Power and Grounding

September 7, 2013 in AV Design Tips, The Basics by Sam Davisson

Isolation, the key to a good grounding structure

This is the first of an upcoming series on “The Basics” of designing the AV systems that high end clients expect and the average client will respect.

Quality AV systems don’t happen by accident. They happen by design. Just like building a house, you’ll never reach that true quality threshold without a proper and strong foundation. With all electronic systems, especially AV, that foundation is the grounding structure.

Most designers and installers of AV systems think of grounding as black magic. How often have you or someone you know said that a cable is “picking up” noise or that the solution is “better” shielding or floating the shielding? (Note: In very high end audio systems floating the shield can be essential but in the typical AV installation, if grounding is done properly, should not be required.) Equipment manufacturers are typically of no help as they don’t have a clue as to what’s going on because they design and test their equipment in a defined, never changing environment.

The basic rules of physics are overlooked, ignored, and / or forgotten. Electrical engineering courses rarely even mention practical issues of grounding. As a result, myth and misinformation have become the normal

The preferred grounding technique for AV equipment, which contributes to this sense of mystery, is isolated grounding. The reason for this is noise interference and particularly what is known as common mode noise. In AC power systems, the difference in potential between neutral and ground is one form of common mode noise, since any change in neutral potential relative to ground also affects all of the other power circuit conductor potentials to ground. A more troublesome form of common mode noise is the differences in ground potentials throughout an electrical system. When AV devices are interconnected byway of audio, video or control cables, any difference in ground potentials between the interconnected pieces of equipment is common mode noise to the audio, video or control circuits.

Properly Conceived Isolated Grounding System

Properly Conceived Isolated Grounding System Through a 3 Phase Transformer

There is much confusion over what an “isolated ground” (IG) is, how an isolated ground technique is implemented, and why it is used. Isolated ground is allowed in the U.S. by the National Electrical Code (NEC) and in Canada by the Canadian Electrical Code (CEC). But it should be noted that in both cases, isolated ground is an exception to the standard grounding requirements. NEC 250-74 and 250-75 allow IG wiring only where required for the reduction of electrical noise on the grounding circuit. In practical terms, what this means is that not all electrical contractors or electricians are versed in building isolated ground systems.

Common Myths About Earth Ground and Wires

Unfortunately, as electronic equipment developed the term “ground” became sort of a generic term. In AC power systems ground refers to a common connection point, typically earth ground. In a DC power system, such as what is powering my laptop, ground also refers to a common connection point for power return. Thus, the very meaning of the term ground has become vague, ambiguous, and often quite fanciful but make no mistake these grounds should never have thier common connection point in common.

Some electricians have a strong urge to reduce unwanted AC voltage differences by “shorting them out” with massive conductors, the results are most often disappointing. Other electricians think that system noise can be
improved experimentally by simply finding a “better” or “quieter” earth ground. Many indulge in wishful thinking that noise currents can somehow be skillfully directed to an earth ground, where they will disappear forever!

Here are some common myths about grounding:

  • Earth grounds are zero volts – presumably with respect to some “mystical absolute” reference point. This leads to fanciful ideas about lots of ground rods making system noise disappear. The fact is soil resistance between ground rods is much higher, often significantly than a wire between them.
  • Wires have zero impedance (is the apparent ac resistance of a circuit containing capacitance and/or inductance in addition to pure resistance) – therefore they can extend a zero-voltage reference to many locations in a system, eliminating voltage differences. In fact, wires are quite limited:
    • The DC resistance of a wire applies only at very low frequencies and is directly proportional to its length.
    • The inductance of a wire is nearly independent of its diameter (gauge) but is directly proportional to its length and increases at bends or loops.
    • A wire resonates (becomes an antenna) when its physical length is a quarter wavelength

Something to consider, are earth grounds even necessary for low noise system operation?

Equipment Rack Treatment for Isolated Ground SystemUpdate 9/11
To answer the above question: no earth grounds are required for safety, not for low noise system operation!

But what I really wanted to update here today, something I left out of my post, was the equipment rack treatment an AV integrator needs to do in order to maintain a nice clean isolated ground system.

Unless your are installing the equipment rack on a wood floor, the rack must be isolated from the floor. There are a number of ways to successfully accomplish this. While not called out, the drawing above used wooden 2×4’s underneath the rack but many equipment rack manufactures also sell ground isolation kits made from polycarbonate strips that work perfectly fine.

If your bolting the rack down you’ll need to use a nylon washer to keep the bolt itself from making the electrical path. If your simply setting the rack on the floor with no requirement to bolt it down be sure to include leveling feet with rubber covers to provide your isolation. Finally, if the rack is portable, the rubber wheels should do a good job of providing the required isolation.

Install a copper bus bar in each rack. This bus bar is connected to the isolated ground system (at the isolated ground sub panel) and then connected to the equipment rack via a 12 gauge stranded wire. The bus bar must be isolated from the equipment rack at all other points. Additionally most AV equipment provides a grounding lug on the back of the equipment. Again use a 12 gauge stranded wire to connect equipment chassis to the copper bus bar. Note: some older equipment may strap signal ground to chassis ground. If this is the case be sure and remove the strapping mechanism.

All AV conduits coming into the rack must be isolated from the rack. Most electrical supply outlets provide isolation bushings for this. In addition your equipment rack vendor may be able to provide isolation knockout panels. Finally, be sure and use an approved IG power strip.

Following these guidelines will ensure that your IG system remains so.

As I discuss future AV basic design topics grounds and dealing with grounding issues will be a constant theme. As I said, we are simply setting the foundation at this point, not only of your AV system but for future posts as well.

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