In last month’s column, we covered the concepts behind common methods of AC power distribution, and a number of power specific terms that have specific definitions in the context of the National Electrical Code (NEC). That article was to lay the groundwork for this month’s discussion of portable power distribution in the language of the code. Readers are encouraged to reference that article, and generally re-familiarize themselves with the definitions within.
I repeat the disclaimer from the October article. 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. This will give a basis for safe shows and good relationships between the industry and AHJs.
No group of articles can hope to be as comprehensive as the code. In the same way that a broad general understanding of a mixing console allows one to quickly get up to speed on an unfamiliar console, we have similar goals in discussing power distribution. Even our rather weighty list of definitions covered last month is a small fraction of the overall list of definitions in NEC Article 100, so there is no way to be comprehensive in this format.
Similarly, it’s virtually impossible to discuss every possible electrical configuration that is seen in the field by the typical sound company or every piece of electrical equipment that will be encountered. We could easily draft a full article on unusual service configurations! Here, though, we will stay largely within the confines of common distribution schemes and emphasize distribution concepts over “cookbook” understanding of specific configurations. We’ll work our way from the electrical source towards the power cords of individual pieces of gear, swooping in on key points as we proceed. For non-USA readers, the concepts in this article remain applicable, though your location will have its own unique voltages and configurations.
It Starts at the Source
No amplified noise is made at the gig without a source of electrical energy, and that energy comes in the form of current flowing as a result of a difference in electrical potential, which we measure by voltage. That electrical energy could come in the form of a service from the utility, or in the form of a generator that is typically operating as a separately derived system. Whether service or separately derived system, the basic interconnections to the power source are through some electrical switchgear that is already tied to the source.
This switchgear could be in the form of temporary breakers added to an existing electrical panel, a dedicated “company switch,” tails under lugs, any number of various NEMA plugs, Camloks, Ceeform, etc. In most circumstances, this connection to the source must be completed and energized by a licensed electrician. Clarification of where and who should be making these interconnections is a great introduction to a productive conversation with your local AHJ. This is a prime point of safety and liability for FRONT of HOUSE readers to address with their production clients and insurers.
Connections to the source (e.g., a generator) are either single-phase or polyphase. By way of review, from last month, if an electrical system utilizes a single voltage supply from the generating system, it is termed a single-phase service. If an electrical system uses multiple voltage sources, each with a fixed time relationship, it is termed a polyphase service. The essentially universal form of polyphase power distribution is three-phase distribution. Both single- and three-phase distribution schemes can produce multiple voltages, depending on how they are wired. Before discussing the specifics of these voltages, we briefly talk about measuring voltages on the electrical distribution system.
Measuring Matters
It is the audio professional’s duty to make measurements of the voltages of the various electrical connections within the portable power distribution system. Before the downstream gear is energized, all of the points along the distribution system should be metered. Multimeters are affordable and ubiquitous; all technicians should have one and be practiced in its use.
Further, some electrical faults in power distribution systems are only realized once substantial current is flowing. Loose connections, in particular, tend to measure fine until placed under a meaningful load. A standard multimeter has very high input impedance, so very little current flows in the circuit during the process of measuring voltage. To test the distribution system under meaningful current flow, a robust load such as a PAR can lamp can first be energized by the distribution system, and then measurements performed with a high impedance meter. Alternatively, a solenoid voltage meter (also called a “wiggy”) can be used to measure circuits. The current draw of a wiggy is much more substantial than that of a digital multimeter, better representing a real load. As a solenoid voltmeter is not as precise as a multimeter, it is usually best to measure the circuits with both types of meters.
Single-Phase Source Voltages
We first discuss the voltage behavior of the typical single-phase power source. This configuration is most common on smaller services or separately derived systems (i.e., generators). Current flow requires a completed circuit, so there must be a conductive path back to the source on which the voltage originated. In a single-phase system, this return path typically takes the form of an un-energized conductor (i.e., no applied voltage) that conducts current only when a load (such as a power amplifier) is placed between the voltage source and the other conductor. In common parlance, this conductor is termed the neutral. Please turn to last month’s article (FOH, Oct. 2013, page 44) for the official definition of the neutral conductor.
The most common configuration for single phase power produces two voltages, typically 120V root mean square (RMS) and 240V RMS. How does a single source produce two different voltages? Good question! Most single-phase supplies utilize what is known as a “center tapped” transformer (see Fig. 1.) This transformer’s secondary has three conductors tied to it. The “center” conductor is a grounded conductor, tied to the middle of the secondary winding, and functions as the circuit neutral. The other two conductors are not grounded, and are tied to either ends of the secondary winding. The voltage between the two “end” conductors will be 240V RMS, and the voltage between an end conductor and the central conductor will be 120V RMS.
So, from a single source voltage tied to the transformer primary, we can produce two secondary voltages. In all cases, the closed circuit current loop is formed by the transformer’s secondary. Center-tapped single-phase services are the dominant services for residences in the USA. Appliances that utilize 120V RMS are connected between an end conductor and the central conductor, and those using 240V RMS are tied between the two end conductors. Loads are configured to operate at both voltages simultaneously. Loads that operate at 120V RMS are distributed across both “sides” of the service. This makes best use of the source’s capacity, and ensures the minimum imbalance current for the neutral to return.
Three-Phase Source Voltages
For three phase sources, loads may either be connected between two phases, or between a phase and an additional un-energized conductor; there are also pieces of equipment designed to be connected to all legs of a three-phase service. Gear designed for exclusively for three-phase power are not the norm in professional audio, but are frequently seen in production lighting and stagecraft. The two most common ways of wiring a three-phase circuit are known as “delta” and “wye” configurations (Fig. 2.) The wye configuration is also sometimes deemed the “star” configuration.
In a delta configuration, the load is connected between two of the three phases, or to all three phases. The voltage between each of the three phase pair combinations is the same. Two common phase to phase voltage values for three phase delta in the USA are 480V RMS and 208V RMS.
The wye configuration, by contrast, adds an additional un-energized conductor to the distribution system. In pro audio applications this additional conductor will be a grounded conductor and function as the service neutral. The voltage between a phase and this conductor equals:
Or 0.577 times the delta-configured phase to phase voltage. Rounding to the nearest volt, this makes the two most common wye voltages in the USA 277V RMS and 120V RMS. The latter will be familiar as the standard 120V RMS from single-phase distribution.
A three-phase service may simultaneously have loads wired up in either wye or delta configurations. For pro audio, the amplifiers might be tasked to operate at 208V, and distributed across the three phases in delta configuration. By contrast, a vintage compressor at FOH might only operate at 120V RMS, and thus be configured to operate between a phase and the neutral in wye. As a further example, a moving light at the same show is delta connected across all three phases, and the control electronics inside the moving head fixture could be wye-connected between one phase and the neutral.
Additionally, there may be a center-tapped transformer between two legs of a three-phase delta service to provide another voltage. These are services are sometimes deemed “wild” or “high” leg, and are outside the scope of this article. We now move on to consider the EGC conductor in the portable distribution system.
The EGC
So far we have considered the behavior of the energized conductors of a portable distribution system, and how they behave with respect to each other, and to an additional grounded conductor commonly termed the neutral. In the portable power context, there will be another conductor, namely the EGC (Equipment Grounding Conductor).
The EGC and neutral are both bonded to the GEC (Grounding Electrode Conductor), which then ties to the grounding electrode. The EGC thus serves the additional purpose of bonding all the electrical equipment associated with the portable distribution system to the grounding electrode, which in turn serves to hold the bonded power distribution hardware at the same electrical potential as the grounding electrode.
Normally the EGC is not an active player in the operation of equipment. It stands alongside the complete circuit path formed by the various combinations of energized legs and the neutral. The EGC is a silent watchman, there to provide a current path back to the bonding point in the event of an electrical problem with operating gear. The EGC is sized to bear the fault current that could possibly occur in the event of a fault. If a fault occurs, the EGC acts as the current return back to the bonding location with the neutral, essentially acting as surrogate conductor for the neutral. Because this fault current has not traveled through the resistance of the load, the low impedance path of the EGC causes the current to spike which generally trips the overcurrent device.
Conclusion
The four or five wires that compose our common portable distribution systems each serve a purpose in transferring energy, or improving distribution safety. When the pro audio professional is familiar with the operation of each conductor, and the conductor’s function, they have the important steps for confirming the setup and safety of the electrical system. Knowing the nomenclature will further prepare them for interacting with the AHJ.
