Choosing between grounded and ungrounded electrical system designs
Ungrounded, resistance grounded systems
Although the NEC requires the majority of electrical systems to be grounded, some are actually required to be ungrounded. There are only five different electrical power systems/subsystems noted in NEC Article 250.22 where the code committee has determined the hazards of grounding to outweigh safety benefits of grounding. One of these system types is an isolated power system, which is a distribution power system of limited size, typically for use in hospital operating rooms. These areas are required to have an ungrounded system because it would be considered unacceptable to have a power outage during a surgical procedure. A typical isolated power system consists of a single-phase 10 kVA isolation transformer in which the secondary side remains ungrounded. The transformer’s electrostatic shield is connected to ground and effectively shunts high-frequency noise to ground. The 120 V equipment connected to these systems will continue to operate after the first fault, just as in an ungrounded system. These power systems are particularly suitable for use in operating rooms where there may be water or fluids present and where a GFCI receptacle (required by the NEC in wet areas) would ordinarily be required to be installed. The installation of the isolated power panel is alarmed locally, so if there is a ground fault, the team will be notified, but any ongoing procedures needn't be interrupted.
During the 1970s, language was added to the NEC to require ground fault trip sensors to feeders 1,000 A and above on 480 V grounded electrical systems. The need for electrical service continuity for the industrial process sector drove the need for a hybrid system to combine the stability and safety benefits of the grounded system with the continuous service benefits of the ungrounded system. During this time, resistance grounded systems began gaining traction. Service continuity makes this type of grounding system very attractive today for the traditional pulp and paper industry as well as for high-tech data centers. An impedance grounded system incorporates the benefits of both the grounded and the ungrounded system. The IEEE Green Book identifies the following benefits:
- Reduces burning and melting effects in faulted electrical equipment
- Reduces mechanical stresses in faulted circuits and cables
- Reduces electric-shock hazards caused by stray ground fault currents in the ground path
- Reduces the arc blast or flash hazard
- Reduces the momentary line-voltage dip caused by a fault and the subsequent clearing
- Controls transient overvoltages and prevents circuit shutdown on the first ground fault.
Impedance grounded systems include high resistance ground (HRG) and low resistance ground (LRG) configurations. For a wye-connected transformer, Figure 5 demonstrates how a known resistance is matched to the facility load profile and inserted directly between the secondary of the service transformer and ground. To accomplish this with a delta secondary transformer, an artificial neutral must be created using a zigzag transformer.
In a wye connected HRG system, intermittent faults that cause so much trouble in ungrounded systems will be eliminated by the neutral system ground resistor because its insertion limits the total current flow to ground.
System continuity is maintained because, although ground fault alarms occur, the overcurrent devices do not operate. This current flow in a low-voltage system (480 V to 600V) will be limited typically to 10 A so that the fault can be located and then repaired at a scheduled time without exposing staff to hazardous fault levels (see Figure 6). While HRG systems are a good fit for large data centers, there are pitfalls, such as misapplication of surge protective devices (they must be rated for ungrounded-neutral circuits), and the UPS must be grounded in a compatible method to its input and output wiring. Tracing faults is somewhat difficult and must be accomplished on live circuits using circuit pulsers.
LRG-grounded systems are typically used for 15 kV medium-voltage applications where the charging current may be too high to match an HRG. LRG systems tend to operate more similarly to the solidly grounded system than the ungrounded system. In this case, the added resistor limits the fault currents between 200 A and 400 A, which is too high to allow continuous operation during a fault. Therefore, ground fault detection equipment must be set to trip as quickly as possible on detection. The advantage of controlling the current is that improved selectivity between overcurrent protective devices in the system may be achieved. It is interesting to note that through the 1999 code cycle, impedance/resistance grounded systems were in the same article as the ungrounded systems because of their operating similarities.
The NEC provides the framework for applying grounded and ungrounded systems. Table 1 summarizes the benefits and drawbacks of these different grounding systems as organized by the NEC. In a facility with a predominant need for line to ground loads, the NEC clearly requires a solidly grounded system. The solidly grounded system is the simplest and the cheapest to implement in the field. It is typically found in commercial buildings of today. In contrast, if a facility only has 3-phase loads and terminating its internal processes is deemed to be too heavy a risk, then an ungrounded system has definite merits. There is a middle ground, however, where service continuity is required, and the benefits of isolating and locating a fault for added safety are required. In these situations, one might consider an HRG system that has a proven track record for use in industrial process plants as well as large data center designs. The HRG system provides a single-point ground system for the facility. However, if and when there is a ground fault, the fault won't cause downtime.
NEC Article 250 has remained largely unchanged over the years, with a few punctuated changes in the 1940s and 1970s. Much credit must be given to the original code committee members for understanding the fundamentals and safety benefits of system grounding. Although grounding is often viewed as being mysterious, adhering to the code will safeguard occupants and facility equipment.
Sharpe is a senior electrical engineer at Affiliated Engineers Inc. She has more than 20 years of design experience in higher education, research facilities, and mission critical projects. Her most recent projects have been for the Fred Hutchinson Cancer Research Center and the University of Washington, School of Medicine Research Facility, both in the South Lake Union neighborhood of Seattle.