Grounding Your Generators
There are many topics to consider when discussing grounding, even when the discussion is centered on emergency systems. Here, we focus on several topics that relate to the most common generator applications found in commercial systems. Some definitions are necessary for an understanding of system components referred to.
There are many topics to consider when discussing grounding, even when the discussion is centered on emergency systems. Here, we focus on several topics that relate to the most common generator applications found in commercial systems.
Some definitions are necessary for an understanding of system components referred to. I will review service grounding to understand how the components will fit together. With this base of knowledge, we can then discuss the important code passages related to generator grounding and ground fault protection. Finally, I will provide a few valuable resources so that you can further enhance your knowledge.
The National Electrical Code (NEC) provides guidance on what systems are to be grounded—and not grounded—and how grounding is to be accomplished. This article is not intended to be a complete listing of the code requirements. The current edition of NEC and any local codes and modifications should be reviewed prior to the design or installation of an on-site power generation system. My purpose here is to give the reader direction to accumulate knowledge needed for this research. Interpretation of the NEC is always left up to the authority having jurisdiction.
The following are several terms defined in NEC. Because they contain variations of the word "ground," these terms are misunderstood if the reader does not pay close attention to context:
Ground . A conducting connection, whether intentional or accidental, between an electrical circuit or equipment and earth or some conducting body that serves in place of earth.
Grounded . Connected to earth or some conducting body that serves in place of earth.
Grounded conductor . A system or circuit that is intentionally grounded.
Grounding conductor . A conductor used to connect equipment or the grounded circuit of a wiring system to a grounding electrode or electrodes.
Equipment grounding conductor . A conductor used to connect the non-current-carrying metal parts of equipment, raceways and other enclosures to the system grounded conductor, the grounding electrode conductor or both, at the service equipment or at the source of a separately derived system.
Bonded . The permanent joining of metallic parts to form an electrically conductive path that ensures electrical continuity and the capacity to conduct safely any current likely to be imposed.
Service . The conductors and equipment for delivering electric energy from the serving utility to the wiring system of the premises served.
Two additional terms that one encounters are not found in NEC. The following are found in The Green Book (IEEE Std. 142-1991- Recommended Practice for Grounding of Industrial and Commercial Power Systems, Section 1.2 Definitions):
Solidly grounded . Connected directly through an adequate ground connection in which no impedance has been intentionally inserted.
Resistance (or impedance) grounded . A grounded system with a purposely inserted resistance (or impedance) that limits the ground fault current such that the current can flow for an extended period of time without exacerbating damage. In high resistance systems, this current is generally accepted to be 10 amps or less. In low resistance systems, a lower ohmic value is selected to generate a higher relaying current.
Solidly grounded systems will allow the greatest control of transient overvoltages but will produce the highest fault currents. Generators lack the ability to withstand the mechanical stresses caused by high fault-current levels. As a result, solidly grounded systems are not always the best choice. Grounding systems utilizing a resistance or impedance can limit the fault current to a point where the emergency system can continue to operate until normal power is restored.
The low-resistance method has the advantage of immediate and selective clearing of the grounded circuit, but requires that the minimum ground-fault current be large enough to positively actuate the applied ground fault relay. High resistance grounding is a method that can be applied to existing medium voltage ungrounded systems to obtain the transient overvoltage protection without the modification expense of adding ground relays to each circuit. (See IEEE Std. 142-1991- Recommended Practice for Grounding of Industrial and Commercial Power Systems, Section 1.4.3.)
Resistance grounded systems are most commonly employed in medium voltage systems, which tend not to use a neutral conductor. Resistance grounding can also be used in 600-volt systems if the loads are balanced or are served by a transformer with a delta-connected primary. This can be cost effective, because ground fault relaying is simpler and three-pole transfer switches can be used.
A solid foundation in service grounding is important. However, I find that even the most seasoned designers occasionally have to pause to consider exactly how services are grounded.
The best way to explain this grounding is through a diagram. Figure 1 is a typical three-phase, four-wire service showing the service disconnecting means required by Article 230 and the various grounded conductors.
The diagram does not show the additional connections to a cold water pipe, grounding mat, service conduit grounding bushing(s) and structural steel as required by NEC, but these should be understood to exist. There are some designers who prefer to make these connections to the neutral bus of the service disconnect. But I prefer to connect these to the ground bus, which allows the removal of the main bonding jumper for continuity testing. This connection can also be used for ground fault sensing. Others will argue that connecting to the neutral bus ensures a good connection to the earth. This is simply a matter of designer preference.
NEC requires the neutral to be grounded as close to the source as possible and only grounded one time. Grounding at other locations in addition to the source can create current flows in the grounded conductors as the potential changes between the multiple locations. Grounding the system at a location other than the source can cause problems if the ground is isolated from the source or left open inadvertently. During line-to-ground fault conditions, it is the equipment grounding conductor that carries the fault current back to earth.
NEC and Emergency Systems
NEC Article 250 covers grounding in everything from telecommunications systems to health-care facilities to pipe organs. In the NEC 1999 edition, significant changes were made to Article 250. In 2002, all sections of the NEC were renumbered, but no major changes to the intent of Article 250 were made.
NEC requires systems to be grounded for three principle reasons, all of which relate to safety:
To limit the voltages caused by lightning or by accidental contact of the supply conductors with conductors of higher voltage.
To stabilize the voltage under normal operating conditions, which maintains the voltage relative to ground so that any equipment connected to the system will be subject only to that potential difference.
To facilitate the operation of overcurrent devices, such as fuses, circuit breakers or relays, under ground fault conditions.
One needs to pay particular attention to these passages in the NEC when designing grounding systems:
230.95 Ground-Fault Protection of Equipment . "Ground-fault protection of equipment shall be provided for solidly grounded wye electrical services of more than 150 volts to ground but not exceeding 600 volts phase-to-phase for each service disconnect rated 1,000 amperes or more."
Section 230.95 (C) FPN No. 3 . "FPN No. 3: Where ground-fault protection is provided for the service disconnect and interconnection is made with another supply system by a transfer device, means or devices may be needed to ensure proper ground-fault sensing by the ground-fault protection equipment."
These sections date to the 1971 Code when ground faults were identified as a significant cause of fires. Prior to 1971, transfer switches only had three poles, with no provisions to switch the neutral. The neutral was typically grounded at both sources. As mentioned earlier, grounding at multiple locations can create current flows in the grounded conductors as the potential changes between the locations. This is most commonly a problem when the grounding locations are separated by significant distances or are in different soil types. In 1978, the definitions for separately derived systems were modified and generators were no longer considered separately derived.
Section 250.20 (D). Separately derived systems, as covered in 250.20(A) or (B), shall be grounded as specified in 250.30. Section 250.30 provides the requirements for bonding and grounding separately derived systems.
Section 250.20 (B). (B) Alternating-Current Systems of 50 volts to 1,000 volts. Alternating-current systems of 50 volts to 1,000 volts that supply premises wiring and premises wiring systems shall be grounded under any of the following conditions: (1) Where the system can be grounded so that the maximum voltage to ground on the ungrounded conductors does not exceed 150 volts; (2) Where the system is three-phase, four-wire, wye-connected, in which the neutral is used as a circuit conductor; (3) Where the system is three-phase, four-wire, delta-connected in which the midpoint of one phase winding is used as a circuit conductor.
50.26 Conductor to Be Grounded . Alternating-Current Systems. For AC premises wiring systems, the conductor to be grounded shall be as specified in the following: (1) single-phase, two-wire—one conductor; (2) single-phase, three-wire—the neutral conductor; (3) multiphase systems having one wire common to all phases—the common conductor; (4) multiphase systems where one phase is grounded—one phase conductor; and (5) multiphase systems in which one phase is used as in (2)—the neutral conductor.
The following sections tell us which systems are to have grounded conductors and which conductors shall be grounded:
250.4 General Requirements for Grounding and Bonding . "(5) Effective Ground-Fault Current Path. Electrical equipment and wiring and other electrically conductive material likely to become energized shall be installed in a manner that creates a permanent, low-impedance circuit capable of safely carrying the maximum ground-fault current likely to be imposed on it from any point on the wiring system where a ground fault may occur to the electrical supply source. The earth shall not be used as the sole equipment grounding conductor or effective ground-fault current path."
This section directs us to provide the equipment grounding conductor. It can't be emphasized enough how important this conduction path is as it facilitates the operation of the ground fault equipment.
Finally, NEC Article 445 addresses generators and states, "445.3 Other Articles. Generators and their associated wiring and equipment shall also comply with the applicable provisions of Articles 695, 700, 701, 702, and 705."
Article 695 deals with fire pumps, articles 700 through 702 enumerate the requirements of legally required emergency systems and Article 705 describes the requirements of the interconnecting normal and emergency systems.
How to Ground
This brings us to the heart of this article. Now that we have provided the ground rules (no pun intended) for generator grounding, we can show how it is accomplished. It may seem obvious, but if the generator is the only source of power on site, then the grounding is relatively straight forward. Grounding is accomplished as shown in Figure 1 (p. 21). The complexity occurs when there is more than one source of power on site.
There are two schemes that can be employed to safely ground an emergency generator. The designer chooses one of these two schemes based on the requirements of the project and on the budget and physical space available for the equipment.
The first grounding scheme in Figure 2 (p. 21) can be used in systems that are not required to have ground fault protection on the normal side (less than 1,000 amps on systems greater than 150 volts to ground) or systems that are not required to comply with Article 700. In this case, we use a three-pole transfer switch with the engine generator set considered a non-separately derived source.
The three-pole switch is the least expensive switch and requires the least physical space. Ground faults that occur may trip the normal source breaker when connected to emergency source. This becomes a problem when normal power is restored. The automatic transfer switch will not detect normal power and will not transfer back. Also, there is no area protection provided and a possibility exists of creating an ungrounded system in the event of cable failure as in Figure 3 (p. 22).
The second grounding scheme, shown in Figure 4 (p. 22), is used in systems that are required to have ground-fault protection on the normal side and systems that are required to have ground-fault indication on the emergency side. Using a three-pole transfer switch with overlapping neutral, the engine generator set is considered a separately derived source. Or, we can use a four-pole transfer switch. Again, the generator set is a separately derived source. Not all manufacturers offer both four-pole and three-pole with overlapping neutral so the configuration of the switch is usually a function of the selected manufacturer.
The four-pole transfer switch has a set of contacts that will arc as the transfer is made from emergency power back to normal power. These contacts need to be maintained just like the phase contacts. The three-pole switch with overlapping neutral avoids this situation and also provides a smoother, in-phase transition for motor loads.
If a cable failure (not shown) is experienced with this configuration, we still have a path for fault current to flow back to earth and the ability to sense this fault. Area protection can also become an issue if the generator is located a distance away from the utility service entrance.
At the generator itself, there are quite a few items that require grounding. Most of the ground connections on the generator skid will be made at the factory, but the designer still needs to identify several items for the contractor to connect. The generator frame, the automatic transfer switch enclosure, the generator enclosure, the conduits feeding from the generator and feeding the generator accessories and all other exposed metal parts must be permanently bonded. It is especially important for the generator frame to be properly grounded as most manufacturers connect the RFI suppressors to the frame. Without proper RFI suppression, today's electronic governors may not operate properly. Most generator manufacturers recommend that the impedance to ground be less than five ohms.
While no list is comprehensive, these are some of the key issues that a designer must consider in designing grounding systems for generators.
NFPA 70 — 2002 The National Electrical Code Article 250
National Electrical Code Handbook - Ninth Edition, 2002 NEC Edition Edited by Earley, Sheehan, Sargent, Caloggero and Croushore
IEEE Std. 142-1991 — Recommended Practice for Grounding of Industrial and Commercial Power Systems (The Green Book), Chapter 1.8, Produced by Power Systems Grounding Subcommittee, Chaired by Gordon S. Johnson
On-Site Power Generation — A Reference Book, Edited by Gordon S. Johnson (The 'EGSA' Book), published in 1998, Chapters 7 and 29.