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.
Grounding 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
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).