Generators, and portable power distribution in general, are central to professional audio. Electricity is a powerful means of transferring energy; it is the lifeblood of all pro audio gear. While electricity is ever-present, it is rarely discussed technically in print, in part because of the liability that accompanies its underlying power.
With that liability in mind, 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. If in doubt about any electrical use, seek professional help. 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.
With the liability caveats clearly stated, the specific details of portable power distribution for pro audio is often outside the daily interactions of the local electrician and/or AHJ. In these circumstances, it can fall to the audio practitioner to clarify the relevant codes and procedures to the electrical professionals in the name of event safety. A solid conceptual understanding of portable electrical power distribution should be a goal for all pro audio professionals.
This article is a first in a series targeted specifically at generators and portable power distribution for professional audio. This article will address two important principles in power distribution systems: bonding and grounding. We will investigate these important concepts as foundation for discussing portable, generator-based power systems.
Before discussing grounding and bonding, we must first dive deeper into Ohm’s law, and then introduce the National Electrical Code (NEC).
Ohm’s Law — A Deeper Look
Many of the situations related to power distribution can be analyzed using little more than Ohm’s law, which famously states:
Where “V” is voltage, “I” is current, and “R” is resistance.
The math of Ohm’s law is simple multiplication, but generally we do not need to calculate mathematical values from the law. Instead, we use the behavior of the underlying terms in Ohm’s law to understand electricity in the context of topics like grounding and bonding.
The voltage in Ohm’s law is a measure of the “push” behind electricity. It is the driving force to create current. Current is the measure of the rate of flow of charge. If voltage is the pressure in a garden hose, then current is the amount of water flowing out of the hose when you open the sprayer valve. Voltage is a measurement of potential, which means that it is the difference in voltage between two points that matters. If two points are at different voltages, then current will flow from the point of higher potential to one of lower potential. If two points are at the same voltage, then no current will flow between them. Voltage has its meaning in comparing the relative potential value between different locations.
The third component of Ohm’s law is resistance. Resistance is the physical property that gets in the way of current. If resistance is very high, then little current can flow for a given voltage. However, if resistance is low, then a small voltage results in a large current. Another useful way to think of resistance is that it defines the voltage difference one must have between two points to drive a given amount of current. Resistance is a property defined for the materials that make up the electrical path for current. Materials that allow ready flow of current are called conductors. Some materials, called insulators, completely prevent the flow of current, and their value for resistance is functionally infinite.
There is an important subtlety to consider that’s not readily apparent from looking at the above equation. A voltage can exist without current, but not the other way around. Voltage is calculated from an inherent property of electrons called the electric field. The electric field defines the force that causes charged particles (e.g. protons and electrons) to attract or repel each other. The electric field travels at the speed of light, and can exist in a vacuum. Thus, the electric field is an underlying property that gives rise to current. Current results from the push of the electric field, which is conveniently quantified by the voltage.
It’s All About Safety
Why do these extra details about Ohm’s law matter? They are important for understanding how electricity behaves in the context of safety. As a practical example, two points both at 1,000,000 volts connected with giant copper wires won’t produce any current, because there is no potential difference between the points. Similarly, one point at zero volts connected to a point of high voltage by an insulating material won’t result in current, either. We must have both a voltage difference, and a conducting pathway between the points, to produce current.
Because current is what ultimately creates the risk of fire or electrocution, it is important to have a clear grasp on circumstances that will produce current. The NEC refers to circuit breakers as “overcurrent” devices, specifically because we are concerned with disconnecting the circuit when there is excessive current. The core of electrical safety is understanding when circumstances will generate undesired electrical current(s), and a clear understanding of Ohm’s law aids in this quest. With Ohm’s law addressed, we now turn to the NEC and continue on to the meat of grounding and bonding.
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 principles, 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. Due to 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, which has been revised extensively over the years. Most of what is discussed in this article was first clarified and/or expanded in the 2002 NEC release, particularly NEC 250.4. When reading the code for yourself, please reference editions 2002 or later (e.g. 2008 or 2011). My code reference for this article series is the 2013 NEC 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. Familiarity with the electrical code should be the goal of all audio professionals, and having a digital copy of the code will allow you to read the sections referenced in this article in their entirety.
Bonding vs. Grounding
In the world of professional audio, the term “grounding” is frequently mentioned. We have a ground wire in XLR cables, ground lifts on direct boxes, and worry about ground loops. It is perhaps natural for us to think about grounding in the context of portable power distribution, even though it can be a term improperly applied.
Before getting into the details of bonding and grounding, it is worth mentioning where the full text can be found in the electrical code. The bonding and grounding terminology is defined in NEC article 100 (NEC 100), and the details of bonding and grounding are outlined in NEC 250. Within NEC 250, the specific requirements for grounding and bonding are outlined in NEC 250.4. Much of the remaining discussion is built on NEC 250.4.
First we define bonding. Bonding is the physical connection of various pieces of the electrical system together to provide a low resistance current path back to the source electrical system [NEC 250.4(A)(3)]. The NEC defines the low resistance current path in NEC 250.2:
“Effective Ground-Fault Current Path. An intentionally constructed, low-impedance electrically conductive path designed and intended to carry current under ground-fault conditions from the point of a ground fault on a wiring system to the electrical supply source and that facilitates the operation of the overcurrent protective device or ground-fault detectors.”
An example of a ground fault current event would be a wire coming loose inside a piece of pro audio equipment and then contacting the case. The case would then be electrified, and pose a current flow risk. By providing a low resistance path back to the source electrical system through bonding, the current would instantaneously spike and cause the circuit breaker to open.
Next, we define grounding. In its essential nature, grounding is the direct physical connection of components in the electrical system to planet earth. In the context of a generator, this is the ubiquitous copper rod driven into the ground. Grounding serves several specific purposes. Quoting from NEC 250.4(A)(1):
“Electrical systems that are grounded shall be connected to earth in a manner that will limit the voltage imposed by lightning, line surges, or unintentional contact with higher-voltage lines and that will stabilize the voltage to earth during normal operation.”
Notice that nowhere in this wording is a mention of fault conditions, circuit breakers, overcurrent, or any of the other things we typically associate with electrical safety. The electrical path to the ground rod is not utilized to provide the fault current path. In fact, NEC 250.4(A)(5) specifically excludes the earth (ground) connection as an “effective ground-fault current path.” Frequently, the resistance of the ground rod connection to earth is too high, and therefore by Ohm’s law the fault current flowing to the ground rod is insufficient to trip the breaker.
If the ground rod is not the path of return for ground-fault current in portable power distribution, what then? On the typical 25 to 250 kW production generator (e.g. MQ Power WhisperWatt), the ground (green) wire and the neutral (white) are bonded to each other and the generator frame. It is this bond point to the generator neutral that completes the circuit, and provides a low resistance ground-fault return path for overcurrent protection. The connection to the external ground rod is also bonded to the frame, but ultimately is not the primary low resistance fault current path.
For portable power distribution, the rated cable jacket (e.g. SOOW) is insulating, and the green ground wire bears the responsibility of providing the ground-fault current return path to the generator. Thus the green wire is bonding the equipment in the portable power distribution system. It is this bonding that causes a circuit breaker to trip. Now, if the green wire is tied to earth through a ground rod, then the green wire is also grounding, but that connection to the earth is not the source of overcurrent protection. We see that there are clear differences between bonding and grounding, and a green wire’s bonding behavior is typically more important than its grounding functionality.
Conclusion
Plainly stated, many of the times that we use the term grounding we really should be discussing bonding. It is bonding that provides the element of safety in our portable power distribution systems. The generator ground rod has its own important functions, but they are perhaps not what audio professionals typically think.
Grounding and bonding are foundational to electrical distribution, so they are the necessary starting point for this series. In part two of this article we will discuss portable generators more specifically, building on this first article. We will look at the requirements for generator grounding, proposed code changes for generator overcurrent devices, and consider the unique requirements of inverter-based generators (e.g., Honda EU series). Part two will launch into the nuts and bolts of safely utilizing generators in the field.
I implore the readers of FRONT of HOUSE to educate themselves on the topic of portable power distribution. Read this article, ask questions, buy an electronic copy of the code, and talk with your local AHJ to learn the unique requirements of the venue and municipalities where you undertake the craft of professional audio.
Clarifying Code Language
The NEC is a large document, and looking at various versions, historical terminology can be found within its pages. While it can be argued that the historical terminology provides deeper technical insight, it is usually more confusing to read and understand. The table below compares some example relevant historical electrical terms with their modern counterparts.
|
Historical Electrical Term |
Modern Terminology |
|
Ungrounded Conductor |
Hot Conductor |
|
Grounded Conductor |
Neutral Conductor |
|
Grounding Conductor |
Ground Conductor |
U.S. Specific Code Vocabulary
In addition to historical terms, the NEC has U.S. specific terms for certain items and activities. Two common examples follow:
• Instead of “Residual Current Devices (RCD’s)” we call them “Ground Fault Circuit Interrupters (GFCI’s).”
• We use the term “grounding” to refer to the physical connection of electrical equipment to planet Earth, a term that is referred to as “earthing” elsewhere in the world.
Phil Graham is FOH’s regular technical contributor and resident scientist. Email him at: pgraham@fohonline.com.
