C7 Grounding & Bonding for Low Voltage Systems 5 — Questions and Answers
Question 1: Which NEC article specifically covers grounding and bonding requirements for network-powered broadband communications systems?
- Article 640
- Article 725
- Article 830 (Correct answer)
- Article 800
Correct answer: Article 830
NEC Article 830 addresses network-powered broadband communications systems, including their grounding and bonding requirements.
Question 2: A low voltage contractor is bonding multiple equipment racks in a data room. The bonding conductor between racks should be connected to the:
- Telecommunications grounding busbar (TGB) in a daisy-chain from rack to rack
- Each rack individually run back to the TMGB using star topology (Correct answer)
- Building steel only, bypassing the TGB
- AC ground bar inside the nearest electrical panel
Correct answer: Each rack individually run back to the TMGB using star topology
TIA-607 recommends a star (home-run) topology where each rack is bonded individually back to the TGB to prevent cascading impedance and ground loops.
Question 3: What is the purpose of an intersystem bonding termination (IBT) device as defined in NEC 250.94?
- To provide a grounding point for multiple low voltage systems at a common accessible location (Correct answer)
- To isolate different low voltage systems from the electrical grounding system
- To measure ground resistance for multiple systems simultaneously
- To connect communication neutrals to the service entrance neutral
Correct answer: To provide a grounding point for multiple low voltage systems at a common accessible location
NEC 250.94 requires an IBT to provide an accessible bonding point for communications, CATV, and other low voltage systems at or near the service entrance.
Question 4: A C7 contractor installs a structured cabling system in a multi-story building. Telecommunications grounding busbars (TGBs) on each floor must be bonded back to the TMGB using:
- A telecommunications bonding backbone (TBB) (Correct answer)
- The building's conduit system as the sole path
- Individual ground rods on each floor
- Shared neutral conductors from the electrical panels
Correct answer: A telecommunications bonding backbone (TBB)
TIA-607 defines the Telecommunications Bonding Backbone (TBB) as the conductor that interconnects TGBs on each floor back to the TMGB.
Question 5: When a coaxial cable for satellite TV enters a building, the outer shield of the cable must be bonded before the cable connects to the first piece of equipment. What is this bonding point called?
- Equipotential plane
- Antenna discharge unit (ADU) (Correct answer)
- Surge protective device (SPD)
- Primary protector
Correct answer: Antenna discharge unit (ADU)
NEC 820.100 requires an antenna discharge unit (ADU) at the entry point of coaxial cable to bond the shield and discharge static or lightning energy before it reaches equipment.
Question 6: In a Class 2 low voltage control wiring installation, equipment grounding conductors are required when:
- The circuit voltage exceeds 30V and metal raceways are used
- Any metallic enclosure or raceway is part of the installation (Correct answer)
- The circuit supplies Class 2 power-limited loads exceeding 100VA
- Grounding is never required for Class 2 circuits
Correct answer: Any metallic enclosure or raceway is part of the installation
NEC 250.86 requires that any metallic enclosure, raceway, or cable armor used in any wiring system be bonded and grounded to prevent shock hazard.
Question 7: Which of the following actions would VIOLATE proper grounding practice when installing a low voltage security system panel?
- Connecting the panel ground lug to a dedicated ground rod outside the building (Correct answer)
- Running the grounding conductor in the same conduit as the power conductors
- Sizing the grounding conductor per the panel manufacturer's listing
- Connecting the panel ground to the building's grounding electrode system
Correct answer: Connecting the panel ground lug to a dedicated ground rod outside the building
NEC 250.52 and 800.100 prohibit using a separate isolated ground rod for communications or low voltage equipment — all systems must tie to the common grounding electrode system to prevent dangerous potential differences.
C7 Grounding & Bonding for Low Voltage Systems 5
Which NEC article specifically covers grounding and bonding requirements for network-powered broadband communications systems?