Understanding Electrical Terms in Wire & Cable Industry

The wire and cable industry often uses various technical terms to describe the electrical characteristics of its products. These terms are essential for understanding and evaluating the performance of wires and cables. Here is a brief overview of some of the most common electrical characteristics terminology in the wire and cable industry:

Basic electrical characteristics testing of wires
Basic electrical characteristics testing of wires

Spark Test

The spark test is used to detect insulation defects in insulated conductors. It is commonly used in the core extrusion or stranding process using a spark tester. It can also be used during the overall stranding process. Generally, cables with shielding (such as braided wire, and aluminium foil facing outwards) use a spark tester to detect defects during the extrusion of the outer sheath. The basic method involves applying a voltage between the electrode in contact with the object being tested and a ground conductor. If the insulation medium is faulty (e.g. too thin or partially missing), the applied voltage will create an arc on the ground conductor. This will trigger the connected indicator (such as a buzzer, light, counter, etc.).

The spark test involves high voltage, so the related equipment must be fully grounded. Generally, the tester can use AC or DC voltage, usually using AC voltage, and can use different frequencies. For safety, the testing current is usually limited to non-lethal levels.

Conductor Continuity

The conductor continuity of wires and cables is a key characteristic. Unless otherwise specified, conduct the continuity test using 100V or lower DC voltage. Manual continuity testing involves connecting a 9V battery in series with a visual or audible indicator. Strip the insulation layer at both ends of the conductor. Separate the conductors. When using an automatic tester, fix each conductor separately on the testing fixture. If using manual testing, usually connect all conductors at one end of the wire to a common test terminal and then sequentially apply voltage to both ends of each conductor. Use indicator lights to indicate whether the circuit is continuous or discontinuous.

Conductor Resistance

The resistance of each conductor in a wire or cable is an important characteristic. However, the measurement of conductor resistance is usually done only during sampling checks of finished products, calculated per unit of wires when shipped (e.g. reel). If the cable contains a very large number of conductors, the conductors can also be sampled during measurement. Unless otherwise specified, conductor resistance measurements are performed at 68°F (20°C). The temperature coefficient is according to ASTM B 193. Resistance varies with cable length. Conductor resistance is usually tested using a voltmeter/ohmmeter or a Wheatstone bridge.

Conductor Resistance Unbalance

The resistance difference between any pair of conductors is a critical characteristic in communication transmission. Unbalance conductor resistance is usually measured simultaneously with conductor resistance measurement. The resistance values are recorded per pair. The absolute difference between the maximum resistance and the minimum resistance is the absolute resistance unbalance per pair. Absolute resistance unbalance is usually expressed as Ω/1000ft or Ω/km. A more commonly used representation is as follows:

Unbalance Resistance = (Maximum Resistance – Minimum Resistance) / Minimum Resistance x 100%

Coaxial Capacitance (Capacitance To Water)

During the manufacturing process, water capacitance is measured by immersing the insulated conductor in a tank, measuring the capacitance between the ground conductor and water, and automatically feeding it back to the control equipment. Coaxial capacitance is the capacitance between the outer surface of a circular metal conductor and the outer surface of the insulation layer. The test method is to immerse a section of the insulated wire in a water tank. Measure the capacitance directly between the ground conductor and water, with water temperature generally at 20°C ± 2°C and frequency at 1000 ± 10HZ.

Mutual Capacitance

Mutual capacitance refers to the effective capacitance between a pair of conductors. In a multi-conductor cable, the mutual capacitance formula is as follows:

Cm = Cab + (Cag)(Cbg)/(Cag + Cbg)

Before measuring, strip the outer sheath, shielding layer, etc. at both ends of the cable, until about 2 feet of the core wires are exposed at both ends. Separate the conductors at one end of the cable. Ensure that the conductors are not short-circuited or grounded. After stripping the insulation layer of the conductors at the other end of the cable, short all conductors, then connect to the ground for measurement. Unless otherwise specified, mutual capacitance refers to the capacitance at an AC frequency of 1000 ± 100HZ.

Capacitance Unbalance – (Pair To Ground)

Capacitance unbalance to the ground is shown in the following figure.

Unbalanced capacitance relationship between pairs of wires and ground
Unbalanced capacitance relationship between pairs of wires and ground

A and B are a pair of conductors. Cag and Cbg are the direct capacitance between conductors A and B with the shield. Cap and Cbp are the direct capacitance between conductors A and B with other pairs of conductors. The formula is:

Cupg = (Cap + Cag) – (Cbp + Cbg)

Attenuation

Attenuation refers to the loss of signal strength as the signal passes through a wire or cable. It is influenced by the insulation conductor material and geometric shape, with units in decibels (dB). The concept and definition of dB values (decibels) are based on the concept of energy or power, where power is equal to the time derivative of energy or the energy output per unit time.

Propagation Delay

Propagation delay refers to the time it takes for a signal to pass through the object being measured. The smaller the dielectric constant, the faster the propagation speed and the smaller the loss.

Velocity of Propagation

The velocity of propagation, also known as the velocity of the waveguide (one of the characteristics of a coaxial cable), is defined as the square root of the product of the dielectric constant in vacuum and the dielectric constant of the insulating medium. Since the dielectric constant in a vacuum is 1, it is expressed as follows:

Propagation rate formula
Propagation rate formula

The ratio of the signal propagation speed in a cable to the propagation speed in free space. The dielectric constants of various materials with air are as follows:

Cross Talk

It occurs when adjacent conductors interfere with each other during signal transmission. The common types of cross-talk are:

Near-End Crosstalk (NEXT)
Power Sum Near-End Cross Talk (PSNEXT)
Equal Lever Far-End Cross Talk (ELFEXT)
Power Sum Equal Lever Far End Cross Talk (PSELFEXT)

Structural Return Loss (SRL)

SRL is derived from the input impedance (square root of open/short circuit impedance), while RL is derived from the termination impedance scanning. SRL compares the input impedance to the characteristic impedance, while RL compares the termination impedance to the load impedance (e.g. 100Ω), so these two measurements are different. Independent characteristic impedance and SRL characteristic curves are usually the preferred methods in the specification. This is because the two curves can be easily distinguished. From a measurement standpoint, the RL method is sometimes preferable because it does not require a function of input impedance, but uses the load impedance as a reference value.

Impedance

It is the sum of resistance, capacitance, and impedance in a certain AC circuit, with units in ohms (Ω).

It is expressed as Z = R2 + (XL-XC)2, where Z is impedance, R is resistance, XL is inductive reactance, and XC is capacitive reactance. The common types are:

  • Input Impedance
  • Characteristic Impedance

There are two test methods for input impedance

Open/Short Method

The basic principle of the open/short measurement method is as follows:

Open and Short Circuit Calculation Formulas
Open and Short Circuit Calculation Formulas

Use a network analyzer for testing, and input the actual open/short data into the above formula to calculate the input impedance.

Matched Load Method

The basic principle of the matched load measurement method is as follows:

Load method formula
Load method formula

Connect a load matching the impedance of the wire to be tested at the end of the test line, use a network analyzer for testing, and the instrument will automatically calculate the input impedance based on the formula above.