Take a Fantastic Voyage Into Cabling
Whether it is a small residential or large commercial system installation, cabling is truly the lifeblood of all modern technology projects.
Like the veins and arteries that transport blood through a human, cabling is the circulatory network for electronic systems. Installers, technicians and integrators could easily be considered the doctors of modern systems technology. Doctors have their anatomy books, with detailed descriptions and pictures of the structural makeup of the human body. In the same spirit, a system’s cabling can be dissected and explained.
Even with the long lengths of multi-colored cable in a typical installation, there actually aren’t that many different types of cables. The trick toward a successful installation is knowing the right specifications for the cable. There are also several conditions that could play a role in the installation, and several standards and guidelines to follow.
Installers don’t need to be shrunk down like Raquel Welch and Donald Pleasance to take a fantastic voyage into the universe inside the installation’s circulatory system. They just need to build a greater knowledge of how cables function to lead to a successful installation.
Going Inside the Types and Specifications of Cables
One can quickly become overwhelmed when looking at the hundreds of cable types and cotwisnfigurations used in today’s security systems. Actually, there are only four standard cable types. They are individual conductor, twisted-pair, coaxial and fiber-optic cables.
Individual conductor cable is what many installers teethed on many years ago when they started out in the security industry. These cables typically come bundled and are still the mainstay for DC- and AC-powered control systems. They are either single- or double-jacketed, have either solid or stranded cores, and almost always consist of copper due to its flexibility and high electrical conductivity.
Conductors of various diameters or gauges are made by drawing copper stock many times through a “die.” However, the wire will be brittle and inflexible after being drawn several times. A manufacturing process called “annealing” is used to heat the drawn brittle wire to a temperature that makes the copper molecules reconnect but does not cause the wire to melt and deform. This annealing will give extra flexibility to the wire.
Wire diameter, or gauge sizes, are measured in AWGs (American Wire Gauge). The smaller the AWG number, the larger the diameter. For example, a wire with an AWG of 16 has a diameter of 0.051 of an inch, while a 26 AWG wire is 0.016 of an inch across.
Wire diameter for low-voltage security installations is typically between 16 and 26 AWG with the larger sizes (16–18 AWG) used for required life-safety circuits and higher-load audible circuits. The smaller sized wires, between 22 and 24 AWG, are used for low-current and communications components.
Twisted pair cables, often referred to as unshielded twisted pair (UTP) and shielded twisted pair (STP), is where the action is in high-tech security systems and networks. They are designed to transmit data at very high rates and are often found in data and telephony networks. The uniform twisting of the these cables allows for low-level signals to travel considerable distances without influence from outside electro-magnetic interference (EMI) or radio frequency interference (RFI).
This is possible due to the balanced nature of the twisted-pair circuits in which the signal in one wire is 180° out of phase with the other wires in the pair. This allows for the interference in one wire to be cancelled out by the other wire in the pair. This canceling process is often referred to as “common mode noise rejection.”
Some of the more popular UTP cables are referred to as “Category” (Cat) cables such as Cat-5, Cat-5e, Cat-6 and Cat-6e (see diagram on page 64 of November issue). Each category indicates a particular bandwidth or frequency range capability of the cable. The wire gauge of twisted cable is typically around 20 to 24 AWG. Greater bandwidth demands are pushing copper twisted pair to the wall and, in some cases, passing the baton over to fiber-optic cabling.
Coaxial is referred to as a cable but actually acts like a wave guide at higher frequencies (100KHz-plus). The construction of coaxial cable is such that a center inner wire acts like an antenna. An outer shield keeps reflected signals in and outside interference out, and provides a return path to ground.
In some cases, this can cause what is referred to as a ground loop if the connection points are far enough apart to have different ground potentials at both ends. Sometimes, the only cure for a ground-loop problem is terminating the shield at one end. A good copper center conductor and a shield with a high braid count is recommended for CCTV installations.
The space between the center conductor and the ground shield is filled with nonconductive foam. This keeps the center conductor equidistant from the shield. Care should be given when handling coax cable as crushing it can cause severe attenuation of the signals being sent.
Voice data video (VDV) technicians typically encountered two coaxial cable impedances, or the manner it resists the flow of electrical current if a given voltage is applied. They are 75 ohms for CCTV installations and 50 ohms for RF applications.
CCTV installers deal with three basic 75-ohm coax configurations. They are the popular R-59/U (the U stands for general utility) for 750 feet or less, RG-6/U for less than 2,000 feet and RG-11/U for up to 10,000 feet. Today, fiber-optic cable is often considered in runs exceeding 2,000 feet.
Fiber optics has become the fastest-growing transmission medium. The transmission is made in light and the path consists of one or more glass fibers. It can carry more data at a better quality than either copper or coaxial. Information is transmitted via a laser beam.
There are many key advantages to using fiber. Since it is not electrical, it can go almost anywhere and will not interfere with other electrical cable or attract lightning. The fiber will not corrode and is not affected by moisture. It weighs considerably less and is smaller in diameter than copper, thereby taking up less duct space. It is a more secure transmission medium and very difficult to physically tap into.
Some of the disadvantages of fiber are that it is more expensive than copper and must have transmitting and receiving interface devices. Fiber termination takes special skills to achieve the required high-quality connections. However, technicians should not fear working with fiber, as it has become much easier to work with and connect using the popular “ST-type” connectors. Testing equipment can be rather costly, but those have also been dropping in price. Extra care must be taken when pulling fiber-optic cable. Fiber-optic cable has a braided “strength member” that is used for pulling the cable.
Knowing Cable and System Specifications Are Important
Cable manufacturers list every type of cable with their respective specifications. Knowing what these specifications mean is important in selecting and specifying the correct cable configuration. Make sure to review the system application with the cable manufacturer’s support specialist.
Attenuation is the decrease in power. Various types of cable combinations can cause signal loss at different frequencies. This is usually tested and specified as a quantity per length. Attenuation is measured in decibels (dB). Remember, this is a linear logarithmic scale. An example would be 1.2dB per 100 feet at 100MHz.
Capacitance of a cable is the capacity to store an electrical charge. It is measured in f
arads. Cables with a high capacitance will negatively affect the shape of the electronic signals being sent down the cable.
Impedance is the total opposition a cable has to the flow of alternating electrical current. This is measured as both resistance and reactance. It is measured in ohms and the lower the measurement, the better the cable. For proper transmission, the impedance of the cable and the equipment must match. An example of this would be a CCTV application in which a 75- ohm coaxial cable must be terminated to a 75-ohm equipment load or the video signals may be reflected.
Resistance is the opposition to the flow of direct current in a cable. This is typically represented at ohms per 1,000 feet. Depending on the overall electrical circuit relationship as defined by Ohm’s Law (voltage [E] = current X resistance [R]), a larger diameter may be needed for a cable to handle the load of the device it is providing power. Resistance per foot decreases as the crosssection diameter of the cable increases. That means if the cable is not large enough, the device connected to it may not work properly. Too much resistance in cabling can also cause it to overheat and create a fire.
Velocity of propagation (VOP) is the speed at which a signal travels over a cable. A cable with a VOP of .75 means that a signal will travel 75 percent the speed of light. The VOP of cable can be important when using a time domain reflectometer (TDR) test device and finding a problem in fiber-optic cable. By referring to the VOP of the cable, the TDR will be able to give a very accurate reading of the location of the problem.
Consider Conditions When Considering Application
In order to pick the correct cable for the application, there are many outside conditions that need to be considered. Some of the key conditions are:
Cross talk is the transmission of a signal from one cable pair to another pair in a multiconductor cable. This becomes an increasing concern with UTP technology as more cable is grouped together at higher demanding frequencies. This is commonly called NEXT or near end cross talk.
Frequency is of great concern in today’s high-speed installations with category, coaxial and fiber-optic cables. Cable manufacturers recommend that cables should exceed at least two times the frequency requirement of the application.
Length of cable can drastically affect performance. Many applications will recommend not only the type of cable, but also the maximum length to be used. Cable length is linked directly to the cable specifications (see diagram on page 64). In some cases, length of cable runs can be extended with the use of amplification electronics.
Environmental variations must be taken into consideration as extreme heat/cold and moisture can affect the installation and operation performance. Some cables are gel-filled for extra protection. One other concern is small animals that may chew through cable, especially delicate, smaller diameter cables such as fiber optic. Armored versions have been designed to resist rodents, such as gophers, from chewing critical small diameter underground cables.
Bandwidth is the range or “band” of frequencies that a device can operate in and is probably the most talked about of the application considerations. This is due to all the work being done in high-speed data networks, including such security applications as digital video over networks.
Interference is caused from RFI/EMI sources outside the cable. This can be something as simple as running a cable too close and parallel with a noise-producing cable. Technicians should learn little installation tricks such as running cable perpendicular to other cables when crossing them. Paying close attention to cable shielding can also be valuable in combating interference.
When Laying Down the Line, Be Sure to Follow the Law
We could not survive in an integrated, cabled society without standards and codes. It is truly the DNA of our cable anatomy. Today’s cabling standards are constantly changing. These include items such as new high-speed gigabit networks and additional required measurements such as “propagation delay skew” and “equal level far end cross talk” (ELFEXT). Even standards for cable coloring have been developed and technicians should be aware of them when working in today’s integrated systems environment (see diagram at left).
Cable standards are proposed and/or maintained by organizations such as the American National Standards Institute (ANSI), Institute of Electrical and Electronics Engineers (IEEE), Electronic Industry Alliance (EIA) and the Telecommunications Industry Association (TIA). While there are too many standards to list in this article, more information can be found at those organizations’ respective Web sites.
Try to Connect with Reference and Training Resources
As with any great doctor or surgeon, one of the most important requirements when installing cable is highquality training and access to documentation. All the cabling specification and considerations do no good if we don’t understand the technology and know where to reference it.
The cabling, security, home automation and VDV industries have made major efforts to make training a priority and available to all. Organizations such as those listed on page 66 can provide training programs that vary in price and availability. Training material is available online at little or no cost.
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