How to Measure Cable Length Accurately in the Field
Field-based cable length measurements using hand-held devices provide accurate billing information, reducing time required to issue invoices. Either Capacitance or TDR technology are the best measurement methods.
The ability to quickly and confidently measure cable length is of critical importance. A quick accurate determination of cable length brings value to custom installers both 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 simply 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 re-installation requests due to 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 as 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.
Generally, 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 that 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 Measurements Accurate to 5%
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). For example, a Cat 5e cable 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 accurate to about 4 percent to 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 Technology Accurate to 3%
Time Domain Reflectometry (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. As 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 at which electrical pulses travels along cables is almost constant for any uniform cable and therefore 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. For example, a Cat 6 cable may have an NPV of 70 percent, which implies that 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 about 1 percent to 3 percent.
One of the major advantages that TDRs offer over Capacitance-based instruments is that they can be accurate to about 1 percent to 3 percent dependent upon the instrument. This is a dramatic two- 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 both open and short faults, whereas, capacitance based instruments can only measure cable length to an open termination or open fault. This TDR feature can be a tremendous 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. Also, 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 in the market today. 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.
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Sean O’Flaherty is senior product manager at Klein Tools in Lincolnshire, Ill. Have a suggestion or a topic you want to read more about? Email Sean at SOFlaherty@kleintools.com
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