Think Twice About Surge Protection & Power Conditioning
Taking the correct steps dealers can solve issues such as ground loops without having to resort to shortcuts that include adapters that lift proper grounding.
July 27, 2015
You’ve probably seen ad copy something like this: “Today’s residential systems contractors face unprecedented challenges where high resolution, trouble-free operation is required. From inducing AC ground loops, video hum bars, static bursts, damage from AC line surges and variable audio and video performance, comprehensive control and conditioning of AC power is no longer an option.”
Although many of these claims can’t be supported in theory or real-world measurement, the assertions are made by so many, so often that many system designers and installers accept them as truth. We are also told that AC power is “dirtier” than ever, with no supporting evidence. Paranoia usually helps when selling protection.
Summarizing the Power Protection/Power Conditioning Category
“Power management” products generally perform functions such as switching or power-up/power-down sequencing of multiple outlets. Some monitor voltage and/or current and can report their data or be controlled remotely.
“Power conditioning” products are intended to protect system equipment from the potentially damaging effects of:
- “Swells” or “surges” (long-term high line voltages)
- “Transients” or “spikes” (short-term high line voltages)
- “Sags” or “brownouts” (long-term low line voltages)
- “Inrush current” (occurs at power up in some equipment)
Implementing these layers of protection is reasonably straightforward engineering. But most “conditioners” also include filters and/or transformers intended to eliminate “noise” that contaminates audio, video, and data signals. Noise issues are widely misunderstood in the A/V industry but have been the focus of this author’s work for the past 25 years. I hope this knowledge and experience will shed some light on this aspect of power conditioning.
Safety Trumps Everything
Since power management and conditioning involve AC power, electrical safety is extremely important. Ignorance of safe practices and regulations, or their cavalier dismissal, causes a substantial number of electrocutions—-deaths by electricity—-in the U.S. every year. Those responsible for defeating safety features are usually sued and often face multi-million dollar judgements.
While some shocks are just annoying, others leave victims paralyzed or dead. Protect yourself by installing equipment that’s certified safe. In the U.S., the certifying body is Underwriters Laboratories (UL) and the equipment is labeled “UL listed.” Weasel-worded phrases like “contains UL recognized components” are not acceptable. Similar bodies in other countries include CSA (Canada), VDE (Europe) and others.
Using non-certified equipment puts both you and your customer at high risk. The most important approval requirement is protection from an internal insulation failure in the equipment. Such a failure can energize all touchable metal parts of the equipment at 120 VAC (volts AC) and result with an electrocution hazard.
Manufacturers get approval two different ways. The first, sometimes called “double insulation,” allows for the use of a two-prong (ungrounded) AC power cord. This approach however, requires passing rigorous tests under extreme conditions of electrical and mechanical abuse, which generally makes the product more expensive to manufacture.
If designing a product to pass the tests is too expensive, agency approval requires a three-prong (grounded) AC power connection. Most importantly, such products are safe only when the AC plug is properly connected to a properly wired three-prong outlet. Since touchable parts of the equipment are grounded, internal failures cause a high “fault current” (in the 150 to 1,000 Amp range) to flow back to the NG (neutral-to-ground) bond at the main electrical panel, which causes rapid tripping of a circuit breaker——and no one getting hurt.
Articles 250.6(D) and 250.114 of National Electrical Code (2008) specifically prohibit interruption of safety grounding for plug-and-cord connected equipment to solve “noise” issues (referred to as “objectionable current” in the NEC language). This brings us to the irresponsible, illegal, and dangerous practice of defeating safety grounding far too commonly done to “quick fix” a system hum or buzz problem. The use of three-to-two prong “adapters” to do this is so widespread that some people believe they’re intended for the purpose.
Even worse, others hide their crime by cutting the grounding prong off a power cord so that it is invisible when plugged into an outlet. If someone is shocked or electrocuted, as a dealer you are legally liable for this action. Dealers can minimize their liability by complying with code and using only agency-approved equipment in installed systems. Legally this demonstrates due diligence.
How Does Noise Actually Contaminate Signals?
Most equipment manufacturers believe, and want you to believe, that simply connecting all the system gear together will result in a clean and quiet system—the “plug-and-play” myth. Therefore, if a system has a noise problem when first powered up, it’s easy to assume the installer “did something wrong.”
Logically, it becomes the dealer’s responsibility to fix it, and of course, they’ll be eager to solve the problem in a quick and simple way. So called “ground loops” [see Figure] are responsible for virtually all audio hum and buzz problems and interference in video and digital signals as well. In spite of rampant myths and legends, ground loops are well understood by some industry professionals.
An unintended side effect of the AC wiring in buildings is the generation of small voltage differences, generally under 100 mV, between the safety ground contacts of the AC outlets. It’s not widely understood that this voltage is created by magnetic induction effects in the building wiring (see Whitlock-Fox paper).
In general, this voltage is proportional to the distance between the outlets and the current flowing in the branch circuit(s). When a signal cable connects two pieces of equipment, this inter-outlet ground voltage difference is applied between the ground connections at the ends of the cable. The inherent nature of unbalanced interfaces means this voltage (the hum or buzz) is directly added to the signal. Solutions to this problem are various ways to open or break the loop at some point.
There are two ways do this:
- Disable the third-prong ground connection. This is exactly why illegal and dangerous “cheaters” are so attractive, they solve the problem. But, as explained earlier, they create an illegal and deadly shock hazard.
- Install a device that eliminates the electrical path through the signal cable but still transfers the signal from one end to the other. This is exactly what a transformer does and there are products called “isolators"made for the purpose. It’s the much preferred solution.
The notion that “dirty power” causes all system noise problems has great intuitive appeal but, in fact, nearly all noise issues can be traced to the premises safety ground wiring. The only exceptions are individual pieces of equipment I call “powerline prima donnas.” They have an internal design error (similar to the “pin 1 problem” you may have heard of) that causes them to be sensitive to powerline noise.
Will an Isolation Transformer Eliminate Noise?
Many power conditioners are based on transformers and claim to reduce or eliminate hum and buzz due to “ground loops.” Every transformer transfers power (or signal) via a magnetic field with no electrical path between input and output. It would seem that an isolation transformer would be a perfect way to break a ground loop. Unfortunately, it creates a deadly shock hazard, too.
National Electrical Code, UL standard 1950, and IEC standard 950 all specifically prohibit separating the input to output safety ground connection of plug-and-cord connected isolation transformers. Since the safety ground can’t legally be interrupted, ground loops and the noise they often couple, will remain when using any legal, UL-listed isolation transformer connected with a plug and cord.
National Electrical Code does allow the establishment of a new, local ground reference point—-like the NG bond at a building’s the main breaker panel—-that’s bonded to the local environment. Code refers to these as “separately derived power systems” and for reasons of safety and complex rules they must be permanently installed by a licensed electrician (see the Middle Atlantic white paper).
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