Installing and operating an ungrounded system is typically a combined effort, including the design or engineering team, the owner, the operators and sometimes the authority having jurisdiction. In addition to the Code allowances, common reasons to operate an ungrounded system are providing electrical operation continuity and minimizing downtime. Common ungrounded delta systems include, but are not limited to, 240-volt (V), three-phase, 3-wire, delta-connected; 480V, three-phase, 3-wire, delta-connected; and 2,300V, three-phase, 3-wire, delta-connected.
The disadvantage of an ungrounded system is a first phase-to-ground fault condition can be difficult to find and can take a considerable amount of investigation and time. In theory, the voltage to ground in an ungrounded system is 0 volts because there is no ground connection from any system conductor. However, there is distributed leakage capacitance present throughout such systems. Phase-to-ground voltage levels that may appear in a test instrument reading results from capacitance coupling effects from the system circuits.
Another important point about voltage-to-ground levels in ungrounded systems is covered in the definition of voltage to ground. The definition clarifies that the voltage to ground in a grounded system is the voltage between the given conductor and the point or conductor of the grounded circuit. For example, in a 120/240V, single-phase system, the voltage is 120V from any ungrounded phase conductor to ground.
For ungrounded systems, however, the greatest voltage between the given conductor and another circuit conductor is also the phase-to-ground voltage. For example, on a 480V, three-phase, 3-wire, ungrounded delta system, the phase-to-phase voltage is 480V, which is also the phase-to-ground voltage for this system and based on the definition in Article 100. The definition recognizes a first phase-to-ground fault event accidentally grounds the system, while a second ground fault on another ungrounded phase conductor creates a phase-to phase condition (480V) in addition to ground-fault condition, which is also 480V to ground.
Although the system is ungrounded, the functions of grounding and bonding are still necessary for the equipment enclosing circuit conductors supplied from these systems. The performance language in 250.4(B)(1) is related to the grounding requirements for equipment installed in an ungrounded system. The noncurrent-carrying parts of raceways, equipment enclosures, etc., must be grounded (connected to the earth) in such a way as to limit over voltages caused by lightning or unintentional contact with higher-voltage lines. Grounding equipment places these conductive parts at or close to the same potential as earth’s. Therefore, if abnormal events should cause a voltage rise on these parts, the rise will be consistent with the rise in the earth’s potential. The objective is to keep the noncurrent-carrying conductive parts at the same potential as the earth.
Grounding minimizes potential differences between the earth and equipment in normal operation and during abnormal events such as ground faults. If equipment is grounded, potential differences are reduced for the time it takes overcurrent protection to operate. The equipment grounding requirements in 250.4(B)(1) can be compared with those in 250.4(A)(2) since the electrical performance expectations are essentially the same. If an electrical system is not grounded, there is no intentional connection between any of the system conductors and the earth or other equipment grounded conductive parts.
When the supply conductors from ungrounded AC systems are installed in grounded metal raceways and enclosures, the effects of capacitance coupling are typically present, creating varying potential (voltage) differences between them. This potential difference can appear as voltage-to-ground readings during test instrument measurements, but this is the result of leakage capacitance. An example of an ungrounded AC system is a single-phase, 2-wire, 480V system. Although this capacitive coupling voltage is present, it offers little or no effect in the event of a ground fault on the system.
Instead, a ground fault from any ungrounded system conductors will often accidentally and ineffectively ground the system. The word “ineffectively” is used because these events are typically ground faults, which are an indication of a pinched wire or other insulation failure creating this condition.
This differs greatly from the intentional system connection to the ground for solidly grounded electrical systems. Thus, a first ground fault on an ungrounded system is usually an intermittent and unstable connection that will manifest through intermittent minor arcing to possibly become attached to the conductive raceway or enclosure. This condition should be identified and annunciated to qualified people by a required ground detection system(s). If a second phase-to-ground fault were to develop on a different phase than the first fault, a short-circuit and ground-fault condition results simultaneously. Ground-detection systems are an important requirement found in Section 250.21(B), which also includes cautionary marking requirements for such equipment.
About The Author
JOHNSTON is NECA’s executive director of codes and standards. He is a member of the NEC Correlating Committee, NFPA Standards Council, IBEW, UL Electrical Council and NFPA’s Electrical Section. Reach him at [email protected].