Generators and portable power distribution are central to professional audio. Electricity is the preferred means of transferring energy. It is the lifeblood of all pro audio gear. The specific details of portable power distribution for pro audio are often outside the daily interactions of the local electrician. 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.
I should 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 guidance. Knowledge gained from an article like this is no substitute for direct experience in practicing safe handling of electrical equipment in the field.
With that out of the way, let’s peer into the future and see what concerts could look like in a world powered by a diversity of newer, cleaner energy sources. Beyond diesel generators or tapping into temporary panels, what could the future of entertainment power hold?
Basics Never Change
Some concepts are central to all power distribution systems of the present and future — and for today, that means bonding and grounding. No matter what our crystal ball reveals, bonding and grounding are going to inform how we will interconnect to these future electrical sources. Grounding and bonding requirements come from the underlying behavior of electrical conductors.
Three commonly referenced electrical properties are often combined to show the familiar Ohm’s law (V = I * R). Where “V” is voltage, “I” is current, and “R” is resistance. The math of Ohm’s law is simple multiplication, but more valuable is to investigate the underlying terms to better understand electricity.
The voltage 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 (i.e. electrons). 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 nearly infinite.
There is an important subtlety to consider that is not readily apparent from the above. 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. As long as you have charged particles you will have an electric field. Whether those charged particles are free to move and create current is a separate question.
The electric field is an underlying property that can give rise to current. Current results when charges are pushed by the electric field, which is conveniently quantified by the voltage. If the electric field pushes on charges that are able to move through a material, then current results. Why does this subtlety matter? They are important for understanding how electricity behaves in the context of safety. As a practical example, two points both at 10,000 volts of potential connected with pure copper wires will not experience any current flow, because there is no voltage difference between the points. Similarly, one point at low voltage connected to a point of high voltage by an insulating material will not result in current either, because charges cannot move in the insulating material. We must have both a voltage difference and a conducting pathway to have current flow.
Current flow is what ultimately creates the risk of fire or electrocution and it is important to have a clear grasp on circumstances that will cause current flow. The electrical code refers to circuit breakers as “overcurrent” devices, specifically because we are concerned with disconnecting the circuit when there is excessive current. At the core of electrical safety is determining when circumstances will generate electrical current between undesired locations. Bounding and grounding together are the tools to equalize voltage potentials between different points. The goal is to keep errant current from flowing or to give that current a traverse away from people and to a safety trip device.
Bonding and Grounding
In the world of professional audio, the term “grounding” is frequently mentioned. We have a ground wire in microphone cables, ground lifts on direct boxes, and worry about ground loops between equipment. It is perhaps natural for us to think about grounding in the context of portable power distribution, even though bonding is usually the more appropriate term.
First we define bonding. Bonding is the physical connection of various pieces of the electrical system together to minimize potential differences between components and 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 portable distribution, this is the ubiquitous copper rod driven into the soil. 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 high. Put this high resistance into Ohm’s law and the fault current flowing to the ground rod can be insufficient to trip a breaker.
If the ground rod is not the path of return for ground-fault current in portable power distribution, what is the current path then? On a typical 25 to 250 kW production generator, 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 back to the generator windings. The connection to the external ground rod is also bonded to the frame, but ultimately is not the primary low resistance fault current path. The ground rod does serve to hold the relative voltage potential of the generator frame to a value close to the adjacent soil.
In an environment with a power panel, the ground and neutral are connected (i.e., bonded) together at the main electrical panel. The neutral then goes from the panel back to the power pole, where the secondary winding of the pole transformer completes the circuit and provide the low resistance ground-fault current path to trip a breaker. Similar to a generator, the transformer on the power pole will often have a ground rod to hold the transformer voltage potential near that of the local ground.
For portable power distribution, the cable jacket is insulating and the green ground wire bears the responsibility of providing the ground-fault current return path to the generator or panel. 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 the soil through a ground rod, then the green wire is also grounded, but that connection to the earth is not the source of overcurrent protection. The fault current path isn’t into the soil, but rather to the transformer or generator windings.
There are clear differences between bonding and grounding, and a green wire’s bonding behavior is more important to tripping a safety device than whether it is grounded to the soil. 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 ground rod has its own important functions, but perhaps not what people think. In a future world, what constitutes a power source will get more complicated. So ,too, will thinking about how the system will be interconnected.
Future Complications
Why all the detail about grounding and bonding, and what does this have to do with the future of generators? As generator fuel efficiency and emissions requirements strengthen, current generators will have to improve their efficiency. One way that efficiency gains can be seen is through the use of solid-state inverter technology from multiple winding generators. Similar inverters will also be the output stage for any solar-based generators, most wind turbines, and any battery-based solution. An inverter lacks the easy visual of a current path through a transformer secondary or generator winding.
Instead, the inverter manufacturer (or the field technician) will decide where and how the inverter bonding and grounding is configured within the unit. Depending on the type of power panel or switchgear connected to the inverter, there could be a number of different grounding and bonding setups for an inverter. Today, we see this complication with configuring internal bonding of diesel generators and with properly configuring inverter based “floating neutral” generators. We covered these scenarios at length in the June to August 2013 issues of FRONT of HOUSE. The ultimate aim is to ensure that bonding has a single voltage reference potential, and that any errant current that can flow travels entirely through a known safety pathway.
Looking at the future of portable power generation, bonding and grounding will be relevant for portable solar arrays. Not only will the output side of the system after the inverter need to be set up properly as a source, the solar panels providing the energy need properly grounded and bonded themselves.
Future Power Sources
What, then, is a likely path forwards for portable power in the production environment? Realistically, diesel generation is going to continue to dominate in the near-term. As generators become less powerful due to emissions requirements of their prime movers, more advanced electrical generators will pick up some of the slack. The logical evolution beyond diesel generators would be to units powered by compressed natural gas, which is cleaner burning and produce less greenhouse gasses, but again with a power output penalty. As the generators produce less output, the production professional will have to become more mindful of their power consumption, especially as it relates to higher draw equipment like lighting and video walls. LED, laser diode, and switch mode technology will continue to improve, so hopefully these two trends will offset each other.
Moving more into the future, a purely electric pathway to event power seems the likely scenario to my mind. This could take the form of battery banks charged by a local solar, wind or small nuclear generation station, which are then moved onsite for the duration of an event. Here, load balancing would be required to smoothly transition new batteries onto the event site while pulling depleted batteries offline during the event. Batteries could take the form of lithium ion, compressed air, molten salt, zinc air, flywheel or any other number of potential storage means.
Another feasible outcome would be local generation of power, backed by batteries, by portable solar arrays. I have seen early versions of these systems deployed at concerts. Unfortunately the ones I have seen are largely “greenwashing” in that they do not provide sufficiently large solar arrays to power the event in real time, but rather lean very heavily on their battery banks. However, recent developments by companies like Renovagen (renovagen.com) are resulting in portable deployments sufficient to truly power small- and medium-sized sound systems and eventually other audio-visual subsystems.
Getting Better All the Time
Energy footprints for the production aspects of concerts are improving every day. Environmentally conscious events and/or providers can provide meaningful, growing improvements into their energy footprint. The energy challenges for large-scale events lie more outside the production realm. The energy footprint of on-site refrigeration, equipment transit, patron transit, beverage packaging and sales, food preparation, carnival rides, trash management, etc. far outstrip anything within the realm of the sights and sounds of the show itself. There is irony that some of the events that most who champion environmental causes also have blatantly negative environmental footprints. While easy solutions to the overall energy consumption of events are not at hand, it should not deter the industry from being mindful to seek these improvements.
