Whether an electrical system is grounded or not grounded, there are specific performance requirements that apply. As discussed in last month’s article, sections 250.4(A) and (B) provide a detailed breakdown of the grounding and bonding performance requirements for grounded and ungrounded electrical systems. If the National Electrical Code or a specification requires or permits an ungrounded system, other rules, including for ground-fault detection systems, are triggered. This month, let’s do a closer review of ungrounded systems and why they are permitted by the NEC in specific applications and under certain conditions.
The first NEC provision to look at is in Section 90.1(A), which clearly indicates that the Code’s purpose is practical safeguarding of people and property from hazards arising from electricity use. The mandatory requirements for grounding electrical systems are provided in Section 250.20, and which conductor of such systems must be grounded is in Section 250.26.
If or when an electrical design or engineering specification requires installation and use of an ungrounded system, concerns usually relate to continuity of power and property protection in addition to safety concerns for industrial systems and processes. Ungrounded systems are often installed and used in industrial facilities where power continuity is desired for assembly operations and other continuous processes that would be damaged or could cause personnel injury if a first phase-to-ground-fault event were to result in interruption of power to the system. The choice to install and operate this type of system is often determined by the process’ unique nature and operational characteristics and the operator’s or owner’s desired method of operation.
Requirements for ground detection
For the systems addressed in Section 250.21(A), system grounding is permitted but not required. If a system is not grounded and operates at 120V through 1,000V, ground detectors must be installed. The requirement for ground detection provides the ability to monitor ungrounded systems to detect a first phase-to-ground fault on the system.
If a system is operating ungrounded, no system conductor is intentionally connected to ground. In that case, a first phase-to-ground fault will not cause overcurrent protective device operation, so continued and uninterrupted power is generally achieved. However, it is important that the ground detection monitoring the system be triggered so qualified people can react to the annunciation, investigate the phase-to-ground condition and remove it.
The term qualified person is defined in Article 100, and it is intended to emphasize that those working on ungrounded systems be trained and experienced in the construction and operation of these unique systems and equipment.
If the first phase-to-ground condition is not cleared and a second phase-to-ground fault develops on a different ungrounded phase of the system, there is a likely consequence of a simultaneous phase-to-phase short circuit and subsequent phase-to-ground-fault event. This type of event can result in personnel injury or a fatality and lead to significant downtime resulting in some destruction of equipment (property).
There are some benefits of operating a system ungrounded if the NEC permits it, but it is important to monitor it and react appropriately if a phase-to-ground condition develops.
Where a ground detection system is installed on an ungrounded system, the sensors for such systems must be located as close to the supply source as possible, as indicated in 250.21(B)(2). Listed ground-detection equipment is available for use on ungrounded systems.
Marking requirements
According to 250.21(C), all enclosures containing equipment and conductors for ungrounded systems must be field-marked “Caution: Ungrounded System Operating—____ Volts” at the source or at the first system disconnecting means. This marking must be durable enough for the environment involved. This marking requirement also applies to switchboards and panelboards that contain ungrounded AC and DC systems, as provided in 408.3(F)(2) and (F)(4). These required markings provide qualified and trained personnel an additional notification of the type of system contained within the enclosure. The phase-to-ground voltage readings of such systems are not as familiar to workers as grounded system voltage readings are. This is also a good example of why training on the use of test instruments is so vital, as emphasized and required by Section 110.6(A) of NFPA 70E, Standard for Electrical Safety in the Workplace.
The marking requirement provides an added level of safety for personnel who must work on ungrounded systems, either to troubleshoot, modify or add to such systems. Those responsible for maintaining ungrounded systems and the associated ground detection systems should also be qualified and familiar with the construction and operation of such systems.
About The Author

Michael Johnston
NECA Executive Director of Codes and Standards (retired)JOHNSTON, who retired as NECA’s executive director of codes and standards in 2023, is a former member and chair of NEC CMP-5 and immediate past chair of the NEC Correlating Committee. Johnston continues to serve on the NFPA Standards Council and the UL Electrical Council. Reach him at [email protected].