Grounded and ungrounded electrical and power system design

Uninterruptible power supply systems are operating ungrounded during power transfer, critical to the overall design of electrical and power systems in a nonresidential building.

06/20/2017


This article is peer-reviewed.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.

Figure 1: The photo shows an uninterruptible power supply installed in a typical equipment room with associated switchgear. This is a good example of an installation where the user is planning for future growth of the UPS, and has allowed space for additional modules to add capacity or redundancy. Courtesy: Eaton

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.

Figure 2: This 3-wire UPS system depicts an ungrounded zone. Ungrounded operation occurs during battery discharge when the UPS isolates the incoming source. Courtesy: CH2MFor 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.

Figure 3: This 4-wire UPS system depicts an ungrounded zone. Ungrounded operation occurs during battery discharge when the UPS isolates the incoming source. Courtesy: CH2MFor 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.”


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