C7 Grounding & Bonding for Low Voltage Systems 4 — Questions and Answers
Question 1: When installing a CCTV system on a metal building, the coaxial cable shield should be bonded to the building steel at which point?
- At every camera location only
- At the equipment rack/head-end only
- At the point where the cable enters the building (Correct answer)
- Bonding is not required for coaxial shields
Correct answer: At the point where the cable enters the building
NEC 820.93 requires coaxial cable shields to be bonded at the point of entry into the building to prevent lightning and fault currents from entering the system.
Question 2: Which conductor color is used for equipment grounding conductors in low voltage DC systems per NEC guidelines?
- White
- Green or bare (Correct answer)
- Black
- Red
Correct answer: Green or bare
NEC 250.119 specifies that equipment grounding conductors must be green, green with yellow stripe, or bare copper.
Question 3: A low voltage technician discovers that two separate audio systems share the same ground bus but experience a hum. The MOST likely cause is:
- Undersized grounding conductors
- A ground loop caused by multiple ground paths at different potentials (Correct answer)
- Missing bonding jumpers at the panel
- Improper wire gauge for the audio cables
Correct answer: A ground loop caused by multiple ground paths at different potentials
Ground loops occur when equipment is connected to ground at multiple points with differing potentials, inducing a hum voltage in signal cables.
Question 4: Per NEC Article 800, the protector for a communications circuit must be grounded to which of the following?
- The nearest water pipe
- The grounding electrode system of the building (Correct answer)
- A separate ground rod installed specifically for communications
- The neutral bar of the nearest branch circuit panel
Correct answer: The grounding electrode system of the building
NEC 800.100 requires communications circuit protectors to be connected to the building's grounding electrode system using an acceptable grounding means.
Question 5: What is the maximum resistance typically acceptable for a telecommunications grounding electrode system (TGPE) per TIA-607?
- 1 ohm
- 5 ohms (Correct answer)
- 10 ohms
- 25 ohms
Correct answer: 5 ohms
TIA-607 recommends a TGPE resistance of 5 ohms or less to ensure effective fault current dissipation and signal integrity.
Question 6: A bonding conductor connecting the telecommunications main grounding busbar (TMGB) to the building steel must be:
- Minimum 6 AWG insulated copper
- Minimum 6 AWG bare copper
- Minimum 2/0 AWG insulated copper (Correct answer)
- Any gauge as long as it is bonded at both ends
Correct answer: Minimum 2/0 AWG insulated copper
TIA-607 specifies that the backbone bonding conductor from the TMGB to building steel shall be at least 2/0 AWG copper.
Question 7: When installing a fire alarm control panel (FACP), the grounding conductor from the panel to the grounding electrode must be:
- Sized per NFPA 72 and installed in a continuous, unbroken run (Correct answer)
- Run alongside power conductors to share conduit
- Connected to the neutral terminal at the panel
- At least 18 AWG per standard low voltage practice
Correct answer: Sized per NFPA 72 and installed in a continuous, unbroken run
NFPA 72 requires the FACP grounding conductor to be continuous with no splices and sized per the listing requirements of the equipment.
C7 Grounding & Bonding for Low Voltage Systems 4
When installing a CCTV system on a metal building, the coaxial cable shield should be bonded to the building steel at which point?