Uninterruptible power supply systems are operating ungrounded during power transfer, critical to the overall design of electrical and power systems in a nonresidential building.
- Learning objectives
- Identify electrical and power systems that require grounding.
- Determine the best methods to ground electrical and power distribution systems.
- Understand when to specify uninterruptible power supply systems.
Ungrounded electrical systems are not often employed due to real and perceived safety concerns. Predominately, commercial systems are solidly grounded (SG). SG systems are characterized by high line-to-ground fault current with reliance on quick overcurrent protection to limit the release of dangerous energy. Ground faults on an SG system are intended to be eliminated by the fastest means practical. Electrical systems designed through the 1940s were ungrounded. The issue discovered during this period was that system faults had gone undetected until a second fault caused fire or injury.
Alternatives to an SG system include low-resistance grounding (LRG), reactance grounding (RG), and high-resistance grounding (HRG). LRG or RG systems are recommended on medium-voltage systems to limit fault currents while overcurrent protection operates. HRG systems, which limit the fault current to a small value, were adopted by the petroleum and chemical industries as an alternative to an ungrounded system. Semiconductor facilities with similar critical process continuity requirements also adopted HRG systems. In recent years, mission critical data centers have been designed with HRG systems. Onsite power generation and uninterruptible power supply (UPS) systems are used extensively where equipment costs can be justified against the losses due to business continuity interruptions (see Figure 1).
Transformerless UPS systems are preferred due to efficiency savings, lower thermal heat rejection, and a smaller footprint as compared with transformer-based UPS systems. These transformerless systems have been introduced in the past decade and are commonly employed on a large scale for data centers and critical manufacturing processes. For domestic, medium- to large-scale applications, engineers are specifying UPS distribution as a 480 V, 3-wire system with 208 V power distribution units (PDUs) at the point of connection. A PDU or isolation transformer is provided when single-phase loads are served. A neutral is not required or advised for this system until single-phase loads are required (see Figure 2).
For smaller systems, such as a 208/120 V UPS input source, a 4-wire system may be specified (see Figure 3). Systems in both figures 2 and 3 operate ungrounded during an event where power is lost. Whether a short circuit is flowing through the neutral or grounding conductor when the UPS is providing power, transistors in the UPS rectifier isolate the input power, opening the supply circuit and interrupting the return path.
For applications that can’t tolerate an ungrounded zone within the electrical distribution system, an isolation transformer inside the UPS is an option. Without an isolation transformer, there is no safe way to connect the direct-current source to ground without introducing a parallel return path. With transformerless applications being the leading choice in the industry, it is important for engineers to mitigate and understand the risks of operating an ungrounded system during power transfer. Electrical systems are not necessarily required to be grounded by NFPA 70-2017: National Electrical Code (NEC). Careful application of grounding continues to rank No. 1 in safety priority. It is a mistake to ground everything by default. Grounding duplication creates parallel paths, which is strictly prohibited for neutral conductors. By design, when connecting exposed metal cabinets and conduit to the grounding system, there are many parallel paths to the source. However, properly grounded systems are only connected once at the source. A grounded conductor is provided to intentionally return unbalanced current back to the source. These grounded conductors are separated from the grounding system to avoid a parallel return path. Most important, the isolation of a grounded conductor from grounding keeps these intended and unintended exposed metal paths from carrying current under normal conditions.
For critical applications, redundant components along with alternative utility and standby sources are normal practices. These separately derived systems are grounded at the source and interconnected by transfer-switch schemes. Grounding interconnection is required and care must be taken to avoid hazards, such as not being able to isolate a ground fault or circulating ground currents. Where 4-wire sources are required, auto-transfer schemes must consider switching the neutral. Refer to the Consulting-Specifying Engineer article, “Choosing between 3-pole and 4-pole transfer switches.”
NEC grounding requirements
Consider that a grounding electrode system is provided to meet NEC Article 250.53 Grounding Electrode Installation. For system ac voltage between 50 and 1,000 V, the following sections of Article 250 require review. First consider Part II System Grounding, Section 250.20, Alternating-Current Systems to be Grounded, which permits this system to be grounded: “Other systems shall be permitted to be grounded. If such systems are grounded, they shall comply with the application provisions of this article. The system source does not have to be a neutral being used as a current-carrying conductor as outlined in Article 250.20 (B) (2) and does not meet other provisions of Part 20.”
Secondly, Article 250.30, Grounding Separately Derived Alternating-Current Systems, must be considered. Without a transformer, a UPS is not considered a separately derived source (SDS). Refer to the definition of SDS in Article 100, Definitions: “An electrical source, other than a service, having no direct connection(s) to circuit conductors of any other electrical source other than those established by grounding and bonding connectors.” Refer to Figure 2 and note that there isn’t isolation between the dc source. The dc system generally operates ungrounded as a connection to neutral requires a midpoint tap with an inductive choke.
For the battery system, NEC Article 250 Part VIII, Direct-Current Systems, applies. Refer to Figure 4 for a typical grounding configuration. For this battery system operating at greater than 500 Vdc, ground is not required to be grounded. Article 250.162, Direct-Current Circuits and Systems to be Grounded, applies to systems operating at greater than 60 V but not greater than 300 V. Grounding for the battery cabinet is per Article 250.169. A dc grounding electrode is required to bond the battery cabinet and other exposed metal parts between the battery and first disconnect. For a large-scale UPS, the default maximum conductor size is 3/0. Note that ground-fault detection is required for an ungrounded system per NEC Article 250.169, Ungrounded Direct-Current Separately Derived Systems. The reader should review the specification and confirm that the UPS manufacturer includes this option.
Switched neutral
Consider this example with a switched neutral. Figure 5 shows a generator backup for the utility with a switched neutral. The UPS neutral is connected to the supply and, therefore, cannot be connected to the grounding system by definition of a separately derived source. During a power loss, the following steps are executed:
- M1 breaker is opened
- Generator starts
- G1 breaker is closed.
During the time it takes to bring the generator online, the UPS is providing power to critical loads. The neutral is disconnected from the grounding system when the 4-pole breakers are open. This puts the UPS into an ungrounded state temporarily. If a line-to-ground fault were to occur prior to the generator breaker closing, provisions must be made in the UPS to continue providing power. During normal operation, a downstream fault would trigger the UPS to go into bypass mode so that the system could clear the fault.While a UPS is providing backup power, it is necessary to block the bypass during a ground fault. This can be accomplished with UPS firmware. Consult with other manufacturers to see what options are available for the manufacturer you are considering. Blocking the bypass allows continued operation without disruption of power. This is desirable for an HRG system.
An option to reduce exposure to these hazards and operational risks is to provide a 3-wire system and, where conditions meet NEC Article 250.36, High-Impedance Grounded Neutral Systems, install a high-resistance ground. For the first fault, the current is limited to less than 10 amps. An alarm is triggered and a detection system is provided. This meets the intent of a grounded system. A grounded system isolates and finds the fault by tripping a breaker.
Risks for ungrounded operation are minimized by operating time. When the alternative source is brought online, the ungrounded condition is eliminated. UPS systems are designed for less than 10 minutes of operation, and a typical standby generator is online in 10 seconds. Standby generator systems can be designed to call the load within 5 to 45 seconds of starting. This leaves the system ungrounded from 15 to 55 seconds. During this short period, there could be a ground fault. This first fault is detected by the UPS but does not trip the system offline. Voltage and 120-deg. phase separation is maintained. A second fault would provide a return path and likely trip the system offline. Has this condition occurred in the 20 years since these systems have been put into operation? This research is not available today, but anecdotal evidence suggests that the risk is minimal.
The system shown in Figure 6 with three sources is not intended to have those sources operate in parallel. There is a redundant UPS not shown that matches the one shown. With this configuration, the UPS can be taken out of service without impacting reliability of the power. For a high-resistance ground application, each source must have its own individual system. This prevents a source transfer onto a fault without current limitation by the HRG. UPS systems shown are 3-wire, similar to Figure 2, and grounded per Figure 5.
Best practices
The focus of this article is on NEC safety requirements and best practices to avoid circulating ground currents for UPS applications. For global applications where a common 400/230 V distribution voltage is used, a neutral is carried throughout the system. HRG is not an option for this configuration. During an outage, the neutral floats, the phases remain 120 deg. apart, and operation is consistent. Consult the manufacturer’s installation manual for additional requirements. These instructions have been updated in recent years. Each manufacturer’s method for detecting a ground fault and sequence of operation varies. The industry is in the process of refining 3-wire UPS systems and is still evolving.
