Three-phase, three-wire, ungrounded systems (DELTA), which are extensively used in industrial establishments, do not require the use of grounded conductors as circuit conductors.

The same network of equipment grounding conductors shall be provided for ungrounded systems as for grounded systems. Equipment grounding conductors are required in ungrounded systems to provide shock protection and to present a low-impedance path for phase-to-phase fault currents in case the first ground fault is not located and cleared before another ground fault occurs on a different phase in the system.

Grounding electrode conductors and bonding jumpers shall be computed, sized, and installed in the same manner as if the system were a grounded system. Apply all the requirements listed in Sections 4.6 through 4.8 for sizing the elements of an ungrounded system.

4.12 GROUNDING A SEPARATELY DERIVED SYSTEM

NEC Sections 250-5(d) and 250-26 cover the rules for grounding separately derived systems. The system grounding conductor for a separately derived system shall be grounded at only one point. That single system grounding point is at the source of the separately derived system and ahead of any system disconnecting means or overcurrent devices. Where the main system disconnecting means is adjacent to the generator, converter, or transformer supplying a separately derived system, the grounding connection to the system grounded conductor can be made at or ahead of the system disconnecting means.

The preferred grounding electrode for a separately derived system is the nearest effectively grounded structural metal member of the building or the nearest effectively grounded water pipe. If neither is available, concrete-encased electrodes or made electrodes are permitted.

In a grounded, separately derived system, the equipment grounding conductors shall be bonded to the system-grounded conductor and to the grounding electrode at or ahead of the main system disconnecting means or overcurrent protection device. The equipment grounding conductor should always be connected to the enclosure of the supply transformer, generator, or converter.

The grounding electrode conductor, the main bonding jumper, the grounded conductor, and the equipment grounding conductor are calculated, sized, and selected by the rules listed in Sections 4.7 through 4.10. (See Figure 4-7.)

Figure 4-7. The grounded (neutral) conductor can be used to carry both normal neutral current and abnormal ground fault current.

4.13 GROUNDING ELECTRODE SYSTEM

If 10 ft or more of metal water pipe is in the earth, the water pipe is considered the grounding electrode, but it shall be supplemented by an additional electrode. NEC Section 250-81 lists four types of electrodes. If one or all are available, they shall be bonded together to make up the grounding electrode system. The bonding jumper that connects these electrodes shall be at least as large as the grounding electrode conductor of the system sized by NEC Table 250-94. The four types of electrodes are as follows:

1. Metal water pipe in contact with the earth for 10 ft. or more. Interior metal water pipe beyond 5 ft. from the water entrance shall not be used as a part of the grounding electrode system or as a conductor to interconnect those electrodes. (See NEC Section 250-81.)

2. Metal frame of the building, where effectively grounded

3. Bare #4 conductor at least 20 ft in length and near the bottom of the concrete foundation (within 2 in.), or 1/2 in. reinforcing steel or rods at least 20 ft in length (one continuous length or spliced together)

4. Bare #2 conductor encircling building at least 2-1/2 ft. in the ground (spliced together at each end).

The grounding electrode conductor at the service equipment can be connected to any convenient interbonded electrodes that provide a solid, effective connection. Metal water pipe shall be supplemented by an additional electrode, which can be any of the following electrodes:

1. A rod

2. Pipe

3. Plate

4. Building steel

5. Concrete-encased electrode.

(See Figure 4-8, which lists some of the different types of grounding electrodes.)