A contractor bonded 30 new racks in a colocation hall with a single jumper daisy-chained from rack to rack, and one lead back to the room busbar. It looked tidy and passed a visual walk-through. It was wrong twice over: the standard prohibits daisy-chaining, and the last rack sat at the end of a long, high-impedance path.
The drawing had a second problem. It labeled the busbar a TMGB. The current edition calls it a PBB.
What TIA-607 Covers
ANSI/TIA-607 is the standard for the telecommunications bonding and grounding infrastructure inside a building, and its full title sets the boundary: Generic Telecommunications Bonding and Grounding (Earthing) for Customer Premises. It specifies the infrastructure itself, plus the way it interconnects to the electrical system and to the telecommunications system.
It comes from TIA’s TR-42 committee, the same family behind the TIA-568 and TIA-606 standards. Where TIA-568 tells you what the cabling performs and TIA-606 tells you how it is labeled, TIA-607 tells you what the metallic infrastructure is bonded to, and how.
The version situation needs care. TIA’s own announcement puts ANSI/TIA-607-E at May 17, 2024, issued to incorporate addendum ANSI/TIA-607-D-1 and update references. Two industry references still cite -D, one dated 2021. Neither is wrong on technical content, but if a specification names an edition, confirm which is current before submitting.
Grounding and Bonding Are Not the Same Thing
Grounding and bonding do different jobs
The two words get used interchangeably and mean different things in the standard, and the difference explains why the requirements look redundant until you see the goal.
Grounding (earthing) is intentionally connecting a circuit or equipment to the earth, establishing a zero-voltage reference and giving fault current and lightning transients a safe path to dissipate.
Bonding is permanently joining metallic parts to form a conductive path that ensures electrical continuity and can safely carry any fault current likely to be imposed. The point of bonding is equalizing potential — no damaging voltage difference between two pieces of metal a person might touch at once.
That second goal is the one engineered away by accident: a technician standing between two racks that are each "grounded" but not bonded can complete a circuit through their own body. Grounding each item separately does not prevent it; only bonding to a common reference does.
The Hierarchy: PBB, SBB and TBB
The TIA-607 bonding hierarchy and the renamed acronyms
The infrastructure is a three-part hierarchy, and the acronyms were renamed in a recent revision. That rename is the most common source of confusion on live projects, because drawings and old specifications still use the previous names.
Component | Older name | Current name | Where it lives |
|---|
Primary busbar | TMGB | PBB (Primary Bonding Busbar) | Entrance facility or main equipment room |
|---|
Secondary busbar | TGB | SBB (Secondary Bonding Busbar) | Each telecommunications room |
|---|
Main conductor to the electrical service | BCT | TBC (Telecommunications Bonding Conductor) | From the PBB to the building electrical service ground |
|---|
Bonding backbone | TBB | TBB (unchanged) | Interconnects all SBBs back to the PBB |
|---|
The PBB is the central hub and functions as the dedicated extension of the building’s grounding electrode system for telecommunications. It is a pre-drilled copper busbar, normally insulated from its mounting surface, and it must accept every bonding conductor that lands on it. The TBC links the PBB to the building’s main electrical service ground. Each SBB gives a local bonding point for equipment, racks and pathways in that room, and the SBBs tie back to the PBB through the TBB.
In a multistory building with more than one TBB, the TBBs are bonded together at the top floor and at a minimum of every third floor in between.
Sizing the Conductors
The sizes are specified, and the rule is finer than the familiar "6 AWG minimum."
The bonding backbone is copper, minimum No. 6 AWG, and sized at 2 kcmil per linear foot of conductor length, up to a maximum of 3/0 AWG. That per-foot rule is the part installers miss — a long backbone needs more copper than a short one, so the size is calculated from the run rather than copied from the last job. If the backbone is insulated, the insulation must meet the fire rating of the pathway it runs in.
The 6 AWG floor applies broadly: to the backbone, to the main conductor, and to the bonding jumpers. A 6 AWG copper equipment grounding conductor is the minimum for rack power circuits up to 60 A under NEC Article 250, rising to 4 AWG up to 100 A. Bonding the main busbar to building steel is commonly specified at 2 AWG minimum, often with a 0.1 ohm target.
Two limits on what the backbone is for. It reduces or equalizes potential differences between telecommunications systems, and while it carries current during an AC ground fault, it is not intended to be the only ground fault current return path. The sizing rule is also not intended to address the reduction or control of electromagnetic interference — nobody should size a bonding conductor to fix a noise problem.
What Must Be Bonded, and What Must Not
The bonded items are the ones you expect once the goal is stated: rack and cabinet frames, cable trays, ladder racks and basket, conduit, patch panel mounting hardware, and the shield systems of shielded cable. Each rack gets bonded; the intent is equipotential, not decoration.
The standard is explicit about what may not be drafted into service as the backbone:
- The interior water piping system shall not be used as the backbone.
- The metallic cable shield shall not be used as the backbone.
- Bonding the steel reinforcement bars of a reinforced concrete building is not required.
The shield prohibition separates two things that look similar. A shield can be bonded, and on shielded cable it must be — but a shield is not a substitute for the backbone. It is a signal-path element, not a bonding conductor.
The concrete clause works the other way and saves money: reinforced concrete rebar is not required to be bonded. If a specification demands it, that is a project requirement, not a TIA-607 requirement.
The Installation Rules That Get Violated
Daisy-chaining racks versus dedicated bonding conductors
Most field failures come from four workmanship requirements.
No daisy-chaining. Bonding jumpers run rack to rack to rack are prohibited. Each rack needs its own dedicated bonding conductor back to the backbone or to a local busbar. A chain makes every downstream rack depend on every upstream connection, and the impedance at the far end is the sum of the whole path.
No shared conduit. The backbone may not run through conduit that also carries current-carrying conductors, to prevent inductive interference. Plan the bonding route alongside the cable pathway.
Remove the paint. Bonding a lug to a painted rack leaves an insulator in the path. Paint comes off to bare metal, and antioxidant joint compound goes on to keep the connection from corroding.
Use two-hole lugs and control the bends. Two-hole compression lugs are recommended for critical connections such as the main conductor to the busbar, because they resist loosening. Bonding conductors should run with large sweep bends rather than sharp 90-degree corners, since a tight corner raises impedance to high-frequency surge current — exactly the current the conductor exists to carry.
One habit matters because it fails silently: torque every lug to the manufacturer’s specification. An under-torqued connection can pass its initial test, then increase in resistance as the joint oxidizes.
Verifying It: One Ohm, and the Right Meter
The acceptance threshold is a maximum resistance of 1 ohm between any two bonded points. That is a measurable number, which makes it checkable rather than a workmanship opinion.
Measuring it correctly is where projects go wrong. A standard digital multimeter cannot resolve milliohm-range resistance — it does not drive enough current through the joint. Verification requires a low-resistance ohmmeter, also called a digital low resistance ohmmeter or ground bond tester, applying roughly 200 mA or more. A meter that reads "0.2 ohms" with no current drive behind it has told you nothing.
Where conductors are insulated, standard practice is green with a yellow stripe, per NEC Article 250.119.
Frequently Asked Questions
What is the difference between TIA-607 and the NEC?
The NEC is code and sets the legally enforceable requirements. TIA-607 details how to build the telecommunications-specific infrastructure, including the busbar hierarchy and conductor sizing the code does not describe at that level.
What do PBB and SBB mean?
Primary Bonding Busbar and Secondary Bonding Busbar — the current names for the components previously called TMGB and TGB. The conductor from the PBB to the electrical service ground is now the TBC, previously the BCT.
What is the minimum size for a telecommunications bonding conductor?
No. 6 AWG copper. The backbone is sized at 2 kcmil per linear foot of length up to a maximum of 3/0 AWG.
Can I daisy-chain bonding jumpers between racks?
No. Each rack requires its own dedicated bonding conductor to the backbone or a local busbar. Chaining them is prohibited.
Does shielded cable need the shield bonded at both ends?
For shielded Cat6A in high-density 10GBASE-T environments, drain wires are bonded at both ends of the run to the same ground reference. That only helps if the grounding system is at equipotential, which is what the rest of TIA-607 builds.
The Short Version
TIA-607 covers the telecommunications bonding and grounding infrastructure: the busbars, the conductors that connect them, and the way that infrastructure ties to the building electrical system. TIA’s announcement dates the current edition, ANSI/TIA-607-E, to May 2024, incorporating the -D-1 addendum; several industry references still cite -D.
Grounding establishes an earth reference and a path for fault current; bonding equalizes potential between metallic parts. Bonding is what stops a person from becoming the path between two "grounded" racks.
The hierarchy is a primary busbar in the entrance facility or main equipment room, a secondary busbar in each telecommunications room, and a backbone tying them together — with the acronyms renamed to PBB, SBB and TBC from the older TMGB, TGB and BCT. Expect old drawings to use the old names.
Conductor sizing follows a rule rather than a table: 6 AWG minimum, 2 kcmil per linear foot, 3/0 AWG maximum for the backbone. The backbone equalizes potential. It is not the only fault return path, and it is not an EMI fix.
Do not daisy-chain racks, do not share conduit with current-carrying conductors, remove the paint, use two-hole lugs, keep the bends sweeping, and torque the joints. Do not draft the interior water piping or the metallic cable shield into service as the backbone, and do not bond concrete rebar on the standard’s account.
Then measure it — with a low-resistance ohmmeter, not a multimeter, against a 1 ohm limit between any two bonded points.
Our communication cable and patch cord ranges are built for installations that get inspected, with construction details documented for submittal. How Ethernet cables work covers the transmission principles underneath, every test and inspection parameter is listed by measurement, and our certifications cover the compliance side. If a submittal needs shielded construction or drain wire data for a bonding scheme, send us the details.
Sources
- Telecommunications Industry Association — ANSI/J-STD-607-A, Commercial Building Grounding (Earthing) and Bonding Requirements For Telecommunications (approved October 22, 2002), covering that the standard replaces ANSI/EIA/TIA-607 Commercial Building Grounding and Bonding Requirements for Telecommunications and that ANSI/EIA/TIA-607 is rescinded, the scope and purpose sections and the note that work area grounding is beyond the scope of the standard, the requirement in 5.4.3.2 that the interior water piping system of the building shall not be used as a TBB, the requirement in 5.4.3.3 that the metallic cable shield shall not be used as a TBB, the requirement in 5.4.3.4 that whenever two or more TBBs are used within a multistory building the TBBs shall be bonded together with a GE formerly known as the telecommunications bonding backbone interconnecting bonding conductor at the top floor and at a minimum of every third floor in between, the design provisions in 5.4.3.1 that the design of a TBB shall be consistent with the design of the telecom system and permit multiple TBBs as dictated by the building size and address routing to minimize the lengths of the TBBs, the sizing requirement in 5.4.4.1 that the TBB shall be a copper conductor and that the minimum TBB conductor size shall be a No. 6 AWG and that the TBB should be sized at 2 kcmil per linear foot of conductor length up to a maximum size of 3/0 AWG and that the TBB may be insulated and if insulated the insulation shall meet the fire ratings of its pathway and that the sizing of the TBB is not intended to account for the reduction or control of electromagnetic interference, the function statement in 5.4.4.2 that the TBB shall be connected to the TMGB and that the intended function of a TBB is to reduce or equalize potential differences between telecommunications systems and that while the TBB will carry some current under ac power ground fault conditions it is not intended to provide the only ground fault return path and that a TBB is not intended to serve as the only conductor providing a ground fault current return path, the bonding to metal building frame provisions in 5.6.4 that when practicable because of shorter distances and where horizontal steel members are permanently electrically bonded to vertical column members the TGB may be bonded to these horizontal members in lieu of the vertical column members, the provision in 5.6.5 that this Standard does not require bonding of the steel bars of a reinforced concrete building to the TGB or TBB, the table of contents structure showing section 5.5 the telecommunications grounding busbar with 5.5.2 bonds to the telecommunications grounding busbar and 5.5.4 installation considerations and section 5.6 bonding to the metal building frame and section 6 telecommunications entrance facility with 6.2 placing the telecommunications main grounding busbar and 6.3 other attachments to the telecommunications main grounding busbar and section 7 telecommunications room and equipment room with 7.2 multiple telecommunications grounding busbars and 7.3 placing the telecommunications grounding busbar and 7.4 other connections to telecommunications grounding busbar and section 9 towers and antennas, and the tower guidance in C.11.8 that the interior ground ring should be bonded to the exterior ground ring with #2 AWG conductor routed as straight as possible using exothermic connections or listed connectors — http://critical-eng.com/wp-content/uploads/2020/09/TIA-607.pdf
- Open Exam Prep — BICSI Installer 2 Copper study guide, section 4.3 Grounding, Bonding, and Surge Protection (last updated July 2026), covering that ANSI/TIA-607-D provides the requirements for telecommunications grounding and bonding infrastructure and is the definitive standard governing this critical infrastructure, the definition that grounding or earthing is the act of intentionally connecting an electrical circuit or equipment to the earth to establish a zero-voltage reference and provide a safe path for fault currents and lightning strikes to dissipate into the earth, the definition that bonding is the permanent joining of metallic parts to form an electrically conductive path that ensures electrical continuity and the capacity to conduct safely any fault current likely to be imposed and that the primary goal of bonding is to equalize potential across all connected metallic components preventing electrical shock and minimizing interference, the statement that the Primary Bonding Busbar formerly known as the Telecommunications Main Grounding Busbar is the central hub of the telecommunications bonding system typically located in the Entrance Facility or the main Equipment Room and that it serves as the dedicated extension of the building grounding electrode system and that it must be a pre-drilled copper busbar typically insulated from its mounting surface and sized adequately to accommodate all necessary bonding conductors, that the Telecommunications Bonding Conductor formerly the Bonding Conductor for Telecommunications is the critical link that connects the Primary Bonding Busbar to the building’s main electrical service equipment ground and must be adequately sized based on the length of the run and the size of the building’s electrical service but that TIA standards mandate a minimum size of 6 AWG, that the Secondary Bonding Busbar formerly known as the Telecommunications Grounding Busbar is located in each Telecommunications Room and provides a local bonding point for equipment racks and pathways within that specific room and that Secondary Bonding Busbars are bonded back to the Primary Bonding Busbar to ensure a unified ground potential, that the Telecommunications Bonding Backbone is the conductor that interconnects all the Secondary Bonding Busbars within a building back to the central Primary Bonding Busbar and equalizes potentials between different telecommunications rooms preventing stray currents from traveling over data cables and must be properly sized with a minimum of 6 AWG and installed without splices whenever possible to maintain a low-impedance path, that in multi-story buildings with multiple Telecommunications Bonding Backbones a Backbone Bonding Conductor is used to interconnect them at the top floor and at least every third floor in between, the acronym comparison table listing Primary Busbar as TMGB becoming PBB located in the Entrance Facility or equipment room and Secondary Busbar as TGB becoming SBB located in the Telecommunications Room and Main Conductor as BCT becoming TBC connecting the Primary Bonding Busbar to Electrical Service Ground and Backbone Conductor as TBB remaining TBB connecting Secondary Bonding Busbars to the Primary Bonding Busbar, the surge protection note that properly grounded Primary Protectors are required by the NEC at the entrance facility for outside plant cables and that their effectiveness relies entirely on a low-impedance connection to the Primary Bonding Busbar via a properly sized bonding conductor, and the best practices for installers that when bonding lugs to painted racks or equipment the paint must be completely removed to ensure a clean metal-to-metal connection and antioxidant joint compound should be applied to prevent corrosion and that TIA-607-D recommends the use of two-hole compression lugs for critical connections like the Telecommunications Bonding Conductor to the Primary Bonding Busbar to prevent the connection from loosening over time and that bonding conductors should have large sweep bends avoiding sharp 90-degree angles which increase impedance to high-frequency surge currents — https://open-exam-prep.com/study-guides/bicsi-inst2-copper/standards-and-code-compliance/grounding-bonding
- Heather Technologies — Server Rack Grounding and Bonding: ESD Protection and Code Compliance, covering that a single ungrounded server rack can accumulate static potentials exceeding 15,000 volts under low-humidity conditions while the human body typically cannot detect a discharge below 3,500 volts, that ANSI/TIA-607-D of 2021 is the primary telecommunications bonding and grounding standard in North America and mandates a Telecommunications Bonding Backbone and a Telecommunications Main Grounding Busbar and requires all metallic rack and cabinet frames to be bonded to the Telecommunications Bonding Backbone using conductors with a maximum resistance of 1 ohm between any two bonded points, that ANSI/TIA-942-B specifies that Tier classifications affect redundancy rather than bonding exemptions and that all Tier I through Tier IV data centers must comply with grounding provisions, that NFPA 70 Article 250 requires equipment grounding conductors for rack power distribution to be sized per Table 250.122 with a minimum 6 AWG copper conductor for circuits up to 60 amperes rising to 4 AWG for circuits up to 100 amperes, that the definitions of grounding as establishing a reference potential that limits voltage rise during fault conditions and lightning transients and bonding as connecting all metallic elements to the same equipotential plane so no voltage difference exists between them and that in a server rack environment bonding means the rack frame and cable trays and PDU chassis and patch panel mounting hardware and raised floor tiles are all connected to the same low-impedance node and that without equipotential bonding a technician simultaneously touching two grounded but unbonded metallic surfaces can complete a circuit through their body, the ANSI/ESD S20.20 requirement that worksurfaces and floor materials and equipment frames in an ESD protected area maintain a resistance to ground between 1 times 10 to the 6th ohms and 1 times 10 to the 9th ohms for personnel protection while equipment frames should measure below 1 ohm to the bonding reference point, that shielded Cat6A installations which TIA-568.2-D permits for high-density 10GBASE-T environments per IEEE 802.3an require that shield drain wires be terminated and bonded at both ends of the cable run to the same ground reference and that this practice eliminates shield-induced noise only when the grounding system is itself at equipotential, the bonding hardware specification table listing the Telecommunications Bonding Backbone under ANSI/TIA-607-D at 6 AWG minimum and 3/0 AWG maximum with 1 ohm point-to-point and the rack-to-backbone bonding jumper at 6 AWG with 1 ohm and lug termination without splices and the busbar to building steel at 2 AWG minimum with 0.1 ohm recommended and the shielded Cat6A drain wire termination under TIA-568.2-D and IEEE 802.3an at less than 1 ohm shield-to-panel bonded at both ends to the same reference, the implementation guidance that the Telecommunications Main Grounding Busbar must be installed adjacent to the main distribution frame or main distribution area before any rack positioning begins, that the Telecommunications Bonding Backbone should be routed in parallel with the cable pathway and that ANSI/TIA-607-D prohibits routing the backbone through conduit that also carries current-carrying conductors to prevent inductive interference, that daisy-chaining rack-to-rack bonding jumpers is explicitly prohibited by ANSI/TIA-607-D and that each rack requires its own dedicated bonding conductor back to the backbone or a local Telecommunications Grounding Busbar, that a common installation failure is under-torqued lug connections that pass initial testing but increase in resistance as the connection oxidizes so calibrated torque tools should be used and values documented in the as-built record, that verification requires a low-resistance ohmmeter rather than a standard multimeter because standard multimeters have insufficient current drive to measure milliohm-range resistance accurately and that a dedicated Digital Low Resistance Ohmmeter or ground bond tester applying 200 milliamperes or greater is required to confirm the 1 ohm threshold, that all bonding conductors should be labeled green with yellow stripe per NEC Article 250.119 where insulated, and the note that grounding and bonding documentation is as critical as the physical installation with inspectors expecting measured resistance values and conductor sizing calculations and a complete as-built bonding diagram — https://heathertechnologies.com/pages/server-rack-grounding-and-bonding-esd-protection-and-code-compli
