Test equipment: Electrical signals and radio signals travel close to the speed of light, which enables a device to use Velocity of Propagation (VoP) to determine cable length. In fact, the speed that a radio or digital signal travels is dependent on the medium through which the signal travels. In practical terms, this means the speed at which a signal travels down a cable is directly related to the type of cable.
Because most cable impedance is relatively constant, it's possible to use this property to calculate the speed of the signal as compared to the speed of light. The ratio of this speed to the speed of light is known as the Velocity of Propagation (VOP). With the right equipment, it is possible to use this information to measure the length of the cable and identify any faults.
Although the VOP is measured as a speed, in practice it is expressed as a ratio of the actual VOP to the speed of light. This means that it's expressed as a number that's less than one or as a percentage. The VOP of most cables is between 0.4 and 0.9.
The theoretical VOP of a cable is calculated by dividing the speed of light by the square root of the relative permeability multiplied by the relative permittivity. Permeability is a measure of the ability of a material to be magnetized, and because most cables have no magnetic materials, its value is 1. The permeability of a cable is a measure of its ability to store a charge that is directly related to the dielectric constant of the cable's insulation.
So in simple terms, the Velocity of Propagation in a cable is directly related to the type of cable insulation and its density. The denser the insulation, the greater is its dielectric constant and the lower the VOP. This is why the insulation thickness on data cables is as thin as possible.
On the other hand, electrical power cables need more insulation so the dielectric constant will be greater and the VOP lower.
Measuring Cable Length
Most cable manufacturers supply their cable VOP, but if it’s not available, it's possible to calibrate the VOP with a known length of cable. It is best to calibrate to a cable length of 100 feet.
Because any discontinuity on a cable will cause part or the entire signal to be reflected, VOP can also be used to identify cable faults and establish the distance to the fault; a property that’s especially useful with power cables.
Use with Cable Testers
Highly accurate and sensitive equipment is required to measure the length of a cable. For many years this required specialized equipment, but developments in digital testers have brought this capability to low-cost hand held testers that use a technique called Time Domain Reflectometry (TDR) to measure the time for a signal to travel down a cable and back. Key to this ability is the fact that a signal will be reflected from the end of the cable so measurement often doesn't require the cable to be terminated, although this helps.
Spread Spectrum Time Domain Reflectometry (SSTDR) functions by evaluating spread spectrum signals, and is the newest and most accurate technology available for fault detection. In addition to having the capability of being effective in high noise environments, SSTDR devices can precisely locate the position of any given fault in a circuit, thereby saving enormous time and energy in troubleshooting.
SSTDR involves sending multiple frequency signals into the circuit, and comparing the returned signals to a copy of the original. When mathematical algorithms are applied to the shape and timing of the returned signals, the location of the fault can be known with accuracy.
T3 Innovation uses this technique in the Snap Shot™, Power Prowler™ and Arc Chaser™ digital testers. The Snap Shot is designed for use on cables operating below 60 volts, while the Power Prowler and Arc Chaser can be used on circuits energized at up to 600 volts.