Navigating Building Power Issues
Whether new or old, that building you're about to start work in could be hiding a problem that will affect your AV installation almost as soon as you commission the system, if not sooner: poor power quality. It's not an issue that screams, ?Hey, look at me,? therefore taking a proactive approach to assessing and managing building power is important to a successful AV project.
Because of the sensitive nature of today's high-tech AV equipment, installers need to work with electricians and building managers to ensure reasonably good power quality. Without good, clean power, AV equipment may malfunction, in the form of system freezes or lockups that require restarting, or may simply fail prematurely, requiring costly follow-up or even replacement.
It's a straightforward proposition to specify power conditioners and surge protectors for every job, but often it's equally important to understand a building's power characteristics from the very beginning, to help guide decisions on remedies or equipment recommendations.THE MAIN ISSUES
For AV installers, power quality issues normally fall into one or more of these categories: circuit overloading and inadequate isolation of sensitive circuits; grounding problems and ground loops; and building noise, harmonics, and electromagnetic interference. In addition, pre-existing electrical code violations are not uncommon and can easily degrade power quality. Examples include unbalanced loads or phases, reverse polarity, and poor mechanical connections.
At the beginning or any significant AV project, a total facility power quality audit or survey can document code violations and other deficiencies. Using handheld power quality meters and analyzers, a technician can begin to document irregularities. These tests are best handled by specialized power quality technicians during the course of a building survey, but call the local electric utility first. It may have already completed a recent survey.
Sensitive AV equipment should always be installed on dedicated branch circuits. The loads on these circuits should be maintained at a level far below the maximum permitted by code. In other words, no more than a 10-amp load should travel on a 15-amp branch circuit. To minimize voltage drops, it's best to go with oversize wiring, the next size up from the normal National Electrical Code (NEC) specification, and work with electricians to run a separate branch circuit for each 10 amps of load. As a rule of thumb, the optimal practice is to design the electrical such that there is no more than a 1 percent voltage drop at full load on branch circuits that feed sensitive equipment.
Under no circumstances should incompatible loads such as faxes, copy machines, and laser printers be on the same circuit as AV equipment. These machines require periodic high current, which can cause an increase in the neutral-to-ground voltage thus damaging sensitive equipment on the same circuit. Connecting projectors on light circuits, for instance, can cause RF feedback to the projector, and an unstable, vibrating image.
See the table “Wiring for Power Quality: Old Dogs, New Tricks” on page 44 for a look at evolving best practices in wiring to achieve a high level of power quality. Many of the “old” practices are still reflected in building codes today. Be on the lookout and avoid them where possible.GROUND RULES
Grounding refers to the connection between building circuits and the earth. Defects in building grounds and related bonding (i.e., the interconnection between points—usually grounds—to reduce voltage differences) are commonplace and potentially disastrous from the standpoint of AV equipment operation. In some cases, defective grounding results in outright loss not covered by manufacturers' warranties.
Stray voltages and currents on ground wires can degrade signals and fry semiconductors. In recognition of this, high-tech equipment manufacturers are of the opinion that so-called resistance-to-ground should be in the range of 3 to 5 ohms, not the NEC-suggested maximum 25 ohms. To meet such stringent standards, however, almost surely requires the services of a power quality technician running sophisticated field tests, particularly in older buildings.
High-quality grounds are so important to power quality that newer buildings are now often built with isolated ground outlets. These outlets, identified by their orange color and a triangle marked on the face, feature an independent equipment ground in which the ground wire homeruns to the circuit breaker panel. This dedicated ground approach helps to keep electrical noise generated elsewhere in the building away from the sensitive devices plugged into the isolated ground receptacle.
AV manufacturers can usually offer grounding instructions that are specific to their equipment and cabling. Moreover, article 645, “Information Technology Equipment,” of the NEC provides guidance on the minimum wiring and grounding of the types of facilities where AV equipment is common. (To read the NEC, visit the Web site of the National Fire Protection Association, which publishes the code, at www.nfpa.org.)
Where an AV pro suspects inadequate building grounds—maybe in an older structure that might have 15-year-old ground electrodes, or where the building owner reports problems with electronics, such as computers, that rely on a good ground for a zero-voltage reference—the best advice is to call in a technician, who can provide retrofit and repair guidance.
After assessing power quality at a jobsite, there are often issues you can't correct—just manage. Everyday building noise, line noise, harmonics, and electromagnetic interference represent three-quarters or more of real-life problems in the field. Noise and interference typically manifest as “static” or “snow” and affect audio systems in particular. Most of these problems are found within the building, on the customer side of the meter. The range of equipment that can generate power quality problems is extensive.
Because it's nearly impossible to find an electrically pure site, the best defense is a good power or line conditioner. A line conditioner will typically offer voltage regulation and filtering for noise and harmonics. In audio and video applications, line conditioners are predominantly electronic tap-switching autotransformers. Such a device constantly measures the input voltage and then electronically sets an output tap on the autotransformer to maintain a constant output voltage. Devices on the market vary in reaction time, and thus quality. You can find a variety of pro-level conditioners from companies such as APC, Belkin, Eaton, Furman Sound/Panamax, Juice Goose, SurgeX, and Tripp Lite.
The better line conditioners electrically isolate individual components (plug loads) from each other by using isolation transformers. This prevents noise generated by one component from traveling back to other components that are plugged into the same line conditioner. These isolated outputs are sometimes referred to as “digital” quality.
A new wrinkle in power conditioning is a product made by PS Audio called Power Plant Premier. Starting at around $2,200, this sophisticated line conditioner creates a fully synthesized AC waveform. Incoming 120-volt power is converted to DC and then passed through an oscillator to generate a nearly pure 60 Hz sine wave, which is then amplified to 120 volts. The maximum output power of this product is a respectable 1,500 watts and it has excellent noise reduction specifications (>80dB). (For more power conditioners and surge suppressors, see “Spec to Spec,” January 2008, page 64, and online at www.proavmagazine.com.)OTHER COMMON SENSE PRECAUTIONS
Even with central protection, current best practices call for all individual devices to have transient voltage surge suppressors (TVSS), due to fact that internal building loads, such as motors, transformers, and photocopiers generate their own surges that then propagate on branch circuits. TVSS devices provide protection against lower energy spikes that occur very abruptly. Note, however, that a TVSS should be connected to a good grounding system to work. Otherwise, it could make matters worse.
Uninterruptible power supplies (UPS) make sense where momentary ride-through capability is absolutely indispensable. If not, you're better off spending your dollars on more sophisticated and capable power conditioning equipment. The noise reduction ability of a UPS, although built-in, is often not as good as that of stand-alone power conditioning products. That said, if you choose to use a UPS system and a power conditioner, the two have to be carefully matched because UPS operation itself is a known cause of power quality problems, including distortion and harmonics. For instance, a local TVSS should never be connected in series with a local UPS (typically a battery system). UPS systems do not output a pure sine wave, thus the TVSS will operate frequently.
At the end of the day, the best practice for all power quality work is to consult manufacturers' guidance both on the AV equipment itself and any proposed power quality remedies. But doing the leg-work and laying the proper groundwork will pay off in the end with satisfied customers.
Kevin Gainer managed the journal Energy from 1995 through 2007; previously he worked at American Electric Power. He's based in Westerville, Ohio.