Journeyman Electrician Exam (NEC)Grounding and BondingMedium
A client requests a new separately derived system for a critical power supply in their facility. Which of the following is NOT an acceptable grounding electrode for this system?
- AA concrete-encased electrode, if encased in at least 2 in. (50 mm) of concrete.
- BA metal underground water pipe in direct contact with the earth for 10 ft (3.0 m) or more.
- CAn isolated, ungrounded metal conduit system within the building.
- DThe metal frame of the building or structure, where effectively grounded.
Show answer & explanationAnswer & explanation
Correct answer: C. An isolated, ungrounded metal conduit system within the building.
NEC® 250.30(A)(4) references 250.52(A) for acceptable grounding electrodes for separately derived systems. An isolated, ungrounded metal conduit system is not listed as an acceptable grounding electrode and would not provide an effective ground path.
Why the other options are wrong
- A. A concrete-encased electrode is an acceptable grounding electrode as per NEC® 250.52(A)(3).
- B. A metal underground water pipe is an acceptable grounding electrode as per NEC® 250.52(A)(1).
- D. The metal frame of a building or structure is an acceptable grounding electrode as per NEC® 250.52(A)(2).
Separately Derived System Grounding Electrodes
Separately derived systems must be grounded to one or more acceptable grounding electrodes as specified in NEC® 250.52(A), which include metal water pipes, building steel, and ground rings, but not isolated conduit.
- Grounding electrodes for SDS must meet 250.52(A) requirements.
- Common electrodes: metal water pipe, building steel, concrete-encased electrode, ground ring, rod/pipe/plate electrodes.
- Isolated metal parts or ungrounded systems are not acceptable electrodes.
Memory trick: SDS needs a solid ground, not just a floating pipe dream.