Choose the best answer:

1. A transformer is installed and used as a separately derived AC system. At what point on the system should you connect the grounding electrode conductor?

(a) At the service panel

(b) At the overcurrent device on the supply side

(c) At any point on the load side of the first disconnecting means

(d) At any point on the separately derived system from the transformer to the first disconnecting means or overcurrent device


2. Where a separately derived AC system is located on the third floor of a shopping mall, your preferred or first choice for a grounding electrode is:

(a) A metal gas pipe

(b) A concrete-encased electrode

(c) A driven ground rod at the first floor of the building

(d) The nearest available effectively grounded structural member of the building


3. The metal water piping system in the area served by a separately derived AC system is not effectively grounded, so a ground rod is used as the grounding electrode. What must you do with the metal water piping in the area?

(a) Bond it to a separate ground rod.

(b) Bond it to the grounded conductor of the separately derived system.

(c) Use it as a supplemental grounding electrode for the separately derived system.

(d) Isolate it from any contact with the electrical system, since it's not effectively grounded.


4. If a separately derived AC system is installed in an industrial building where the conditions of maintenance and supervision ensure that only qualified persons will service the installation, then:

(a) The resistance of a single-made electrode may exceed 25 ohms.

(b) An exposed metal water pipe is permitted to serve as a grounding electrode.

(c) A metal underground gas piping system is permitted to serve as a grounding electrode.

(d) Building steel that is not effectively grounded is permitted to serve as a grounding electrode.

Answers and Discussion

1. (d). The grounding electrode conductor is sized in accordance with Sec. 250-66 (based on the derived phase conductors supplying the panel, switch, or other equipment connected to the secondary of the transformer) and connected at any point on the separately derived system from the source to the first disconnecting means or overcurrent protection device per Sec. 250-30(a)(2).

2. (d). The grounding electrode conductor shall be located as near as possible and preferably in the same area as the grounding electrode conductor connection to the system. Your first choice should be to connect to the nearest structural steel member of the building structure per Sec. 250-30(3)(a).

3. (b). An ungrounded metal water piping system, located in the vicinity of a separately derived system, which uses a grounding electrode, must be bonded to the derived system's grounded conductor. Regardless of whether the water piping system is supplied by nonmetallic piping to the building or structure, proper bonding of the interior metal water piping system is required. Stallcup's Code Loop: Sec. 250-30(a)(3)(c) and FPN 250-104(a)(4).

4. (b). An exception to Sec. 250-30(a)(3)(b) permits you to use an exposed metal water piping system as a grounding electrode for the separately derived system, as long as the entire length of the interior metal water pipe is exposed.