Choosing between grounded and ungrounded electrical system designs

Understanding both grounded and ungrounded electrical systems enables engineers to apply the appropriate grounding topology for the electrical system requirements.

10/01/2013


Figure 1: This diagram shows several solidly grounded system configurations. The grounded wye system is the most prevalent. As noted, there is a direct connection between a neutral point and ground. Courtesy: AEI/Affiliated Engineers IncGrounding and shielding electrical systems are of key importance to electrical engineers. Understanding the basic operations between grounded and ungrounded electrical systems is necessary for matching the appropriate grounding topology to the desired electrical system performance. 

Selecting the proper grounding topology for an electrical distribution system is important to ensure facility occupant safety and health as well as reliable and safe electrical equipment operation. According to NFPA 70: National Electrical Code (NEC), Article 250.4(A)(1), the purpose of electrical system grounding is, “To limit the voltage imposed by lightning, line surges, or unintentional contact with higher-voltage lines that will stabilize the voltage to earth during normal operation.” The focus of Article 250 is to describe the grounding topologies available among grounded and ungrounded systems and how they operate. 

The purpose of grounding the electrical system as stated in NFPA 70: National Electrical Code (NEC) is, “To limit the voltage imposed by lightning, line surges, or unintentional contact with higher-voltage lines that will stabilize the voltage to earth during normal operation.” To achieve these goals, the NEC provides the framework for the selection of grounding methodologies in Article 250. The focus of this article is to describe the grounding topologies available among grounded and ungrounded systems and how they operate. 

The importance of providing a solidly grounded circuit for safety was recognized in the early editions of the NEC. According to “IAEA Soares Book on Grounding,” 100 years ago, the 1913 NEC committee required that "transformer secondaries of distributing systems must be grounded, provided the maximum difference of potential between the grounded point and any other point in the circuit does not exceed 150 V and may be grounded when the maximum difference of potential between the grounded point and any other point in the circuit exceeds 150 V." The code committee recognized that when a fault occurs on a grounded circuit, the grounded conductor maintains the system voltage at a stable source voltage rather than floating up to a higher potential. This protects individuals from being exposed to a potentially lethal shock were they to touch a faulted line, equipment, or chassis. 

Solidly grounded systems

Figure 2: This diagram shows a solidly grounded wye in a fault condition. Fault current flows through ground impedance (junction boxes, enclosures, and ground electrode) to the service neutral point. Courtesy: AEI/Affiliated Engineers Inc.Today, because grounded systems offer greater voltage stability, most of the systems described in Article 250.20 of the NEC require a grounded system, whether it is a solidly grounded system or an impedance grounded system. Historically, the most commonly used system is the solidly grounded system (see Figure 1).

The NEC allows up to 25 ohms of ground resistance, recognizing different soil resistivities found across the U.S. However, the lower the ground resistance (or higher the ground conductivity), the better the ground fault detection system will operate. Typically, 5 ohms is a good design basis for commercial buildings. Lower ground impedance may be required for some medical imaging equipment. In a solidly grounded system, the ground fault system performs better with smaller ground electrode resistance. Article 250.2 of the NEC states that an effective ground fault current path consists of “an intentionally constructed, low impedance, electrically conductive path designed and intended to carry current under ground fault conditions.” Therefore in a solidly grounded system, it is the design intent to provide an earth reference to open a circuit as quickly as possible to isolate the fault based on high current flow. This prevents the fault from escalating and also protects connected motors and equipment from damage (see Figure 2).


<< First < Previous 1 2 3 Next > Last >>

Anonymous , 10/17/13 11:57 AM:

The impedance to ground for separately deriving the neutral of a system and referencing the system to ground should not be confused with the low impedance ground fault path required to clear faults. The earth should not be the ground fault path for clearing faults and is not part of the path for ground fault detection systems. I would appreciate hearing from the editor of CSE about this article.
MEYNARDO , GU, United States, 11/08/13 08:21 AM:

Grounding is a very broad field and has deeper character than normally though off. In the building electrical system, when grounding is being emphasized it must first make it initially known whether the discussion is about "system" grounding or if it is about "equipment" grounding because the logic involved is not the same. When "lightning protection" grounding comes in the discussion becomes more interesting, and in some cases where the building is handling ordnance materials, the grounding becomes more seriously complicated. Consider more telecommunincations, grounding, power quality grounding, isolated systems grounding, etc, etc, then the discussion becomes more of an expertise subject. I believe the discussion in this article is for "systems" grounding only and i would like to make the discussion on equipment grounding not be confused with the systems grounding.

Meynardo Custodio P.E.
Consulting Engineer
The Engineers' Choice Awards highlight some of the best new control, instrumentation and automation products as chosen by...
Each year, a panel of Control Engineering editors and industry expert judges select the System Integrator of the Year Award winners.
The Engineering Leaders Under 40 program identifies and gives recognition to young engineers who...
Learn how to increase device reliability in harsh environments and decrease unplanned system downtime.
This eGuide contains a series of articles and videos that considers theoretical and practical; immediate needs and a look into the future.
Learn how to create value with re-use; gain productivity with lean automation and connectivity, and optimize panel design and construction.
Go deep: Automation tackles offshore oil challenges; Ethernet advice; Wireless robotics; Product exclusives; Digital edition exclusives
Lost in the gray scale? How to get effective HMIs; Best practices: Integrate old and new wireless systems; Smart software, networks; Service provider certifications
Fixing PID: Part 2: Tweaking controller strategy; Machine safety networks; Salary survey and career advice; Smart I/O architecture; Product exclusives
The Ask Control Engineering blog covers all aspects of automation, including motors, drives, sensors, motion control, machine control, and embedded systems.
Look at the basics of industrial wireless technologies, wireless concepts, wireless standards, and wireless best practices with Daniel E. Capano of Diversified Technical Services Inc.
Join this ongoing discussion of machine guarding topics, including solutions assessments, regulatory compliance, gap analysis...
This is a blog from the trenches – written by engineers who are implementing and upgrading control systems every day across every industry.
IMS Research, recently acquired by IHS Inc., is a leading independent supplier of market research and consultancy to the global electronics industry.

Find and connect with the most suitable service provider for your unique application. Start searching the Global System Integrator Database Now!

Case Study Database

Case Study Database

Get more exposure for your case study by uploading it to the Control Engineering case study database, where end-users can identify relevant solutions and explore what the experts are doing to effectively implement a variety of technology and productivity related projects.

These case studies provide examples of how knowledgeable solution providers have used technology, processes and people to create effective and successful implementations in real-world situations. Case studies can be completed by filling out a simple online form where you can outline the project title, abstract, and full story in 1500 words or less; upload photos, videos and a logo.

Click here to visit the Case Study Database and upload your case study.