IEC Grounding and Bonding 3 — Questions and Answers
Question 1: What is the maximum resistance to ground allowed for a single ground rod electrode before a supplemental electrode is required per NEC 250.56?
- 5 ohms
- 25 ohms (Correct answer)
- 50 ohms
- 100 ohms
Correct answer: 25 ohms
NEC 250.56 requires a supplemental electrode if a single rod, pipe, or plate electrode has a resistance to ground exceeding 25 ohms.
Question 2: Which conductor is used to bond the structural steel of a building to the grounding electrode system?
- Equipment grounding conductor
- Main bonding jumper
- Grounding electrode conductor (Correct answer)
- System bonding jumper
Correct answer: Grounding electrode conductor
The grounding electrode conductor connects the grounding electrode system to the electrical system, including structural steel when used as an electrode.
Question 3: Per NEC 250.104(A), interior metal water piping must be bonded to which of the following?
- The nearest equipment grounding conductor only
- The service equipment enclosure, the grounded conductor at the service, the grounding electrode conductor, or the grounding electrodes (Correct answer)
- The structural steel of the building
- The neutral bar of the nearest subpanel
Correct answer: The service equipment enclosure, the grounded conductor at the service, the grounding electrode conductor, or the grounding electrodes
NEC 250.104(A) requires interior metal water piping to be bonded to the service equipment enclosure, the grounded conductor at the service, the grounding electrode conductor, or one of the grounding electrodes.
Question 4: What type of grounding electrode is considered 'made' rather than 'existing' per NEC 250.52?
- Metal underground water pipe
- Concrete-encased electrode (Ufer)
- Rod and pipe electrode (Correct answer)
- Metal frame of a building
Correct answer: Rod and pipe electrode
Rod and pipe electrodes are intentionally installed (made) electrodes, whereas water pipes, building steel, and concrete-encased electrodes are naturally occurring or part of the structure.
Question 5: What is the minimum length of a ground rod required by NEC 250.52(A)(5)?
- 6 feet
- 8 feet (Correct answer)
- 10 feet
- 12 feet
Correct answer: 8 feet
NEC 250.52(A)(5) requires ground rods to be at least 8 feet in length when installed in the earth.
Question 6: In a separately derived system (such as a transformer), the bonding jumper connecting the grounded conductor to the equipment grounding conductor is called a:
- Main bonding jumper
- System bonding jumper (Correct answer)
- Equipment bonding jumper
- Supply-side bonding jumper
Correct answer: System bonding jumper
The system bonding jumper performs the same function as a main bonding jumper but is used at a separately derived system rather than at the service.
Question 7: Bonding of gas piping systems in a building is required by NEC 250.104(B) to ensure:
- The gas pressure remains constant during electrical faults
- Electrical continuity so that fault current can flow and trip overcurrent devices (Correct answer)
- The gas pipe acts as an auxiliary grounding electrode
- The gas meter housing is at the same potential as earth
Correct answer: Electrical continuity so that fault current can flow and trip overcurrent devices
Gas piping must be bonded to ensure electrical continuity so that any fault current can flow through the system and operate the overcurrent protective device.
IEC Grounding and Bonding 3
What is the maximum resistance to ground allowed for a single ground rod electrode before a supplemental electrode is required per NEC 250.56?