NEC Chapter 8, Communications Systems-2

By David Herres

The stated purpose of the National Electrical Code is the practical safeguarding of persons and property from hazards arising from the use of electricity. Low-voltage cabling, while considered by most technicians to be relatively benign in regard to fire and shock potential, nevertheloess carries with it certain design and installation issues, which need to be addressed. Many commercial occuupancies contain huge amounts of data, telephone and other cabling, which contribute to an already heavy fire load, even in a concrete and structural steel environment.

This fact becomes increasingly true if successive generations of abandoned cable are not removed as required by the Code.

Generally, the lower voltage and power levels associated with data, telephone and other communications cabling are not sufficient to cause electric shock to humans. But there is always the potential for contact between low-voltage cabling and ordinary power and light wiring, which in an industrial setting can go up to 480 volts or higher. Adherence to Code strictures goes a long way to minimize this possibility.

Article 810 begins with a statement of scope. Since the Code does not cover internal wiring within factory-made equipment (generally required to be listed by a recognized organization such as Underwriters' Laboratories), the scope is limited to wiring outside of such equipment. Antennas for radio and television receiving equipment and some aspects of transmitter safety are within the scope of Article 810. There are a number of antenna types -- multi-element, vertical rod, and so forth. Any sort of parabolic dish, such as used for satellite TV and satellite Internet connection (actually a combination transmit/receive device) are covered and in jurisdictions where NEC has been enacted into law the mandates must be followed.

There is a story that somewhere an Electricians' Board decided to look at a large number of completed satellite TV dish installations to see what percent complied with the Code in regard to grounding, cabling and so forth. It was found that not a single installation was fully compliant, but this may be an urban legend.

But it is true that many otherwise knowledgeable professionals miss out on some of the fine points. For example, if you install cabling in a building, you could be construed as being required to remove all abandoned cabling. Unlike many other NEC articles, 810 contains no unique definitions, but all global definitions, contained in article 100, are applicable. Also, any power wiring to radio and television equipment or between units must comply with NEC chapters one through four, which concern ordingary power wiring, usually 120/240-volt, single-phase AC in the United States. These rules are in certain instances modified by such articles as 640, Audio Signal Processing, Amplification and Reproduction Equipment.

Coaxial cables that connect antennas to equipment are covered in Article 820, Community Antenna, Television and Radio Distribution Systems, which will be the subject of the third in this series of four articles. In the case of a community television antenna, the antenna is covered in Article 810, the distribution system in 820. Any equipment that is utility owned is outside the scope of the Code.

In its introductory material, Article 810 specifically requires that radio interference eliminators, interference capacitors and noise suppressors that are connected to power supply leads must be listed by a recognized organization, such as UL. They musst not be exposed to physical damage. All of the foregoing is in Part I of Article 810, which serves as an introduction. Part II, Receiving Equipment -- Antenna Systems gets to the heart of the matter.

The National Electrical Code is not a design manual. It does not provide theoretical information regarding antenna design or selection. The sole focus is electrical safety. What is covered is choice of materials and installation guidelines for antenna and lead-in conductors not in terms of what would provide good reception, but rather from a standpoint of structural integrity so that the end product is not a hazard. Of prime concern are:

• Keeping antenna and lead-in from contacting electrical power lines.
• Proper grounding to limit voltage due to lightning or contact with an energized object.

The section begins by requiring that antennas and lead-in conductors be hard-drawn copper, bronze, aluminum alloy, copper-clad steel or other high-strength, corrosion resistant material. An exception permits soft-drawn or medium-drawn dopper lead-in conductors for spans under 35 feet.

Both antennas and lead-in conductors are to be securely supported. Minimum intervals between such supports are not stated nor are other mechanical details. (In general, the Code provides exact specifications in such matters in the secitions on power and light wiring, but not low-voltage cabling. The same is true of conduit fill.) You could either field-fabricate appropriate supports or obtain hardware made for the purpose. Neither antenna nor lead-in conductors are permitted to be attached to a service mast and of course the lead-in conductors are not permitted to occupy the service conduit or any other conduit occupied by electrical power wires.

Antennas and lead-in conductors may not be attached to poles supporting open electric light or power wires or trolley wires of over 250 volts between conductors. It is further mandated that insulators supporting lead-in conductors be mechanically strong enough to safely support these wires. Lead-in conductors are to be securely attached to the antenna.

An important section follows, which is echoed in other low-voltage cabling articles. It concerns avoidance of contact with conductors of other systems. The primary means of accompolishing this objective is spatial separation. It is specifically required that outdoor antennas and lead-in conductors do not cross over open conductors of electric light and power circuits. They are to be kept well away from these circuits to avoid accidental contact. In strong winds or under conditions of heavy ice loading antenna or lead-in conductors could break or supports could fail. If these wires were to lay over the electric light or power lines, insulation chafing could occur, energizing the radio or television equipment indoors to dangerous levels.

If light or power conductors carry a potential of less than 250 volts between them, the antenna and lead-in conductors must have a clearance of at least two feet. Where practicable, antenna and lead-in conductors are not to cross over or under open electric light or power conductors. In other words, it is better not to cross, but if such crossing cannot be avoided, antenna and lead-in conductors are to be under, not over the open power and light conductors. Don't even thinks of crossing either over or under high-voltage transmission lines.

In keeping with the principle of structural integrity, the Code requires that any splices or joints in an antenna span be made mechanically secure with approved splicing devices. Here again, an alternative is offered: Other means that will not appreciably weaken the conductors are permitted. It is okay to field fabricate a splice as long as the end result is as strong or stronger than an unspliced wire segment.

The whole notion of grounding is critical in this article. Requirements are not all in one place, but pop up here and there in vaious contexts. As we shall see, there are right ways and wrong ways to ground radio and television equipment and this is especially true of the antenna and lead-in conductors.

The first mention of groujnding is the requirement that masts and metal structures that support an antenna are to be grounded in accordance with a later section titled Grounding conductors -- Receiving Stations. (Transmitters are discussed later).

The section on grounding has eleven specific requirements, lettered A through K. Here is a run-down of the key points:

• Grounding conductors are to be corrosion resistant. Copper, aluminum, copper-clad steel and bronze are permitted. Aluminum and copper-clad aluminum (the Code always groups these together with equivalent requirements) are not to be in contact with masonry or earth. Copper, in contrast, can be encased in concrete or direct buried. If aluminum or copper-clad aluminum is used outside, it must be kept 18 inches above grade.
• Grounding conductors may be solid or stranded, bare or insulated. These conductors must be securely fastened to a building surface or other supporting structure. Insulating supports are not required.
• The grounding conductor is to be protected where exposed to physical damage. Installers, designers and inspectors should draw from personal experience and judgment in this area. If a grounding conductor is recessed in an interior corner on the backside of a building, simply fastening it to the building surface may suffice. Adjacent to a parking area where vehicle damage could occur, EMT (Electrical Metallic Tubing) or conduit should be used. In all cases, where metal raceway is used to protect a grounding conductor, both ends must be bonded to the ground wire by means of a grounding bushing or other effective means. It is for this reason the UL listed gray PVC conduit is widely used in this application.
• The grounding conductor for an antenna mast or antenna discharge unit is to be run in as straight a line as possible to the grounding electrode. This is because lightning, with an extremely fast rise time, in some ways resembles a high frequency current. Any sharp bends in the grounding conductor introduce inductive reactance so that the current does not see a low impedance path to ground. You can think of it as a railroad train going too fast around a sharp curve and flying off the track.

• Metal underground water pipe at least 10' in length including any metal well casing bonded to the pipe.
• Metal frame of a building that is connected to earth.
• Concrete-encased electrode -- typically steel reinforcing bars bonded together by commonly-used steel tie wires.
• A ground ring encircling the building consisting of at least 20 feet of bare copper wire 2 AWG or larger.
• Rod or pipe electrodes at least 8' long, either pipe or conduit 3/4" minimum or listed copper clad steel ground rod 1/2" diameter minimum.
• Plate electrodes consisting of 1/4" thick steel (or .06" thick nonferrous metal) with two square feet of surface exposed to exterior soil.
• Other underground metal structures such as underground piping systems, tanks and well casings.

Not permitted to be used as ground electrodes are aluminum structures and gas piping. These items should be bonded to the grounding system but may not be counted as ground electrodes.

Concerning electrode spacing, it is provided that grounding electrodes must be at least six feet apart. If they are closer, they lose effectiveness and it is better, if possible, to have them farther apart than the six-foot minimum. Ground rods should be driven into the drip line from a roof where the soil is damper and more conductive. Beware of well-drained gravel soils where additional measures may be needed.

Part III of Article 810 covers amateur transmitting and receiving stations. It concerns antenna and lead-in conductor clearances and grounding procedures. Part IV presents interior installation specifications for transmitters (not just amateur). It covers several safety requirements including grounded metal enclosures and grounding of controls and interlocks for access doors where voltages of over 350 volts between conductors are present.

These are the basic NEC mandates for wiring radio and television equipment. Before undertaking this type of work a careful review of NEC Article 810 will provide the basis for a safe installation.

Next: Article 820, Community Antenna, Television and Radio Distribution Systems.