Choosing between grounded and ungrounded electrical system designs


Fault types

There are several types of faults that an electrical system must be designed to withstand. The worst case but less common fault is a 3-phase bolted fault with little or no circuit impedance in the fault path. Equipment is typically sized and noted with a fault current rating based on fault calculations for these situations. With little impedance in a grounded circuit, high fault current levels are possible and arc flash hazards may be present in a solidly grounded system. The high fault current levels are considered one of the main downsides of a solidly grounded system. For example, in a 3-phase line-to-ground fault, voltage remains constant and because the impedance of the system is intentionally minimized, a direct result from the application of Ohm’s Law predicts high fault current flow. A benefit is high fault current will cause the upstream overcurrent protective devices to sense and operate quickly to isolate the faults as they return to the source within pathways designed to have the least resistance. It is up to the designer to provide an adequate pathway to guide the fault properly back to the source with strategies such as compression couplings on raceways, bonding to steel and periodic testing of the ground electrode system.

Because of the importance of this current flow being high enough to trip overcurrent devices, the NEC requires that the neutral-to-ground bond be made within the service entrance equipment. This is essential for the ground fault detection scheme to operate correctly. If the ground is made outside the equipment, the reactance of the circuit will increase. The total impedance of the circuit is expressed as (R+Xj), where Xj is the system reactance. When the total impedance of the system is too high, the overcurrent protective device may not operate as desired. Grounding at a single location at the source also provides benefits for the overall electrical system by preventing circulating currents. 

Although a designer must account for the worst-case scenario, the 3-phase fault is quite rare. In fact, line-to-ground faults account for 90% to 95% of all recorded fault events in industrial settings. These faults can manifest themselves as arcing faults, which can cause current flow at a lower level than the overcurrent device rating. This is considered a serious drawback of the solidly grounded system because these faults may go undetected until equipment damage is done. The design remedy is to introduce ground fault detection into the circuit. During the 1970s, the NEC recognized this issue and added language to require that feeders rated 1,000 A or more on solidly grounded 480 Y/277 V wye-connected systems be equipped with ground fault detection. Ground fault detection can get complicated, especially if multiple levels are used within a system. Similar to circuit breaker coordination, it is necessary to coordinate the time-current curves for ground fault overcurrent protection to prevent upstream breakers from tripping prior to the GFI breaker closest to the fault. Otherwise, more systems than desired will be brought offline. 

Figure 3: This diagram shows several ungrounded system configurations. The ungrounded delta system is the most typically used because the NEC requires grounding of WYE systems. Courtesy: AEI/Affiliated Engineers Inc.Modern low-voltage transformers are primarily designed and constructed with delta primaries and wye secondaries. In most commercial and industrial applications, the standardized voltage is 480 Y/277 V on the secondary side. Early versions of the NEC didn't require systems to be grounded on the secondary side for voltages higher than 150 V. Grounding the secondaries of these service transformers for safety and to minimize equipment risk didn't gain momentum until the mid-1930s. A cost-effective solution was to ground a corner of the delta secondary. Therefore, many historic structures still have operating delta-delta service transformers where one corner of the transformer has been grounded to provide 120 V/240 V power within the facility. 

The primary goal for a solidly grounded system is to open the circuit as quickly as possible to limit damage and risk to life. For large process and industrial plants, stopping the process can be equally hazardous. Prior to the mid-1930s, the concept of an ungrounded system was still in favor because of the service continuity benefits that the ungrounded system provided. A fault on an ungrounded system doesn't cause the source circuit breaker to trip. In fact, the system will keep operating until the operator tracks down the fault or until a second fault causes a major component in the electrical system to fault to ground, during which large magnitudes of current flow (see Figure 3). While theoretically this system is ungrounded, in reality the three phases are capacitively coupled to ground (see Figure 4). 

Figure 4: This diagram shows an ungrounded system effectively grounded via its system capacitance. Courtesy: AEI/Affiliated Engineers Inc.Rather than a true ground, it is the system capacitance that helps to stabilize the voltage during normal operating conditions. However, during a fault—typically from line to ground (via the system capacitance)—there is no direct ground connection, and there is no high current flow that would otherwise trip the circuit breaker to isolate the fault. Instead, it causes the phase voltage to rise 1.73 times the voltage on the other phases without tripping the breaker (from “Ground Fault Protection on Ungrounded and High Resistance Grounded Systems,” Post Glover). If cable systems and motor systems were not specified to withstand these higher voltage levels, the electrical systems would be subjected to undesirable stresses that would take their toll over time. Moreover, if an intermittent fault occurs, such as an arc fault, which can strike and restrike, overvoltage of up to 6 times greater than typical line voltage can occur, which can severely damage cable insulation and sensitive equipment. As equipment ages, it becomes more vulnerable to these strikes until, ultimately, it fails and faults to ground through equipment cases—or worse—through a person. Because circuit breakers don't trip, faults in an ungrounded system are difficult to trace and often go undetected until major equipment damage occurs during a second fault. Because of these issues, some industrial plants in the 1930s began converting their electrical infrastructures to grounded systems.

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 Top Plant program honors outstanding manufacturing facilities in North America. View the 2013 Top Plant.
The Product of the Year program recognizes products newly released in the manufacturing industries.
The Engineering Leaders Under 40 program identifies and gives recognition to young engineers who...
The true cost of lubrication: Three keys to consider when evaluating oils; Plant Engineering Lubrication Guide; 11 ways to protect bearing assets; Is lubrication part of your KPIs?
Contract maintenance: 5 ways to keep things humming while keeping an eye on costs; Pneumatic systems; Energy monitoring; The sixth 'S' is safety
Transport your data: Supply chain information critical to operational excellence; High-voltage faults; Portable cooling; Safety automation isn't automatic
Case Study Database

Case Study Database

Get more exposure for your case study by uploading it to the Plant 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.

Maintaining low data center PUE; Using eco mode in UPS systems; Commissioning electrical and power systems; Exploring dc power distribution alternatives
Synchronizing industrial Ethernet networks; Selecting protocol conversion gateways; Integrating HMIs with PLCs and PACs
Why manufacturers need to see energy in a different light: Current approaches to energy management yield quick savings, but leave plant managers searching for ways of improving on those early gains.

Annual Salary Survey

Participate in the 2013 Salary Survey

In a year when manufacturing continued to lead the economic rebound, it makes sense that plant manager bonuses rebounded. Plant Engineering’s annual Salary Survey shows both wages and bonuses rose in 2012 after a retreat the year before.

Average salary across all job titles for plant floor management rose 3.5% to $95,446, and bonus compensation jumped to $15,162, a 4.2% increase from the 2010 level and double the 2011 total, which showed a sharp drop in bonus.

2012 Salary Survey Analysis

2012 Salary Survey Results

Maintenance and reliability tips and best practices from the maintenance and reliability coaches at Allied Reliability Group.
The One Voice for Manufacturing blog reports on federal public policy issues impacting the manufacturing sector. One Voice is a joint effort by the National Tooling and Machining...
The Society for Maintenance and Reliability Professionals an organization devoted...
Join this ongoing discussion of machine guarding topics, including solutions assessments, regulatory compliance, gap analysis...
IMS Research, recently acquired by IHS Inc., is a leading independent supplier of market research and consultancy to the global electronics industry.
Maintenance is not optional in manufacturing. It’s a profit center, driving productivity and uptime while reducing overall repair costs.
The Lachance on CMMS blog is about current maintenance topics. Blogger Paul Lachance is president and chief technology officer for Smartware Group.