Last month, we left off describing closed circuit TV (CCTV) design coverage. This month, we'll plow ahead to video cabling and installation. In this article we will cover, in some depth, the installation of coaxial cable for video signal transmission.
The high bandwidth of video signals requires the performance of coaxial cables for transmission. Coaxial cable has always been the industry standard for high-bandwidth applications, although it is being replaced by fiber optics for long distance or higher bandwidth applications. Microwave transmission is used only where required, as its cost is significantly more than the other two methods.
Using a high-quality coaxial cable is essential. For CCTV installations of less than 1,000 ft, RG-59U cable is fine. But for distances of 1,000 to 2,000 ft, RG-11U should be used. Installations of more than 2,000 ft in length require the use of amplifiers to keep the signals at usable levels, or the use of optical fiber as the communication media. Most CCTV installations use BNC connectors, a bayonet mount connector or occasionally an “F” connector like CATV.
CATV uses the large RG-8 for its trunk lines and the smaller RG-59 or RG-6 for drops to the home and within it. Larger buildings with longer cable runs use Series 7 or RG-11 cable for its lower loss. The standard connector for CATV is the “F” connector.
Installing video cabling is relatively simple. Coax cables must be installed with care; that is, they must not be pulled beyond their tension limits and must not be sharply bent. In addition, they must remain safe from physical damage and from environmental hazards. Care must also be taken when strapping or (especially) stapling cables to structural surfaces, such as walls or ceilings. If the staple or strap is cinched too tightly, it will deform the cable and alter its transmission characteristics. If the staple or strap is overly tight, the system may not work properly, or even at all.
As mentioned earlier, surveillance cameras must be firmly mounted. In addition to standard wall-mounting brackets, cameras may also be mounted in vandal-resistant cases, which are recommended in trouble-prone areas. Mountings are also available with built-in panning and tilting mechanisms. Obviously, these will cost a bit more than the standard mountings but will provide an additional benefit. However, they cannot be used in all installations, particularly not with video motion detector devices.
Coaxial cable connections include a center contact and an outer shell that connects to the shield on the cable. CATV systems use “F” connectors, a screw thread connector, while CATV may also use BNC, bayonet mount connectors. The “N” connector is also used on larger coax cables. Note that the “F” connector uses the center conductor of the cable as the center contact, minimizing installation time and cost.
COMMON COAXIAL-CABLE CONNECTORS
Coaxial connectors are so simple to install that most of us have done many terminations — such as for connecting our VCRs — using simple “screw-on” F connectors. For building wiring with coax, especially for CATV, we need to be more careful to insure high performance and low signal leakage. Crimp connectors are preferred and cable should be top quality. When making up coaxial connectors the important things are to make sure that the connector type matches the cable type, and that they are made up securely.
The process of terminating these cables is as follows:
The outer insulating jacket must be stripped away, exposing the braided shield.
The braided shield must be pulled back, over the outer jacket, leaving the inner insulation and its foil shield exposed.
The end of the inner insulation should then be removed, exposing the center conductor.
The connector is then placed on the end of the cable and crimped or tightened down. (A number of different connectors are available, with slightly different termination styles. You must follow the directions for the given connector precisely.)
Cable TV and security monitoring circuits are covered by Article 820 of the NEC. You will notice that the title of Article 820 is “Community Antenna Television.” Because very few of us have ever done a community antenna installation, and because we are relating this article to cable television and security monitoring, here is a brief explanation:
Community antenna television (CATV) started decades ago as a means of providing television signals to communities that could not receive broadcast stations, either because of distance or shadow areas where the signal was too weak. Community antennas were installed at a remote location (such as on top of a nearby hill), and signals from it were fed to the homes in the area.
Later, when television signals transmitted via satellite became common, cable TV systems were able to provide a much wider variety of programming than was available via broadcast. Once programming from all over the world was available, the demand for the services became enormous, and cable TV companies began to provide services. The new services were based on the same standards and methods as the community antenna systems from which they evolved.
Today, cable television has developed into a huge system that serves at least 40 million homes in the United States; and it is growing steadily.
Article 820 is therefore very broadly defined, covering all radio frequency signals sent through coaxial cables. While the term radio frequency (often abbreviated as RF) is not defined in the NEC, it would generally include every frequency from several kilohertz to hundreds of megahertz. This would include all types of radio signals, television signals and computer network signals. Article 820 does not, however, cover TV cabling that is not coaxial (see fine print note to 820-1); it applies to coaxial cable only.
The primary safety requirements of this article are first of all that the voltage applied to coaxial cables cannot exceed 60V, and the power source must be energy-limited.
Grounding is mentioned in both parts B (Protection) and C (Grounding) of Article 820. Grounding is required for coaxial cables run outside of buildings. While the NEC does not specifically state that all outdoor cables must be grounded, almost all outdoor runs must be grounded to meet the requirements of 820-33. The concern with outdoor cable runs is that they will be exposed to lightning strikes. In addition to direct strikes, outdoor runs of conductors can have substantial voltages induced into the conductors from nearby lightning strikes.
Another concern for grounding is accidental contact with power conductors. Whenever an outdoor coaxial cable could accidentally come into contact with power conductors operating at over 300V, grounding is required. It is always the outer conductor of the coaxial cable that is grounded.
The rules are generally as follows:
The cable must be grounded as close to the cable's entrance to the building as possible.
The grounding conductor must be insulated.
The grounding conductor must be at least #14 copper; and must have an ampacity at least equal to that of the coaxial sheath.
The grounding conductor must be run in a more or less straight line to the grounding electrode. The run must be protected if subjected to damage.
The grounding electrode can be any suitable type. Section 820-40(B)(1) of the NEC specifically mentions the following:
- The building grounding electrode.
- Water piping.
- Service raceway.
- A service enclosure.
- The grounding electrode conductor or its metal enclosure.
The connection to the grounding electrode can be made with any suitable means, as detailed in Section 250-64.
If the coaxial cable is grounded to an electrode other than the building electrode, the two electrodes must be bonded together with a #6 or larger copper conductor.
The grounding connection may not be made to any lightning protection conductor, whether it is a grounded conductor or not. You may, however, bond the coaxial grounding electrode to a lightning protection grounding conductor. Pieces of unpowered metallic equipment such as amplifiers and splitters that are connected to the outer conductor of grounded coaxial cables are considered to be grounded.
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