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Protection from EMF and Electro-Stress - Environmental Energy Products for the 21st Century from The Energy Store |
All about EMFs (cont’d) |
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One of the more common transmission line configurations is called a "vertical double-circuit," where a set of three cables is attached, one on top of each other, to each side of the transmission tower. The three cables in each set comprise the "three phases" of the power network, with each cable carrying current. The current peaks in each cable are intentionally out of phase with each other (i.e., they don't peak at the same time) by 1/3 of a cycle. Electric utilities use the letters A-B-C to denote a three phase circuit, with each letter representing one cable and its phase. EMF can be reduced by 50 percent or more with very little expense by reversing the phase order in one circuit with respect to the other (i.e., C-B-A). This configuration causes both the electric and magnetic fields to partially cancel each other. In early 1989, the Bonneville Power Administration adopted this scheme for implementation on both old and new transmission lines. This configuration is not used by most utilities, however, because it creates interference with nearby TVs and radios, and it causes snapping and buzzing noises.
A single-circuit transmission line still has three cables, one for each phase. Typically the three cables are strung in a flat configuration, with all three cables in the same plane. Significant cancelling can be achieved by merely changing from a flat configuration to a "delta" configuration, with the three cables forming a triangle. Moving the cables closer together also helps to cancel the fields, but it reduces safety for the maintenance workers and degrades the line's performance during lightning.
Sometimes burying electric power lines can reduce EMF, but this is not necessarily the case, as magnetic fields travel through dirt, rocks and cement. Unless the underground lines are configured to reduce EMF, simply hiding the lines out of sight may create a false sense of security. If the underground service is just a single phase wire, radiation levels on the ground directly over the wire will be higher than from overhead lines because you will be closer to the source. On the other hand, some underground lines have several circuits which can be balanced to cancel the magnetic field.
In a 1991 study conducted by the Electrical Systems Division of the Electric Power Research Institute, researchers found that magnetic fields produced by underground cables vary by as much as 10 to 1, depending on the method of installation and cable construction. According to the study, a person standing directly over an underground cable with the worst configuration (from an EMF perspective) will be exposed to the same level of EMF as a person standing at the edge of the right-of-way for an overhead transmission line. Unfortunately, the study also found that the best configurations for the lowest EMF are less efficient for electric power transmission.
With concern about EMF in mind, new and different underground cable systems are being developed. The lowest field underground design has three insulated cables lying adjacent to each other in an oil-filled pipe that cools the cables. This configuration can result in magnetic fields 1/10 to 1/20 of the equivalent overhead line. The EMF can be reduced even further, sometimes to near ambient background levels, if the pipe is grounded in a special way.
SUBSTATIONS A substation is an assemblage of circuit breakers, disconnecting switches, and transformers designed to change and regulate the voltage of electricity. Primary distribution lines, carrying high voltages typically of 115,000 volts to 230,000 volts, bring the current from the power plant to the substation, where the transformers reduce it to lower voltages, typically 4,000 to 13,800 volts. The transformers give off magnetic fields because they depend upon magnetic fields to operate. (See discussion of transformers under "The Nature of Electromagnetic Radiation.") Further compounding the problem, the incoming and outgoing currents at a substation are generally unbalanced. High magnetic fields from substations have been blamed for causing cancer clusters among nearby residents.
Paul Brodeur wrote about several such cancer clusters in the July 9, 1990, issue of the New Yorker. Citing evidence that a cancer cluster had occurred among the residents of Meadow Street in Guilford, Connecticut, Brodeur pointed out that during a twenty year period, seven tumors - four brain tumors, an eye tumor, an ovarian tumor, and a bone tumor - were recorded among the residents. This was particularly extraordinary since the street has only nine houses. The cancer victims lived in five of six adjacent houses located near an electric-power substation and next to a pair of 115,000 volt high-current distribution lines, called feeders, which carry current to the substation. Measurements of magnetic fields taken at that time near the peripheral fence around the Meadow Street substation showed magnetic fields ranging from 20 mG to several hundred mG.
NEIGHBORHOOD TRANSFORMERS A key component of a utility's electrical distribution network depends upon numerous, small transformers mounted on power poles. A transformer looks like a small metal trash can, usually cylindrical. Even when the electrical service is underground, you will often see a metal box (usually square) located on the ground near the street. Many people don't realize that when they see a transformer, the power line feeding the transformer is 4,000 to 13,800 volts. The transformer then reduces the voltage to the 120/240 volts needed by nearby homes. Since these transformers can be seen in almost every neighborhood, they are a source of popular concern.
The ELF magnetic field near a transformer can be high, but due to its small structure, the field strength diminishes rapidly with distance, as it does from a point source. In fact, measurements at street level directly underneath a power pole transformer are no greater than underneath the power lines themselves. Ground level transformers may have readings as high as 200 mG right next to the box, and 50 mG at 4 inches away. Fortunately the fields drop off quite rapidly, with a 3 mG reading at 2 feet, and near ambient levels 10 feet away. For this reason, having a transformer located near your home is not usually a major source of concern, although just to make sure, you should measure the field strength around it.
WIRING INSIDE THE HOME AC magnetic fields can be found inside everyone's home. These fields can come from power lines outside the home, wiring inside the home, and appliances. Some experts feel a background level of less than 1 mG is desirable, but many homes have readings much higher than this level. If your home has high EMF readings, it is important to determine the sources of the magnetic field so that remedial action can be taken, if possible. Often the source of a high AC magnetic field is incorrect wiring, so it is important to understand how you can correct this problem.
The most important consideration in wiring a house is that the ground and neutral wires be kept separate and run directly back to the panel box (either a fuse box or a circuit breaker box), where they are grounded. Under no circumstances should the neutral or ground wires be grounded to the plumbing or any other ground except at the panel box.
Electric current needs to flow through a closed loop in order to work. This closed loop is referred to as a circuit. To understand how the current is supposed to flow in a correctly wired circuit, let's examine a circuit used to power a refrigerator. From the panel box electricity flows through the hot wire to the refrigerator, where it turns the motor. The electricity then flows back through the neutral wire to the panel box. With the loop closed in this way, the field is canceled out because the hot and neutral wires are close together. A ground wire runs from the panel box to the refrigerator, but if everything is wired correctly then the ground carries no current. The ground is for safety reasons, so that you will not get electrocuted in case the insulation on the hot wire becomes worn and the hot wire comes into contact with the frame of the refrigerator. The frame of the refrigerator is connected to the ground, so that any stray current from a worn or loose wire will flow back through the ground instead of through your body. If the neutral has been grounded to your plumbing instead of running back to the panel box, your house is wired incorrectly, and this may result in a significant magnetic field. Suppose this is the case. Tracing the flow of the electric current from the panel box to the refrigerator, after the electric current powers the refrigerator it will run to the neutral and, if wired incorrectly, through the plumbing where it is grounded. Since it is no longer paired with the hot wire, the magnetic field will not cancel out. Instead, there will be a magnetic field around the hot wire that is connected to the refrigerator, and another field may surround all your plumbing. Just one incorrectly grounded appliance can send electricity through all your water pipes, and create a magnetic field throughout your entire house! Changing the plumbing from metal to plastic is not a proper solution, because electric current is not supposed to flow through the plumbing. The only solution is to rewire correctly, with all hot and neutral wires paired closely together, and without any current flowing through the ground wire or through your plumbing. Ground currents from underground non-electric utility lines have also been implicated in as a major source of EMF in the home. Present regulations in the United States require that utility lines such as gas, cable TV, telephone, and water be connected at each residence to the same ground as used for electric current. This practice "provides an alternate path for the [neutral return] current to flow from your house back to the distribution system," says Gary Johnson, an executive at a General Electric facility doing EMF research for the Electric Power Research Institute. As a result, an imbalance is created which reduces the cancelling effect of the neutral's field on the hot conductor. This little-known fact can be an eye opener for explaining mysterious EMF in some homes. According to Johnson, you could create fields in your neighbor's house when you switch your appliances on and off, and your neighbor could create them in your house, too. This phenomenon can also account for fields outside of the home and in overhead distribution lines.
Still another source of EMF comes from the power line where it enters your home. The area of your home near this feeder line will have a reading even if the rest of the house is properly wired. If your supply line enters your home with an overhead wire, as opposed to underground, you may want to avoid using a corner of your home, or part of a room, for any prolonged period of time.
To test your home for magnetic fields, simply walk through your home with an ELF Gauss meter. If the reading is generally below 1.0 mG except near appliances, your home is wired correctly. If you find extensive zones of higher readings, you need to first determine if the EMF is coming from your own wiring or from a source outside your home. To start, walk outside and see what the readings are around your home. Then turn off your electricity at your panel box and check inside your home. The results will tell you if you need to go further and check your wiring.
If you suspect that your home is wired improperly, obtain the services of a licensed electrician. Ask the electrician to disconnect all circuits at the panel box and test one circuit at a time. If your home has circuit breakers, you can just turn off all the circuit breakers and turn on one at a time. Then take a reading throughout the house with the Gauss meter.
As an alternative, your electrician can test for the presence of unwanted ground currents with a clamp-on ammeter attached to your plumbing (it should read zero), but a Gauss meter is still recommended as it is generally more sensitive and doesn't require open access to the plumbing. This way, you'll be able to determine which circuits or appliances are causing the problem. Hopefully only a single circuit will be responsible for most of the trouble, but sometimes the house is in need of complete rewiring.
Automatic ice makers in refrigerators and in-sink disposal units are often the source of unwanted EMF since these devices are usually connected through copper piping to your plumbing. It is important that these devices be wired so that no current flows through the ground.
COMPUTER DISPLAYS A video display terminal (VDT) is used to display information from a computer, either in the form of text or graphics. A VDT can be one of several different types: cathode ray tube (CRT), liquid crystal display (LCD), gas plasma display, and electroluminescent display. By far the greatest percentage of video displays are of the CRT type, and for this reason the term "VDT" is generally used to mean the CRT-style VDT.
CRT-STYLE VDTs A CRT-style VDT uses the same type of picture tube as a television set. The cathode ray tube is a large vacuum tube made of glass, and coated with phosphor on the inside. An electron gun shoots a beam of electrons from the back of the tube toward the front of the screen (i.e., toward the computer operator) until it hits the phosphor. The phosphor gives off visible light when it is excited by the electrons. A full screen image is comprised of thousands of dots, each one of which is refreshed (re-excited by a burst of electrons) between 50 and 80 times per second.
CRT-style VDTs give off all sorts of electromagnetic radiation: radio waves, infrared radiation (heat), visible light, ultraviolet light, microwaves, X-rays, ELF and VLF radiation. The radio waves are typically shielded with a layer of conductive material in order to meet the limits set by the Federal Communications Commission. The infrared radiation in the form of heat is not a health hazard, and of course the visible light is necessary in order to see the screen. The levels of ultraviolet light are substantially less than indoor fluorescent lights or outdoor sunlight, and the amount of microwaves is so small that it is almost undetectable. X-rays were once a problem, but strict guidelines in effect since 1970 have reduced the level of X-rays to less than what is naturally present in the environment. Most experts now agree that X-rays from CRT-style VDTs pose no problem unless the display is defective.
It is the ELF and VLF electromagnetic radiation from CRT-style VDTs which is presently raising concern. The ELF radiation (50 Hz to 80 Hz) comes from the vertical deflection coils, and the VLF radiation (15 kHz to 85 kHz) results from the horizontal deflection coils. CRT-style VDTs also have a power transformer which creates a 60 Hz field, and a flyback transformer which steps up the CRT's voltage to tens of thousands of volts and emits VLF electromagnetic radiation.
The levels of EMF emitted by a VDT can be quite high, but the measurements drop off rapidly with distance. That's why it is important to sit back at least an arm's length from the front of the screen. Measurements taken from a typical color VDT (a popular 13 inch color display was used for this test) show 37 mG of ELF at 6 inches, 12.6 mG at 12 inches and 4.5 mG at 20 inches. The VLF field (which contains several hundred times more energy than an ELF field at the same mG reading) is 6.3 mG at 6 inches, 2.0 mG at 12 inches, and .66 mG at 20 inches. At 6 to 7 feet the ELF level drops to background, but the VLF level is still measurable 10 feet away.
Because the EMF comes from the internal components, the EMF levels on the back and sides of a VDT are higher than in front, often by a factor of 2. This means you must distance yourself further away from the back and sides of a VDT (at least 3 to 4 feet, respectively) in order to achieve the same level of exposure. Smaller VDTs are not necessarily better, either. A 15 inch VDT might well generate a stronger magnetic field than a 21 inch one, because the field's strength depends more on the internal design of the deflection coils and electronic components than on the screen size.
The electric components of a VDT consist of an electrostatic potential and alternating electric fields at ELF, VLF and radio frequencies. The electrostatic potential results from a build-up of an electric charge on the surface of the screen. Its effect is similar to what most of us have experienced when we get a static shock by walking across a carpet and touching a metal object in a dry environment. This static may attract dust on your screen and cause eye irritation. On some occasions, skin irritations have been reported, although this is infrequent and the cause has not been proven. Fortunately, no long-term or serious health effects have been attributed to the electrostatic or alternating electric fields. Moreover, the electric fields can easily be blocked by incorporating a grounded conductive layer into an anti-glare shield.
On the other hand, ELF and VLF magnetic radiation is not easy to block. Low frequency magnetic fields can easily travel through layers of solid aluminum, copper or steel with little reduction in strength. Further, unlike an electric field which travels in a straight line, a magnetic field loops outward in curves, forming an irregular, rounded envelope of energy. Adding to the problem is the source of the EMF, which is not the front of the screen but the deflection coils, flyback transformer, and power supply inside the VDT. The EMF travels up and over the top of the screen, around the sides, and underneath in all directions.
"Screen savers" designed to blank out the screen after a short period of inactivity are useful to prevent "burn in" or damage to the VDT's phosphor coating from constant use, but even if the image is blank, the components which generate ELF and VLF emissions are still active. Similarly dimming the display will do nothing to reduce the fields. Shields placed in front of a VDT's screen do not block ELF magnetic fields. They do block electric fields, but the ELF magnetic field is the main concern.
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