Entrances, protectors and outside plant

Every building's communication system has a weak point where cables come in from outside. Inside, the cables are protected by the building; outside, on poles, along walls and underground, they can be struck by lightning, touched by a fallen power line or raised in voltage by a fault on a nearby power system. The codes deal with that risk at the entrance: protectors that divert surges and power crosses to ground, a bonding conductor that ties the protector to the building's grounding system, and rules for how far outdoor cable may run inside. This lesson follows Maya through the entrance room at Riverside school, where the expansion adds a fiber link and copper pairs to a new portable classroom building, and through the outdoor plant that connects them.

Why entrances are dangerous

Lightning does not have to strike a cable to put a surge on it. A strike nearby induces voltage in long outdoor conductors, and a strike to a pole or the ground lifts the earth potential around it. A telephone or data pair running a few hundred meters between buildings can carry thousands of volts for a few microseconds. A power line falling across a communication cable on a pole, or a fault in a buried power cable sharing a trench, can put power voltage on it for much longer. Without protection, that energy enters the building on conductors designed for a few volts, damages equipment, starts fires and can kill someone using a handset or working on the cabling.

Two measures handle it. A primary protector, installed where the conductors enter, clamps the voltage on each conductor and diverts the current to ground before it reaches the inside wiring. Bonding then ties the protector's ground, the cable shields and the building's electrical grounding together, so the whole building rises and falls together during a surge instead of having one part at a high voltage relative to another.

When protectors are required

Under 800.90(A), a listed primary protector is required on each circuit run partly or entirely in aerial wire or cable not confined within a block, and on each circuit, aerial or underground, located within the block containing the building served so as to be exposed to accidental contact with power conductors operating at over 300 V to ground. In practice, nearly every outdoor copper communications circuit is treated as exposed, and the telephone company installs protectors on its own lines. Where Cobalt Line runs copper between buildings on a campus, it is the installer of those conductors and must provide the protection at both ends. Whether an interbuilding run is exposed is a decision for the designer and the inspector, not a field assumption.

Protectors come in two main types. Gas tube and solid-state protectors for each pair are mounted in a building entrance terminal, a block with a ground lug that the bonding conductor connects to. Fused protectors also open the circuit if a sustained power cross flows. Data and coaxial circuits use protectors designed for their signals, so the protection does not degrade a 1 gigabit link or a video feed. Primary protection at the entrance is often followed by secondary protection at sensitive equipment, but the secondary device does not replace the primary.

Where the protector goes

Under 800.90(B), the protector must be located in, on or immediately adjacent to the structure served, as close as practicable to the point of entrance, so that the unprotected length inside is as short as possible. It must not be located in hazardous locations or near easily ignitable material, and the location should allow the bonding conductor to run short and straight to the intersystem bonding termination.

The bonding conductor

Under 800.100, the bonding conductor from the protector must be insulated copper or other corrosion-resistant conductor, at least 14 AWG, with a current-carrying capacity at least that of the grounded metallic sheath or protected conductor, and run in as straight a line as practicable. In one- and two-family dwellings its length may not exceed 20 ft (6.0 m); if the protector cannot be located that close, a separate ground rod is driven and bonded to the electrical grounding electrode system with a 6 AWG conductor. It connects to the intersystem bonding termination, which 250.94 requires at the service of most buildings, or to another point the code lists, such as the service equipment enclosure or the grounding electrode conductor. Sharp bends and long routes raise the conductor's impedance to a fast surge, so a protector mounted beside the service is far better than one at the other end of the room.

The cabling standards add to the code. TIA-607 requires the telecommunications bonding system, described in lesson 11, to start at a primary bonding busbar in the entrance room, bonded to the electrical service, and the protectors bond to that busbar. Where the protector and the busbar are close together, one short conductor serves both.

# Entrance bonding conductor check: size, length and bends against the job rules
MIN_AWG = 14             # smallest size allowed (a larger number is a smaller wire)
MAX_LEN = 20             # ft, length limit where it applies (dwelling) or job target
runs = [  # (entrance, size AWG, length in ft, number of 90-degree bends, dwelling?)
    ("Riverside main entrance", 6, 8, 1, False),
    ("Riverside portable building", 14, 13, 3, False),
    ("Caretaker's house", 14, 28, 2, True),
]
for name, awg, length, bends, dwelling in runs:
    notes = []
    if awg > MIN_AWG:
        notes.append("too small")
    if dwelling and length > MAX_LEN:
        notes.append("too long for a dwelling: add a separate electrode, bonded to the service")
    if bends > 2:
        notes.append("reroute to reduce bends")
    print(f"{name:28s} {awg:3d} AWG {length:5} ft {bends} bends  {'OK' if not notes else '; '.join(notes)}")

Riverside main entrance        6 AWG     8 ft 1 bends  OK
Riverside portable building   14 AWG    13 ft 3 bends  reroute to reduce bends
Caretaker's house             14 AWG    28 ft 2 bends  too long for a dwelling: add a separate electrode, bonded to the service

The dwelling limit in the check is the 20 ft rule of 800.100(A)(4). In other buildings the code asks for as short and straight as practicable, and the inspector judges what is practicable. The point of the check is the same everywhere: long, looping bonding conductors defeat the protector.

Shields, sheaths and metallic members

Outdoor cables often have metal in them besides their conductors: an aluminum or copper shield, a steel armor, or the strength members and armor of some fiber cables. That metal can carry surge current just as conductors can. The codes require the metallic sheath or shield of a cable entering a building to be bonded to ground as close as practicable to the entrance, or interrupted by an insulating joint at the entrance. Fiber cables with metallic members are treated the same way, which is one reason designers prefer all-dielectric fiber cable between buildings: with no metal, there is nothing to bond and no path for a surge.

Outdoor cable inside the building

Outdoor cable is built for sunlight, water and burial, not for fire performance, and most of it has no indoor listing. The NEC, in 800.48 for communications and 770.48 for optical fiber, allows unlisted outside plant cable to run up to 50 ft (15 m) into a building from the point of entrance, provided it terminates in an enclosure; beyond that it must be spliced to listed indoor cable or run in raceway. Riverside's specification calls for every outdoor cable to end in the entrance room within a few feet of the wall sleeve, at a splice enclosure for fiber and at the protector block for copper. Indoor-outdoor cables listed for indoor use as well, such as OFNR or OFNP indoor-outdoor fiber, can run further because they meet the indoor requirement.

Outside plant between buildings

On a campus, the owner usually owns the pathways and cables between buildings, called customer-owned outside plant. TIA-758 is the standard for its design. The main elements are underground conduits, called duct banks when several are encased together, handholes and maintenance holes where cables can be pulled and spliced, direct-buried cable in some cases, and aerial cable on poles where underground is not practical.

• Conduits are buried below the frost line where the climate requires it, with enough cover to protect them from traffic and digging, and sloped to drain toward handholes so water does not sit at the building wall.
• Every conduit entering a building is sealed at the wall with a duct seal or a mechanical plug designed for it, to keep water, gas and rodents out. A gas leak in a street can travel along an unsealed conduit into a building.
• Power and communication cables in a shared trench are separated by the distances the utilities and the codes require, often with a barrier of concrete or a specified depth of earth between them.
• A warning tape is laid above buried conduits, and the route is recorded with GPS coordinates for future locates.

Before any digging, the crew must call Texas 811 so the utilities mark their lines, and a private locator must mark the owner's own lines, which the public utilities do not locate. Riverside's own buried lines include an old copper cable to the portable classrooms, the fire sprinkler main and a parking lot lighting circuit, none of which the public locate would show.

Copper or fiber between buildings

Copper between buildings brings every problem this lesson describes: lightning, power crosses, ground potential differences between the buildings and protectors at both ends. All-dielectric fiber brings none of them. That is why most campus designs now use fiber for data between buildings and keep copper only for circuits that must be copper, such as some fire alarm, intercom or elevator phone lines, or analog lines required by the telephone company. Riverside's expansion runs a 12-strand singlemode fiber to the portable building for data and a 6-pair copper cable for the intercom and the elevator emergency phone, with protectors at both ends of the copper.

Coaxial and broadband entrances

Cable television and broadband coaxial cables entering a building follow the same principles under Article 820: the outer conductor of the coaxial cable is bonded to the intersystem bonding termination or another permitted point, as close as practicable to the point of entrance, usually through a grounding block, with the same 14 AWG minimum and dwelling length limit as communications. Antenna masts are bonded under Article 810. Cobalt Line installs coaxial distribution for the condo tower at Juniper Point and bonds the provider's grounding blocks to the PBB in the entrance room.

Records at the entrance

The entrance room is where future trouble is most likely to be investigated, after a lightning strike or a power cross, so its records matter. Cobalt Line photographs each protector and its bonding conductor route, labels the protector blocks with the cable and pair counts they serve and records the length of every outdoor cable from the wall to its splice or termination. The protector bonding conductors are labeled at both ends, and their connection to the PBB is shown on the bonding riser diagram. When the owner's electrician later replaces the service, those records tell him exactly what must be reconnected.