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This article originally appeared in Cabling Business Magazine --

NEC Article 830, Network-Powered Broadband Communications Systems


By David Herres


How the National Electrical Code Covers Network-Powered Broadband Communications Systems--

Broadband and the associated term bandwidth are slippery concepts indeed. Like a photon in quantum mechanics, the more closely we look at these words, the more their meanings elude us, partly because they are used in such a range of contexts. In analog radio broadcasting, bandwidth becomes applicable when a relatively low frequency audio signal modulates a much higher carrier. The modulating frequency is added to and subtracted from the carrier frequency and that is what constitutes the bandwidth. To transmit a higher pitch audio signal, as is required for what used to be called high fidelity, greater bandwidth is needed. Governing bodies such as the FCC in the United States allocate a precise frequency range and broadcasters are not allowed to exceed those limits. Otherwise they would interfere with adjacent stations.

In the case of digital transmission, bandwidth has a different though related meaning. It is a measure of the amount of information that can transverse a medium and is enabled by the frequency level of the system. Higher frequency means greater bandwidth. Traveling over copper, higher frequencies experience greater loss due to parallel capacitance and series inductance, phenomena which make broadband difficult to achieve. Broadband networks may be fiber optic, DSL or Ethernet over cable. These are much faster than a telephone modem, which typically operates at a mere 56,000 bits per second. A Broadband DSL or cable connection is almost a requirement within today's homes. A telephone modem is no longer sufficient to handle the day to day use of streaming live TV, a Broadband DSL or cable connection is almost a requirement within today's homes. A telephone modem is no longer sufficient to handle the day to day use of streaming live TV, online RPG games, music and constant on connections required by families. online RPG games, music and constant on connections required by families.

Broadband in data communication systems often involves multiple signals simultaneously transmitted across a medium in order to achieve greater capacity. It is this type of system that NEC Article 830 addresses when the system is also network-powered meaning that an AC or DC voltage is supplied along with the multiple signals. Broadband cable, under exclusive control of a communications utility, typically comes onto privately owned property. Beyond the utility-defined point of connection, the same type of cable is run to a Network Interface Unit (NIU), which can be located on the outside of a building or within. The NIU converts the broadband signal into component video, audio and interactive signals, which are routed on separate cables as needed throughout the building.

NEC Article 830 has fairly limited albeit important application. As with power and light wiring, the utility-controlled portion is not under NEC jurisdiction. Article 830 covers the cable from the utility-defined point of connection to and including the NIU. If the utility owns or controls the NIU and cable to it, there will be no Article 830 coverage. Nevertheless, the Code would have jurisdiction over output circuits by virtue of other articles. For example:

NEC Article 725 covers Class 2 and Class 3 Remote Control and Signaling Circuits.

Article 770 covers Optical Fiber.

Article 760 covers power-Limited Fire Alarm.

Article 800 covers communication Circuits.

Article 820 covers Coaxial Cable.

Article 830, conforming to the general NEC template, begins with a statement of scope, followed by definitions. The statement of scope emphasizes that the article covers systems that are network-powered. Typically carried over coaxial cable, there is power (AC or DC) on the center conductor with the outer shield as a ground-referenced neutral. Additionally, any combination of voice, audio, video, data and interactive services terminate at a Network Interface Unit, a unit that converts a broadband signal into components. It also blocks network power so that premises equipment is not damaged.

Also defined is: Point of Entrance, the point within a building where cable emerges from an external wall, concrete floor slab, or from rigid metal conduit (RMC) or intermediate metal conduit (IMC), grounded of course. A section on other NEC articles references hazardous (classified) locations, ducts, plenums and other air-handling spaces and NEC coverage of output circuits. A section on power limitations defines low- and medium-powered systems. Low-powered systems go to 100 volts whereas medium-powered systems may go to 150 volts. There is an additional power limitation of 250 volt-amperes for both categories and a current limitation of 1000 amps divided by volts. Thus, a 100-volt system could go up to only 10 amps. Above these limits, there is such a thing as a high-powered network-powered broadband communications system but the National Electrical Code does not cover it.

Notice that these voltage limits are substantially higher than those in Article 820, Community Antenna Television and Radio Distribution systems, which cannot be over 60 volts. Consequently, design and installation requirements are more stringent. If it were contemplated that an Article 820 system may in the future be upgraded to Article 830 status, it would be worthwhile building it to Article 830 standards at the outset. The remainder of Part I repeats mandates applicable to low voltage cabling in general. Access to electrical equipment behind panels, mechanical execution of work, removal of abandoned cabling and mitigation of fire hazard by means of fire stopping Ð these are familiar topics but should be reviewed before undertaking jurisdictional work.

Part II covers Network-Powered Broadband Communications Systems involving cable outside and entering buildings. Medium-power and low powers take different cable types Ð BMU, BM and BMR for medium power and BLU and BLX for low power. As usual for low voltage cabling, substitutions are permitted. Generally, medium power cable can be substituted for low power cable; plenum can be substituted for riser, and so forth. See Table 830.154 for details.

Aerial cables must comply with rules regarding pole installations, climbing space, lead-in clearance, clearance from ground, over pools and above roofs.

Also final spans, spans between buildings and on buildings are covered.

Underground circuits entering buildings are to conform to Section 830.47. Table 830.47 provides exact minimum cover requirements for various situations and includes protocol for when solid rock is encountered.

Part III, Protection, is central to any work you may do on a network-powered broadband communications system. Primary electrical protection is to be provided for a system that may be exposed to lightning or accidental contact with power conductors. Most installations are prone to lightning damage. Along the U.S. Pacific coast there are areas having five or fewer thunderstorm days per year and earth resistivity of less that 100 ohm-meters. Also, in large cities buildings may be close enough and sufficiently high so as to intercept lightning. Locations that do not fall into one of these categories, however, are prone to lightning. Even the lightning free areas require primary protection for network-powered broadband cabling if accidental contact with electric light or power conductors operating at over 300 volts to ground is a possibility. If aerial cable is run partly or entirely outside a single block and the conductors are neither grounded nor interrupted, primary protection is required.

Primary protectors may be either fuse-less or fused.

Fuse-less primary protectors are permitted if fault currents are limited to a value no greater than the current-carrying capacity of the primary protector and associated grounding conductor. Where fused protectors are used, they must consist of an arrestor between each conductor and ground, and a series fuse.

If a primary conductor is required, it must be located properly.

A listed primary protector is generally on each network-powered broadband communications cable external to the network interface unit at the input. Integral primary protector and network interface units are available and must be so marked and listed.

Another possibility is to have network interface unit output protectors and these also must be listed as suitable for network-powered broadband communications systems.

Regardless of type, the primary protector is to be located as close as practicable to the point of entrance, as previously defined.

The primary protector is not to be located within a hazardous location (unless certain Chapter Five conditions are met) and never in the vicinity of easily ignitable material.

Where network-powered broadband communication cable enters a building, the shield and any other metallic members of the cable assembly must be grounded or interrupted. The grounding or interruption must be close as practicable to the point of entrance.

In instances where the network-powered broadband communications cable terminates outside the building, shield and metallic members are to be grounded or interrupted in the same fashion.

Part IV lays out grounding methods, paralleling other coaxial applications. The network interface unit with protectors, metal enclosures, primary protectors and metallic members of the cable that are intended to be grounded must conform to the standard low voltage protocol.

The grounding conductor is to be listed, insulated, copper (or other corrosion-resistant material). It can be stranded or solid. It cannot be smaller than 14 AWG and must have current carrying capacity approximately equal to that of the grounded metallic members and protected members of the network-powered broadband communications cable. But in no case does it have to exceed 6 AWG.

The grounding conductor must be as short as practicable, not to exceed 20 feet for dwellings. Where it is not possible to keep the length from going over 20 feet because of the distance to a grounding means, drive a communications rod at the network interface location or point of entrance. When this is done, it is still necessary to bond to the power system grounding means using 6 AWG copper, bare or insulated.

The grounding conductor is to be run in as straight a line as practicable so as not to introduce inductive reactance.

The grounding conductor is to be protected if exposed to physical damage and if metal raceway is used for this purpose, both ends must be bonded to the grounding conductor. (To avoid this requirement, PVC conduit is often used for grounding conductor protection.)

If, in accord with latest practice, the building has been provided with an intersystem bonding termination, which is the way to go. Otherwise, tie into the grounding electrode conductor or grounding electrode using a listed clamp, not a stainless steel hose clamp or other impromptu solution.

It is to be emphasized that bonding together of all ground systems including lightning protection if present is required by the Code and necessary for a safe installation so that dangerous voltage differences are not present.

Part V, Installation Methods Within Buildings, presents requirements for installations where the network interface unit is inside the building. If it is some distance from the point of entrance, there will be a significant amount of cable under Article 830 jurisdiction. Since there are two distinct types of these systems covered by the Code, low power and medium-power, mandates vary depending on which of these is applicable. For example, conduit fill restrictions do not apply to low-power systems, but they do apply to medium-power systems since in the latter case heat generation may become a factor.

Low- and medium-power network-powered broadband communication cables are permitted in the same raceway, cable tray or enclosure. Low-power cables of this type can co-exist with Class 2 and Class 3 remote control, signaling and power-limited circuits, power-limited fire alarm systems, communications circuits, optical fiber and CATV and radio distribution systems. Medium-power network-powered broadband cables are not permitted to share facilities with those low voltage types.

Neither power level is permitted to occupy the same raceway, cable tray, compartment, outlet box, junction box or similar fitting with electric light, power, Class 1 or non-power-limited fire alarm circuit cables unless certain stated conditions exist (see exceptions 1 and 2).

Several other mandates familiar to cabling technicians also apply. Network-powered broadband communications cables are not to be attached to the exterior of conduit or raceway as a means of support.

Furthermore, the familiar plenum, riser etc. cable hierarchy exists. Table 830.154 provides permitted substitutions.

These are the principle NEC guidelines regarding network-powered broadband communications systems. The basic ideas are similar to those of other low-voltage cabling systems but, as we have seen, there is some variation in the details. If this is new territory, proceed with caution and consult the National Electrical Code plus any local statutes that may apply.


--END--

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Here is a selection of the most significant electricians' books available online today, at the best prices around. Clicking on any logo provides access to reviews and ratings by electricians. A good place to start is with the 2008 NEC Handbook, which contains the complete text of the current code plus extensive commentary, diagrams and illustrations. Other books of interest for the electrician are available as well.

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This site is created and conducted By David Herres, NH Master Electrician License #11335M

E-mail: electriciansparadise@hughes.net


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