In part one of this series on generators (FRONT of HOUSE, June 2013, page 50), we investigated some underlying details behind Ohm’s law, and that led to investigating when and why current flows in electrical circuits. We then investigated two key concepts in power distribution: bonding and grounding. We showed that bonding — not grounding — creates the primary low-impedance path for fault current in the circumstance where a ground-fault event occurs. In this second article on portable power distribution, we build on the concepts in part one and investigate generator grounding and bonding in light of the existing 2011 National Electrical Code (NEC).
The concepts in this second article should not be viewed as independent of part one, and readers are encouraged to read part one in the June 2013 FRONT of HOUSE before digesting this second article. Successful, safe operation of equipment and/or secondary power distribution attached to a portable generator starts with understanding and applying proper bonding and grounding at the generator.
I repeat the disclaimer from the first article in the series. Let me clearly state up front that: I am not a licensed electrician, and implore readers to consult with an experienced electrician or their Authority Having Jurisdiction (AHJ) for further clarification about electrical matters. Equally important is realizing that knowledge gained from an article like this one is no substitute for direct experience in practicing safe handling of electrical equipment in the field. A solid conceptual understanding of portable electrical power distribution should be a goal for all pro audio professionals, especially when discussing the code with AHJs.
Background on the NEC
Throughout the remainder of this article we will reference sections of the National Fire Protection Association’s (NFPA) NFPA 70: National Electrical Code. Thus, this article will be particularly U.S.-centric in its discussion. Thankfully, physics behaves the same all over the world, so the underlying grounding and bonding behavior, if not the specific code details, will be applicable to readers worldwide.
The NEC is not adopted universally in all U.S. jurisdictions. Some places have exceptions to the NEC, and others have additional requirements. Because of the way the code is adopted, your local AHJ should be considered the ultimate resource for understanding code compliance of your pro audio endeavors. Further, the NEC is an evolving document and has been revised extensively over the years. When reading the code for yourself, please reference code cycle 2002 or later (e.g., 2008 or 2011). My code reference for this second article is either the existing 2011 NEC code cycle or the proposed 2013 code draft to be officially released in 2014. The NEC 2008 code is an affordable digital download ($9.99 U.S.) for Apple devices on the iTunes store, and I encourage readers to purchase a digital copy of it. Having a digital copy of the code will allow you to read the sections referenced in this article in their entirety.
Bonding and Grounding of Generators
The details of bonding and grounding are outlined in NEC 250. Within NEC 250, NEC 250.1-4 set the requirements for bonding and grounding function. NEC 250.26 then defines the requirements for a grounded conductor such as the neutral. NEC 250.34 covers grounding and bonding of portable generators specifically, and those bonding requirements then reference NEC 250.30. (The full text of NEC 2011 250.34 is included in the sidebar to the left.)
The NEC authors presume the reader has general familiarity with the rest of NEC 250 preceding 250.34. As such, NEC 250.34 is focused first on variances for baseline grounding behavior of generators. It then turns to prescriptive bonding requirements in 250.34(C). For our case, it is therefore most prudent to discuss the baseline requirements of grounding and bonding for the typical portable generator in the pro audio context.
The typical 20-250 kW production generator (e.g., MQ WhisperWatt) has an automatic voltage regulator (AVR) based alternator providing the electrical power output. These generators have a bond point between the grounded (i.e., neutral-white wire), grounding (i.e., ground-green wire) and generator frame. The neutral terminates back at the generator’s alternator and completes the electrical circuit. It is the neutral/ground/frame bond point that provides a low resistance ground-fault return path for the overcurrent protection (i.e., the generator’s circuit breakers).
Current that returns via the green ground wire after a fault event finds its way back to the alternator via the neutral, because of the neutral-to-ground bond point on the generator. The green wire in the downstream electrical distribution cable is bonding the equipment in the portable power distribution system. It is this bonding that causes a circuit breaker to trip by providing a low-impedance fault-current path to the alternator via the bond point. Note that this process occurs in the absence of a ground rod.
Separately Derived System (SDS)
With bonding covered, and overcurrent devices (e.g., circuit breakers) therefore functioning properly without a physical connection between the generator and the Earth, what about grounding requirements? Here the NEC is clear. If the power source acts as a separately derived system (SDS), then it must be connected to the Earth via a grounding electrode. The 2013 NEC code draft, Article 100 defines an SDS as follows:
“A premises wiring system or portion of a premises wiring system other than a service. Power for such systems is derived from a source of electric energy or equipment with no direct connection from circuit conductors of one system to circuit conductors of another system, other than connections through the earth, grounding electrode(s), grounding electrode conductors, bonding jumpers used to connect grounding electrodes, equipment grounding conductors, metal enclosures or metallic raceways.”
If a power source is deriving electrical output without being tied directly into another power system, it operates as an SDS. In the pro audio context, the very requirement of a portable generator is usually a sufficient indicator that the generator is acting as an SDS.
For pro audio applications, one should assume that the generator is operating as an SDS. Therefore, bonding the ground rod to the neutral/ground/frame bond point on the generator should be considered mandatory.
Note that the connection with ground rod is not the primary low resistance fault current path, and the Earth is not the source of overcurrent protection, but rather forms a current divider with the neutral circuit. Since the neutral path is usually much lower impedance than the ground rod, the majority of the fault current travels to the alternator, not to the ground rod. (Please refer to the first article in this series for details on the purpose of grounding.)
Returning to NEC 250.34, we see that certain exceptions are made for generator grounding, removing the ground rod requirements. These are for “cord-and-plug-connected” equipment or equipment mounted on the generator. Examples would include things like plugging an electric power tool directly into a generator, or having a welder with an integrated generator on a work truck. In pro audio environments, there are almost always downstream power distribution cables and receptacles. Even if it is only a short SOOW (600-Volt oil- and weather-resistant) cable that terminates in a quad box, we have still created a SDS and therefore code mandates a “grounding electrode” (i.e., ground rod).
Conclusion
The first practical nugget from this jaunt through the code is to always confirm the neutral/ground/frame bond point on any generator that you are using. This bond must be solidly established to provide a fault current path during generator operation. A second in the field detail is mandatory use of a ground rod bonded to the generator frame in the pro audio environment. While readers may be familiar with these practical principles, it is important to uncover their genesis, and understand how they fit in the context of code-compliant generator operation for professional audio. Bonding and grounding are important enough concepts in electrical distribution to warrant extensive coverage for FRONT of HOUSE readers.
In the third article of this series, we will investigate proposed code revisions for generators in the context of the future NEC code revision coming into effect at the start of 2014. Proposed changes include the broader use of ground fault circuit interrupters (GFCIs) on generators. We will also consider the attachment of other power distribution elements downstream of the generator.
“Floating Neutral” Generators
There are millions of small generators, typically less than 10 kW, that are known in industry terminology as “floating neutral” generators. The Honda EU6500is (Fig. 1) is an example of such a floating neutral generator. Essentially all inverter-based generators, and many home backup-oriented generators, adopt this configuration. Floating neutral simply means the generator neutral is not bonded to the generator frame or the grounding conductor.
When these generators are used for house backup, the bonding is performed by the home’s panel, rather than on the generator. The home, generator, and power company neutrals are bonded at a single point in the home’s main service panel. The grounding conductors are also bonded to the neutrals at this same point in the panel. This provides the fault current return path, via the generator neutral, for the home’s branch circuits when running off generator power.
When floating neutral generators are used in a standalone setting, they are intended to be operated without a ground rod. The lack of conduction to Earth, combined with the lack of neutral to ground bonding means that there is no ground fault-current return path, so no current flows in the event of a fault event. Standalone operation without a ground rod limits code-compliant operation to “cord-and-plug-connected equipment” per NEC 250.34.
With no current return circuit, a fault event does not present a shock hazard, barring the presence of a secondary current path. As there is no fault current return path, the floating neutral configuration will not trip a conventional circuit breaker. Thus, if equipment attached to a floating neutral generator experiences a fault event, there will be no breaker tripping to indicate the fault’s presence.
Operation of a floating neutral generator in the pro audio context first requires external bonding of the neutral and ground, similar to the circumstance of using these generators for home backup service. Details of this bond are discussed in NEC 250.30. After the bond is established, a low impedance fault current path via the neutral is created. At this point the generator frame can then be tied to a ground rod, and the generator operated as a SDS. As an example, the Honda EU6500is in Fig. 1 can be externally bonded, connected to earth and operated as a SDS.
Note that some small inverter generators do not hold their neutral at ground potential, but rather have the hot +60V RMS to ground, and the neutral -60V RMS to ground. These generators cannot have a neutral to ground bond, and are only intended to operate cord- and plug-connected equipment.
Portable Generator Requirements (OSHA and Construction)
Portable generators used in construction environments, or in circumstances where OSHA has jurisdiction, have additional code requirements. We mention the relevant code sections here so the interested reader can familiarize themselves with them. It is conceivable that an AHJ would reference and/or enforce these regulations in a pro audio production environment:
For the NEC in construction environments, see NEC 590.6 and specifically 590.6(A)(3) Receptacles on 15 kW or less Portable Generators.
In OSHA regulated environments, see OSHA regulation 1926.404(b)(1) and specifically 1926.404(b)(1)(ii) for generator GFCI requirements.
2011 NEC 250.34 Full Text
(A) Portable Generators. The frame of a portable generator shall not be required to be connected to a grounding electrode as defined in 250.52 for a system supplied by the generator under the following conditions:
(1) The generator supplies only equipment mounted on the generator, cord-and-plug-connected equipment through receptacles mounted on the generator, or both, and
(2) The normally non—current-carrying metal parts of equipment and the equipment grounding conductor terminals of the receptacles are connected to the generator frame.
(B) Vehicle-Mounted Generators. The frame of a vehicle shall not be required to be connected to a grounding electrode as defined in 250.52 for a system supplied by a generator located on this vehicle under the following conditions:
(1) The frame of the generator is bonded to the vehicle frame, and
(2) The generator supplies only equipment located on the vehicle or cord-and-plug-connected equipment through receptacles mounted on the vehicle, or both equipment located on the vehicle and cord-and-plug-connected equipment through receptacles mounted on the vehicle or on the generator, and
(3) The normally non—current-carrying metal parts of equipment and the equipment grounding conductor terminals of the receptacles are connected to the generator frame.
(C) Grounded Conductor Bonding. A system conductor that is required to be grounded by 250.26 shall be connected to the generator frame where the generator is a component of a separately derived system.
Informational Note: For grounding portable generators supplying fixed wiring systems, see 250.30.
