Choosing between grounded and ungrounded electrical system designs
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.
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).
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.