NEVER ASSUME anything about the wiring of power outlets
Aug 1, 2000 12:00 PM, Bill Whitlock
You have probably heard the saying that to assume makes an ass of you and me. When trying to track down and correct A-V system noise or interference problems, false assumptions can lead to plenty of wasted time and frustration. One assumption that is all too easy to make is that power outlets are wired correctly. Ken Fause, a colleague with extensive experience designing A-V systems for large theatrical and stadium venues, says that it is common to find 20% of outlets wired improperly in older buildings.
Partly because of the existence of low-cost outlet testers, the percentage is much lower in newer construction. Although these simple $10 outlet testers will find such dangerous problems as hot-neutral or hot-ground reversals, they cannot detect a neutral-ground reversal. This insidious wiring error, shown in Figure 1, can create nightmarish symptoms in an audio or video system.
In a correctly wired power system, all load current flows in the hot (or phase) and neutral wires. The safety ground wiring normally carries only small leakage currents from devices on the branch circuit having grounding (three-prong) plugs. This current is generally only a few milliamperes at most. Because of normal resistance in the ground wiring, small voltage differences will exist between the grounds of any two outlets on the branch circuit. Remember that these voltage differences, even though they are only a few millivolts, will be impressed across signal cables that connect equipment plugged into these outlets. For systems with two or more pieces of equipment with grounding power plugs, connected by unbalanced signal cables such as coax video or RCA audio, even this tiny voltage can sometimes cause hum problems.
Note that when an outlet is improperly wired as in Figure 1, the load current for any device plugged into it flows in the ground conductor. Depending on the device, this current could be up to 10 amperes or 15 amperes. Again, because of normal resistance in the ground wiring, voltage differences will be created between outlet ground pins, but in this case, they can be a high as a volt or two. This would virtually guarantee hum bars in a video display if it were grounded to one of these outlets and its video source were grounded to another. This is in spite of following the generally recommended practice of powering all system equipment from a single branch circuit whenever possible.
Although there are certainly procedures involving current probes on internal distribution panel/breaker box wiring and test loads on branch circuits (to be performed only by qualified personnel), which would reveal such mis-wiring, probably the simplest procedure is to inspect the suspected outlet for correct wiring visually with the power off, of course.
In the United States, standard 120 V outlets may be grounded in two ways. In the first, metallic junction boxes and conduit serve as the ground conductor. When a standard saddle-grounded outlet (mounting tabs electrically tied to the ground socket) is mounted to the metallic box, the ground connection is made. It is nearly impossible for this style of wiring to suffer from a neutral-ground reversal. The second method uses a wire, either green, green-yellow or bare, as the ground conductor. This wire is connected to a terminal (generally colored green or marked "ground") on the receptacle, which may also connect to its metallic mounting saddle. This method may use either metallic or plastic junction boxes.
Another assumption that can lead to trouble is that devices that are supposed to be grounded, even those required by regulatory codes, really are. This should include metallic water pipes and building entry points for telephone and cable television services. We rely on these connections for safety, but they are made by installers who are often, to be charitable, in a hurry and do less than professional work. I recently moved, and in the process of tracking down some telephone wire problems (that is another story), I removed some wallboard in the garage. I was delighted that power, telephone and cable TV lines entered within inches of the earth ground rod (just behind the large conduit at the right in Figure 2). This is the ideal situation for safety grounding. A closer inspection, however, revealed that the large green grounding wire from the cable television box (at the upper left corner of Figure 2 on the outside of the wall) did not connect to ground (or anything) at all. It had simply been pushed into the wall and left there. I have since corrected this and made all the wiring considerably neater, too.
Such a floating ground on a cable television feed can result in several volts of ground noise being delivered via the shield and/or splitters to TV and VCR system components. The circulating ground current often causes hum and buzz problems throughout an A-V system. It is a good idea to check for voltage between the shield of a cable TV feed and the ground of a nearby power outlet. If it is more than about a tenth of a volt, check out the ground where the CATV feeder enters the building. Remember, it is wise never to assume anything, especially when it comes to tracking down A-V system noise.
Acceptable Use Policy blog comments powered by Disqus
