The decision to install and operate an ungrounded system is typically a combined effort that includes a design or engineering team, the owner, the operators and sometimes the authority having jurisdiction. Ungrounded systems can provide continuity of electrical operation and limit system outage downtime.Common ungrounded, three-phase electrical systems are as follows: 240-volt (V), three-phase, 3-wire, delta-connected; 480V, three-phase, 3-wire, delta-connected; 2,300V, three-phase, 3-wire, delta-connected; 4,600V, three-phase, 3-wire, delta-connected; and 13,800V, three-phase, 3-wire, delta-connected.
One disadvantage of an ungrounded system is that a first phase-to-ground fault condition can be difficult to find. The voltage-to-ground in an ungrounded system is 0V (in theory) 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 can appear during voltage testing and are usually the result of capacitance-coupling effects from the system circuits.
Voltage to ground
The definition of “voltage-to-ground” covers another important point about voltage-to-ground levels in ungrounded systems. It clarifies that the voltage-to-ground of a grounded system is the voltage between the given conductor and the grounded circuit point or conductor.
For example, in a 120/240V, single-phase system, the voltage is 120V from any ungrounded phase conductor to ground. For ungrounded systems, the greatest voltage between the given conductor and any other circuit conductor is also the phase-to-ground voltage.
On a 480V, three-phase, 3-wire, ungrounded delta system, the phase-to-phase voltage is 480V. Based on the definition, this is also the phase-to-ground voltage for this system. This vital information must be applied when selecting circuit breakers for ungrounded systems. The circuit breakers must typically have a straight voltage rating (e.g., 480V) as opposed to slash-rated breakers (e.g., 480/277) (see 240.85 and the associated informational note). The phase-to-ground voltage for ungrounded systems is the phase-to-phase voltage of the system.
For the systems addressed in Section 250.21(A), grounding is not required. Where a system is not grounded and operates at not less than 120V and at 1,000V or less, ground detectors are required. The ground-detection requirement provides the ability to monitor ungrounded systems to detect a first phase-to-ground fault on the system. The first phase-to-ground fault will not cause overcurrent device operation, so continued service is achieved. However, personnel responsible for monitoring the system must react to the annunciation, investigate the first phase-to-ground condition and remove it. If the first phase-to-ground condition is not cleared and a second phase-to-ground fault develops on a different phase, the result is a simultaneous phase-to-phase short circuit and phase-to-ground fault event, which can lead to significant equipment destruction and downtime.
There are some benefits of operating a system ungrounded where the National Electrical Code (NEC) permits, but it is important to monitor it and react appropriately if a phase-to-ground condition develops. Ungrounded systems are often installed and used in industrial facilities where power continuity is desired for assembly lines and other continuous processes that would be damaged or could cause personal injury if a phase-to-ground fault event were to result in power interruption.
The choice to install and operate this type of system is determined by the process’ nature, the process’ operational characteristics and the desired operation method. Where ground detectors are installed on an ungrounded system, the sensors must be located as close to the supply source as possible [250.21(B)(2)]. Listed ground-detection equipment is available for use on ungrounded systems.
According to Section 250.21(C), all enclosures containing equipment and conductors for an ungrounded system must be field-marked, “Caution: Ungrounded System Operating – ____ Volts,” at the source or at the first system-disconnecting means. This marking must meet the requirements in 110.21(B) and be durable enough for the environment in which it is installed. This marking requirement also applies to switchboards and panelboards that contain ungrounded systems, as covered in Section 408.3(F)(2).
This marking gives qualified people an additional notification of the type of system contained within the enclosure. The phase-to-ground-voltage readings are not as familiar to workers as grounded system voltage readings. The marking requirement provides an additional level of safety for personnel who must work on ungrounded systems either to troubleshoot or to add to the systems. Sections 110.2(D)(1) and 110.4(A) of NFPA 70E, Standard for Electrical Safety in the Workplace, provide requirements for qualifications, training and use of test instruments.