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Service Maintenance Company, Inc. Electrical Contractors |
| NEC Article 820, CATV and Radio Distribution Systems | 2/14/2003 |
| Mike Holt for
EC&M Magazine Be careful with limited-energy coaxial cable. It's easy to misinterpret 820.1 as limiting Article 820 to television or cable TV applications. But, it applies to any work you do installing coaxial cables to distribute limited-energy high- frequency signals. For example, if you're installing coax for a closed-circuit television in a security system, Article 820 applies (see Figure 820-1 un820-01 820-01 01.cdr). If you use coax to connect antennas to equipment [810.3], or for local area networks, you must follow Article 820 (see Figure 820-2 un820-02 820-01 02.cdr). Perhaps the most important definition in Article 820 is "Point of Entrance." Knowing this is critical to meeting various grounding requirements, such as those in 820.33 and 820.40. The point of entrance is where the cable emerges from an external wall or a concrete floor slab, or from a rigid metal conduit or intermediate metal conduit grounded to an electrode per 820.40(B). See Figure 820-3 un820-03 820-02.cdr. You must also know the point of entrance to determine the length of unlisted cable inside a building [820.50, Exception No. 3]. Fire When installing coax through fire-resistant rated walls, partitions, floors or ceilings, use approved firestop methods and materials to maintain the fire-resistance rating. Remove the accessible portion of abandoned cables, to limit the spread of fire or products of combustion within a building. Install only plenum-rated cables in plenums, unless you run the cables in metal raceways (see 820.61 and 820.53(A)) or if the plenums are in habitable rooms or areas of a building where the primary purpose is not air handling [300.22(C)]. Coax installed beneath a raised floor doesn't need to be Type DP or plenum rated (see 300.22(D) and 645.5(D)(5)(c)). Mechanical Install cables in a neat and workmanlike manner. Where they run exposed, support them with straps, staples, hangers, or similar fittings designed and installed so as not to damage the cable. Don't attach coaxial cable to, or support it with, raceway [300.1]. Support via "cable tie to conduit" is not an acceptable method (see Figure 820-19 un820-19 820-52D.cdr). There is one exception: You can support overhead (aerial) coax to a raceway mast intended for the attachment and support of communications cables (see Figure 820-20 un820-20 820-52Dx 820-10c 230-28.cdr). Don't support coax by, or attach it to, the power service mast. Route cables so suspended-ceiling panels don't interfere with access to electrical equipment (see Figure 800-4 un800-04 800-05.cdr). If you install cables near framing members, protect them against physical damage from penetration by screws or nails by 1 1/4 in. separation from the face of the framing member, or by a suitable metal plate per 300.4(D). See Figure 820-5 un820-05 802-06 02 300-04D.cdr. If you install cables in hazardous (classified) locations, follow Chapter 5 requirements. Where practicable, leave a separation of at least 6 ft between communications wires and cables on buildings and lightning conductors. CATV coax must be no less than 10 ft over swimming and wading pools, diving structures and observation stands, towers or platforms [680.8(B)]. See Figure 820-6 un820-06 820-10F3 680-08B.cdr. Grounding [820.33] Ground the metallic sheath of coax to the earth (electrode) as close as practicable to the point of entrance to the building or structure (see Figure 820-7 un820-07 820-33.cdr). This doesn't mean you should drive a separate electrode. The practice of driving a ground rod at a convenient location and not bonding that ground rod to the power grounding electrode system is not permitted. As the 820.33 FPN explains, one purpose of 820.33 is to limit the potential differences between CATV and other metallic systems (see Figure 820-14 un820-14 820-40D.cdr). So, connect the coax shield to the main grounding system. If there's a separate grounding electrode for the radio and television equipment, bond it to the power grounding electrode system with a conductor not smaller than 6 AWG (see Figure 820-13 un820-13 820-40D 01.cdr). Not bonding the electrode to the power grounding electrode system creates differences in potential between the CATV and other systems, such as power and telephone-resulting in current flow from lightning strikes and high-voltage surges. This shock and fire hazard can easily destroy equipment connected to multiple systems (i.e., the cable tuner is common to power, CATV and phone). [820.40] When grounding the coax sheath: Use an insulated grounding conductor listed for the purpose. Use a grounding conductor made from a corrosion-resistant conductive material. If copper, it must not be smaller than 14 AWG. Make the primary protector-grounding conductor as short as practicable. In one- and two-family dwellings, it must not exceed 20 ft in length (see Figure 800-8 un820-08 820-40A4 01.cdr). If that length isn't practicable, you can connect it to a driven rod if you bond the rod to the power grounding electrode system with a conductor not smaller than 6 AWG (see Figure 820-9 un820-09 820-40A4x.cdr). Run the grounding conductor to the grounding electrode in as straight a line as practicable. Guard the grounding conductor from physical damage, as necessary. Where the grounding conductor runs in a metal raceway, bond each end of the raceway to the grounding conductor, or the same terminal or electrode to which the grounding conductor is connected (see Figure 820-10 un820-10 820-40A6.cdr). If the building or structure has no grounding means, terminate the grounding conductor to any of the individual grounding electrodes described in 250.52. Otherwise, connect the grounding conductor to the nearest accessible location of one of the following (see Figure 820-11 un820-11 820-40B1.cdr): The building or structure grounding electrode system as covered in 250.50. The grounded interior metal water-piping system, within 5 ft from its point of entrance to the building [250.52(A)(1)]. Accessible bonding means, such as 6 in. of 6 AWG copper conductor connected to the service equipment or raceway [250.94]. The metallic service raceway The service equipment enclosure. The grounding electrode conductor or the grounding electrode conductor metal enclosures. Listing and Markings [820.50] When installing coax in a building, use cables listed for the purpose and marked per Table 820.50, unless: The cable enters the building from the outside and runs in rigid metal conduit or intermediate metal conduit, and the raceway is grounded to an electrode per 820.40(B). The length of the cable within the building, measured from its point of entrance, does not exceed 50 ft, and the cable enters the building from the outside and terminates at a grounding block. Many electrical components and materials give off poisonous toxins when burned. In 300.22 (B) and (C), we find restrictions on the wiring methods and material you can use in areas of a building used for handling environmental air. These restrictions help reduce hazards that arise from the burning of components of an electrical system. Thus, you must pay careful attention to the listings and markings of the materials you use in a given application in a given location. Type CATV coax is suitable for general-purpose use, only-refer to Table 820.50 for other applications, such as risers and plenums. Installation Coax can be in the same raceway or enclosure with cables of any of the following (see Figure 820-16 un820-16 820-52A1a1.cdr): Class 2 and Class 3-Article 725. Power-limited fire alarm systems-Article 760. Nonconductive and conductive optical fiber cables-Article 770. Communication circuits-Article 800. Low-power network-powered broadband communications circuits-Article 830. Coaxial cable cannot be in any raceway or enclosure with conductors of electric light, power, or Class 1 circuits, unless: The coaxial cable is separated from the power or Class 1 conductors by a barrier (see Figure 820-17 un820-17 820-52A1bx1.cdr). If the power circuit conductors are introduced solely for power supply to the coaxial system distribution equipment. The power circuit conductors must have a minimum of 0.25 in. separation from the coax. In other applications, you must separate coax by at least 2 in. from any electric light, power or Class 1 circuit conductors-unless you install those electric light, power or Class 1 circuit conductors per a Chapter 3 wiring method (raceway, metallic or nonmetallic sheath, or UF cable). See Figure 820-18 un820-18 820-52A2x1.cdr). To succeed with Article 820 installations, keep three primary concepts in mind. First, these are low voltage (under 60V) applications. Keep the proper separation between coaxial cabling and other systems. Second, don't violate the fire integrity of a structure. Use approved firestop methods and materials, and take the necessary steps to minimize any loss of fire-resistance. Third, there is no special set of grounding physics for CATV or other coax applications. The basic engineering principles that apply to power installations also apply to limited-energy high- frequency coax installations, such as CATV. So, be sure you eliminate any potential differences between your coax installation and other metallic items. Keeping these three concepts in mind will help you eliminate any potential violations of Article 820. |
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