The telecommunications bonding backbone (TIA-607)

The electrical code requires communication protectors and shields to be bonded to the building's grounding system. It does not say much about how to bond the racks, trays, shielded panels and equipment spread across a dozen telecommunications rooms on different floors. ANSI/TIA-607, the standard for generic telecommunications bonding and grounding, fills that gap. It describes a bonding infrastructure that starts at the electrical service and reaches every telecommunications room through a backbone of large copper conductors, so that all the communication equipment in the building sits on one low-impedance system bonded to the power system. The current edition is ANSI/TIA-607-E, published in 2024; many specifications still name the D edition of 2019, and the terms and principles are the same. At Kellerman, Maya's crew is building the whole system new, from the entrance room to the twelfth floor.

The parts of the system

The standard names each part, and recent editions renamed several of them, so both the old and new names still appear on drawings:

• The primary bonding busbar, PBB, formerly the telecommunications main grounding busbar, TMGB. There is normally one per building, in the entrance facility or the main equipment room, close to the electrical service.
• The telecommunications bonding conductor, TBC, formerly the bonding conductor for telecommunications, BCT. It connects the PBB to the electrical service's grounding, at the service equipment or the bonding point the electrician provides.
• Secondary bonding busbars, SBB, formerly telecommunications grounding busbars, TGB. There is one in each telecommunications room and equipment room other than the room with the PBB.
• The telecommunications bonding backbone, TBB, a conductor that runs from the PBB through the building to each SBB.
• The backbone bonding conductor, BBC, formerly the grounding equalizer, which ties together two or more TBBs in a large building, at the top floor and at least every third floor.
• Equipment bonding conductors, which connect racks, cabinets, cable trays and equipment in each room to that room's busbar.

Busbars

Busbars are flat bars of bare or tin-plated copper, mounted on insulated standoffs so they do not contact the wall, and drilled for two-hole compression lugs. The standard sets minimum sizes: a PBB at least 1/4 in thick and 4 in wide, an SBB at least 1/4 in thick and 2 in wide, each long enough for the connections it serves plus room for growth. They are mounted where they are accessible, usually on the backboard or wall of the room, close to the panelboard that serves the room and to the cable entrance. Tin-plating reduces corrosion and the need for antioxidant, and many specifications require it.

A busbar is planned like a patch panel: count the connections it will need now and in future, and pick a length with enough hole pairs. At the PBB those connections include the TBC to the service, each TBB, the protectors, the cable shields entering the building, the entrance room's racks and trays, the building steel and the panelboard serving the room. At each SBB they include the TBB, any BBC, the racks, cabinets and ladder racks, the panelboard, the building steel and spares.

# Busbar planning: two-hole lug positions needed now and with spares, and the bar length that holds them
import math
PITCH = 1.0             # in per two-hole lug position, including spacing
rooms = {  # room: list of connections now
    "Entrance room (PBB)": ["TBC to service", "TBB riser A", "TBB riser B", "protectors", "OSP shields",
                            "rack 1", "rack 2", "ladder rack", "panel LP-B", "building steel"],
    "TR 7 (SBB)": ["TBB riser A", "BBC to riser B", "rack 1", "rack 2", "ladder rack", "panel LP-7", "building steel"],
}
for room, conns in rooms.items():
    now = len(conns)
    need = math.ceil(now * 1.5)               # half again for growth
    length = need * PITCH
    print(f"{room:22s} {now:2d} connections now, {need:2d} positions with spares, bar about {length:.0f} in long")

Entrance room (PBB)    10 connections now, 15 positions with spares, bar about 15 in long
TR 7 (SBB)              7 connections now, 11 positions with spares, bar about 11 in long

Sizing the backbone

The TBB must be large enough to keep its impedance low over the distance it runs, which means it grows with length. The minimum for any bonding conductor in the system is 6 AWG copper. The TBB is sized by its length, and recent editions of the standard express the rule as about 2 kcmil of conductor per linear foot of length, with a maximum of 750 kcmil. Earlier editions and many specifications use a table of lengths and sizes; a typical version is shown below. The TBC from the PBB to the service must be at least as large as the largest TBB, and a BBC at least as large as the TBBs it connects.

# TBB size by length: kcmil-per-foot rule of recent editions, rounded up to a standard size
SIZES = [("6 AWG", 26.24), ("4 AWG", 41.74), ("3 AWG", 52.62), ("2 AWG", 66.36), ("1 AWG", 83.69),
         ("1/0", 105.6), ("2/0", 133.1), ("3/0", 167.8), ("4/0", 211.6), ("250 kcmil", 250), ("300 kcmil", 300),
         ("350 kcmil", 350), ("500 kcmil", 500), ("750 kcmil", 750)]
def tbb_size(length_ft):
    need = min(2 * length_ft, 750)
    for name, kcmil in SIZES:
        if kcmil >= need:
            return name, need
    return "750 kcmil", need
runs = [("TR 2", 30), ("TR 7", 88), ("TR 12", 150)]   # length from the PBB along the route, ft
for room, length in runs:
    ft = length
    name, need = tbb_size(ft)
    print(f"{room:6s} {length:5} ft -> needs {need:5.0f} kcmil -> {name}")

TR 2      30 ft -> needs    60 kcmil -> 2 AWG
TR 7      88 ft -> needs   176 kcmil -> 4/0
TR 12    150 ft -> needs   300 kcmil -> 300 kcmil

The two methods give different answers for long runs, which is why the specification must say which applies. Kellerman's specification names TIA-607-D, so Sanjay sizes each TBB by the kcmil rule and checks it against the table, and uses one TBB size for the whole riser, sized for the farthest room, so the conductor can run continuously past each floor with taps to each SBB. A continuous TBB with listed tap connectors at each floor is the usual design; splicing is allowed only with listed connectors or exothermic welds and kept to a minimum.

Routing and installing the backbone

• The TBB runs as directly as practicable from the PBB to the SBBs, usually in the riser beside the backbone cabling, supported every meter or so and protected from damage.
• It is insulated, with a green jacket or green marking, and continuous, without sharp bends; bends are kept to a radius of at least 8 in and an angle of at least 90 degrees where possible.
• It is not run in ferrous metal conduit unless the conduit is bonded to the conductor at both ends, because a steel conduit around a single conductor acts as a choke and raises the conductor's impedance to surges.
• Metallic cable shields, water pipes and building steel are not substitutes for the TBB. Building steel is bonded to the busbars where it is accessible, as an additional path, but the TBB is a separate conductor.
• Every conductor and busbar is labeled as TIA-606 and the specification require. The usual warning label reads in effect: if this connector or cable is loose or must be removed, call the building telecommunications manager.

Bonding inside the room

In each telecommunications room, the SBB is the hub. Each rack or cabinet is bonded to it with a rack bonding conductor, usually 6 AWG, either individually or through a rack bonding busbar running along the top of a row. Equipment in the rack bonds to the rack through its mounting with paint-piercing hardware or through its own bonding conductor, and shielded patch panels bond through the rack. Ladder racks and cable trays are bonded with jumpers across every joint that is not mechanically continuous, and to the SBB. The panelboard serving the room is bonded to the SBB, so that the power ground and the telecom bonding in that room are tied together at a known point.

The connections in the room use the same practices as lesson 10: listed two-hole compression lugs on busbars, paint removed under every lug, antioxidant where required and no screws through paint. Conductors are kept short, and taps to the busbar are made with listed tap connectors rather than by looping the TBB through the busbar.

Testing the bonding system

Bonding connections are inspected visually and tested electrically. The visual check covers lug type and crimp, tightness, removal of paint, conductor size and labels. The electrical check is usually a two-point bonding test, measuring the resistance between each busbar and the PBB, or between equipment and its busbar, with a low-resistance ohmmeter. TIA-607 leaves the limits to the design; many specifications require no more than 100 milliohms between any point in the bonding system and the PBB, and much less between a rack and its own busbar. A high reading points to a loose or corroded connection or a missing bond.

Testing is done before the system is energized and again at closeout. The results go in the closeout records with the conductor sizes, routes and busbar locations, and they become the baseline for the owner's maintenance: a bonding system that measured 25 milliohms at handover and 400 milliohms five years later has a failing connection somewhere.

Large buildings and data centers

Buildings with several risers have a TBB in each, tied together by BBCs at the top floor and at least every third floor in between, so that the backbones stay at the same potential. Data centers go further, with a mesh-common bonding network of conductors under the raised floor or overhead, bonded to every rack, and supplementary bonding grids. Those designs are covered by TIA-942 and the data center specification, and they follow the same principles: everything bonded, conductors short and large, and one connection to the building's grounding.

Coordinating the bonding system with the electrician

The TIA-607 system meets the electrical system at three kinds of point, and each needs agreement on the drawings. At the service, the TBC lands where the electrical engineer has designated, and the electrician usually makes that connection. In each telecommunications room, the panelboard serving the room is bonded to the SBB, a conductor the electrician may provide. And at building steel, the bond is often an exothermic weld made by a specialist. Kellerman's interface list from lesson 1 gives each of these an owner, and Maya checks every one of them on the pre-inspection walk in lesson 12.