Free TN LLE Grounding and Bonding Questions and Answers 1 — Questions and Answers
Question 1: According to the NEC, what is the primary purpose of bonding an electrical system?
- To limit voltage imposed by lightning.
- To stabilize voltage to earth during normal operation.
- To establish a low-impedance path for fault current to facilitate the operation of overcurrent devices. (Correct answer)
- To connect the electrical system to a grounding electrode.
Correct answer: To establish a low-impedance path for fault current to facilitate the operation of overcurrent devices.
The primary purpose of bonding is to create a continuous, low-impedance path for fault current to travel back to the source. This ensures that enough current will flow during a ground fault to trip the circuit breaker or blow the fuse, protecting personnel and equipment. While grounding helps with lightning and voltage stabilization, bonding is specifically for clearing faults.
Question 2: A residential service is supplied by 2/0 AWG copper service-entrance conductors. What is the minimum size copper Grounding Electrode Conductor (GEC) required for a connection to a ground rod?
- 8 AWG
- 4 AWG
- 2 AWG
- 6 AWG (Correct answer)
Correct answer: 6 AWG
According to NEC Article 250.66(A), the grounding electrode conductor connected to a rod, pipe, or plate electrode is not required to be larger than 6 AWG copper unless it continues on to bond other electrodes. Even though Table 250.66 would require a larger conductor based on the 2/0 service conductors for other types of electrodes, the connection to a ground rod has this specific exception.
Question 3: In a residential dwelling, an electrician is installing a new subpanel in a detached garage fed from the main service panel at the house. Which of the following is a critical requirement for grounding and bonding at the subpanel?
- The neutral bus and the equipment grounding bus must be connected together by a main bonding jumper.
- A separate grounding electrode, such as a ground rod, must be installed at the detached garage. (Correct answer)
- The equipment grounding conductor must be oversized by 25% to account for voltage drop.
- The grounded (neutral) conductor can be used to ground the subpanel enclosure.
Correct answer: A separate grounding electrode, such as a ground rod, must be installed at the detached garage.
NEC Section 250.32(A) requires a separate building or structure supplied by a feeder or branch circuit to have a grounding electrode system. This means a detached structure like a garage must have its own grounding electrode (e.g., ground rod). The neutral and ground buses must be kept separate at a subpanel to prevent objectionable current from flowing on the equipment grounding conductors.
Question 4: When a metal underground water pipe is used as a grounding electrode, the connection of the grounding electrode conductor must be made:
- At any accessible point on the interior piping system.
- Within the first 5 feet of the pipe's point of entrance into the building. (Correct answer)
- Only on the street side of the water meter.
- After the main water shut-off valve.
Correct answer: Within the first 5 feet of the pipe's point of entrance into the building.
NEC Section 250.68(C)(1) specifies that the connection point for a grounding electrode conductor to a metal underground water pipe electrode must be located within the first 5 feet of where the pipe enters the building. This ensures a reliable connection to the portion of the pipe that is in direct contact with the earth.
Question 5: Which of the following BEST describes an 'effective ground-fault current path'?
- A connection to the earth through a grounding electrode with a resistance of 25 ohms or less.
- An intentionally constructed, low-impedance conductive path designed to carry current under fault conditions from the point of a ground fault to the electrical supply source. (Correct answer)
- The structural steel frame of a building that is in direct contact with the earth.
- A path that utilizes the grounded (neutral) conductor to clear a fault.
Correct answer: An intentionally constructed, low-impedance conductive path designed to carry current under fault conditions from the point of a ground fault to the electrical supply source.
An effective ground-fault current path is a core concept in Article 250. It is an intentionally created, permanent, and electrically continuous path of low impedance designed to safely carry fault current back to the source, which allows the overcurrent protective device to operate. It is composed of equipment grounding conductors, raceways, and enclosures.
Question 6: An electrician is running a 60-amp branch circuit to supply an appliance. According to NEC Table 250.122, what is the minimum size copper equipment grounding conductor (EGC) required for this circuit?
- 12 AWG
- 8 AWG
- 10 AWG (Correct answer)
- 6 AWG
Correct answer: 10 AWG
NEC Table 250.122, 'Minimum Size Equipment Grounding Conductors for Grounding Raceway and Equipment,' dictates the size of the EGC based on the rating of the overcurrent protective device. For a circuit with a rating of 60 amps, the table requires a minimum of a 10 AWG copper equipment grounding conductor.
Free TN LLE Grounding and Bonding Questions and Answers 1
According to the NEC, what is the primary purpose of bonding an electrical system?