RCDD Grounding, Bonding, and Electrical Protection 5

Question 1

A designer is sizing a single telecommunications bonding backbone (TBB) in a new building. The routed length from the primary bonding busbar (TMGB/PBB) to the farthest secondary bonding busbar (TGB/SBB) is 18 m (59 ft). Under the ANSI/TIA-607 sizing table, what is the minimum TBB conductor size?

Accepted Answer

2/0 AWG

Answer

The ANSI/TIA-607 table sizes the TBB by length. It is 6 AWG under 4 m and grows from there: 1 AWG for 10–13 m, 1/0 AWG for 13–16 m, 2/0 AWG for 16–20 m, and 3/0 AWG over 20 m. An 18 m run falls in the 16–20 m band, so the minimum is 2/0 AWG. 3/0 AWG is the largest size the table requires and only applies above 20 m.

Question 2

A 12-story building has three separate vertical TBBs, one in each riser stack. Which practice meets ANSI/TIA-607 requirements for interconnecting them?

Accepted Answer

Bond the TBBs together with a grounding equalizer (GE) at the top floor and at least every third floor in between

Answer

When a multistory building has more than one TBB, ANSI/TIA-607 requires them to be bonded together with a grounding equalizer (GE, formerly called the TBBIBC). The GE goes at the top floor and at least every third floor in between. Bonding only at the entrance leaves long parallel conductors that can develop potential differences between them at the upper floors. A GE on every floor is allowed but goes beyond the minimum. Building steel can be bonded to the system, but it does not replace the GE interconnection.

Question 3

In a one-family dwelling, the primary protector can only be mounted where the bonding conductor run to the intersystem bonding termination would be 9 m (30 ft). Under NEC Article 800, which solution complies?

Accepted Answer

Drive a separate ground rod at least 1.5 m (5 ft) long for the protector and bond it to the power grounding electrode system with a conductor no smaller than 6 AWG

Answer

For one- and two-family dwellings, NEC 800.100 limits the primary protector bonding conductor to 6.0 m (20 ft). If that limit cannot be met, the code allows a separate communications ground rod (at least 1.5 m / 5 ft long) near the protector. That rod must be bonded to the power grounding electrode system with a conductor no smaller than 6 AWG copper, so that the two systems do not develop a difference of potential. A larger conductor does not cancel the length limit. Conduit does not remove it either, and a ferrous enclosure must be bonded at both ends.

Question 4

A carrier-grade equipment room uses an isolated bonding network (IBN) for one equipment lineup, while the rest of the room uses a mesh common bonding network (mesh-BN). How must the IBN connect to the common bonding network?

Accepted Answer

Only through a single point connection window (SPCW), with no other metallic paths between the IBN and the CBN

Answer

An IBN connects to the common bonding network (CBN) at only one place, the single point connection window (SPCW), often a dedicated busbar. This keeps lightning and fault currents from flowing through the isolated equipment. Any extra metallic path, including stray contact with cable trays, racks or conduit, defeats the isolation. Completely isolating it from the building ground (choice D) would be a safety hazard.

Question 5

What is the minimum size requirement for the bonding conductor for telecommunications (BCT) that connects the TMGB/PBB to the electrical service equipment ground?

Accepted Answer

At least as large as the largest TBB in the building

Answer

Under ANSI/TIA-607, the BCT must be at least as large as the largest TBB it serves. The BCT carries the combined currents of every backbone that ends at the TMGB/PBB, so it cannot be smaller than any single TBB. 6 AWG is only the minimum for a short TBB, and matching the power grounding electrode conductor is not the TIA rule.

RCDD - Registered Communications Distribution Designer Practice Test

RCDD - Registered Communications Distribution Designer Practice Test

RCDD Grounding, Bonding, and Electrical Protection 5