

About the author: Sean O'Flaherty is senior product manager for Klein Tools.

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