IEC - Independent Electrical Contractors Grounding and Bonding Questions and Answers — Questions and Answers
Question 1: What is the primary purpose of bonding in an electrical system according to NEC Article 250?
- To connect the electrical system to the earth for lightning protection.
- To ensure all metallic components are at the same electrical potential, creating a low-impedance path for fault current. (Correct answer)
- To function as the primary conductor for returning neutral current to the source.
- To reduce the overall resistance of the grounding electrode system to below 25 ohms.
Correct answer: To ensure all metallic components are at the same electrical potential, creating a low-impedance path for fault current.
Bonding connects all non-current-carrying metallic parts of an electrical system to create a continuous, low-impedance path. This path is crucial for allowing enough fault current to flow to trip the overcurrent protection device (like a circuit breaker) quickly, preventing metal parts from becoming dangerously energized.
Question 2: An electrician is installing a service for a new commercial building. Which of the following is an essential component for connecting the grounded conductor and the equipment grounding conductor to the service enclosure?
- A supplementary grounding electrode
- An isolated ground receptacle
- A main bonding jumper (Correct answer)
- A ground fault circuit interrupter (GFCI)
Correct answer: A main bonding jumper
The National Electrical Code (NEC) requires a main bonding jumper at the service to connect the grounded conductor (neutral) to the equipment grounding conductor(s) and the service disconnect enclosure. This connection establishes the effective ground-fault current path.
Question 3: Which of the following is NOT a primary function of an Equipment Grounding Conductor (EGC)?
- To provide a low-impedance path for fault current to facilitate the operation of overcurrent devices.
- To connect normally non-current-carrying metal parts of equipment together.
- To carry the unbalanced neutral current during normal circuit operation. (Correct answer)
- To keep equipment enclosures at or near earth potential during a fault.
Correct answer: To carry the unbalanced neutral current during normal circuit operation.
The Equipment Grounding Conductor (EGC) is a safety conductor and should only carry current during a ground-fault condition. The grounded conductor (neutral) is the conductor designed to carry the unbalanced current in a circuit during normal operation.
Question 4: In a new installation, a single ground rod is driven 8 feet into the earth. A resistance measurement shows 40 ohms. According to the NEC, what is the required next step?
- Replace the ground rod with a plate electrode.
- Install a second ground rod, spaced at least 6 feet away, and bond it to the first. (Correct answer)
- Treat the soil around the rod with a chemical agent to lower its resistance.
- The installation is acceptable since one 8-foot rod is the minimum requirement.
Correct answer: Install a second ground rod, spaced at least 6 feet away, and bond it to the first.
NEC Section 250.53(A)(2) states that if a single rod, pipe, or plate electrode has a resistance to earth of 25 ohms or more, it must be supplemented by an additional electrode. For rod electrodes, the supplemental rod must be installed no less than 6 feet from the first.
Question 5: When installing a feeder to a detached garage with its own subpanel, which practice is critical for safety?
- Connecting the subpanel's neutral bar to the grounding bar.
- Driving a ground rod and connecting it only to the subpanel's neutral bar.
- Running a separate equipment grounding conductor from the main panel and keeping the neutral and ground bars isolated in the subpanel. (Correct answer)
- Using the metal water pipe in the garage as the sole grounding means for the subpanel.
Correct answer: Running a separate equipment grounding conductor from the main panel and keeping the neutral and ground bars isolated in the subpanel.
In a subpanel located in a separate building, the grounded (neutral) conductor and the equipment grounding conductor must be kept separate. A separate equipment grounding conductor must be run with the feeder conductors to provide the effective ground-fault current path back to the main service. Bonding the neutral and ground at the subpanel would create objectionable current on the grounding conductors. A grounding electrode system is also required at the detached structure.
Question 6: According to NEC Article 250, which of the following can be used as a grounding electrode?
- A metal underground gas piping system.
- The steel reinforcing bar (rebar) in a concrete foundation, if it is less than 20 feet long.
- An aluminum ground rod.
- A concrete-encased electrode (Ufer ground) consisting of at least 20 feet of 1/2-inch rebar. (Correct answer)
Correct answer: A concrete-encased electrode (Ufer ground) consisting of at least 20 feet of 1/2-inch rebar.
NEC Section 250.52(A)(3) permits a concrete-encased electrode, consisting of at least 20 feet of either bare copper conductor (not smaller than 4 AWG) or one or more bare or zinc-galvanized or other electrically conductive coated steel reinforcing bars of not less than 1/2 inch in diameter, to be used as a grounding electrode. Metal underground gas piping is explicitly forbidden, and rebar must be at least 20 feet long.
