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Possible Health Effects of Exposure to Residential Electric and Magnetic Fields
TABLE 2-2 Average and Percentile Values for the Magnetic Field in Homes According to Room
Magnetic Flux Density, µT
Room Center (n = 992 homes)
All rooms (average)
—, not provided.
SOURCE: EPRI 1993a.
In the EPRI study, the power-line fields were found to be the dominant source of average and median fields. For most low-voltage power lines, the load current on the three wires is not always balanced. To analyze the effect of the imbalance, the load current can be mathematically divided into the balanced and unbalanced parts; the balanced part of the current produces a field that decreases approximately as the inverse square of the distance from the power line; the unbalanced part (the zero-sequence current) causes a field that decreases with the inverse of the distance. Therefore, at greater distances, the field associated with the zero-sequence current dominates. When the median field in a house is greater than 0.16 µT (1.6 mG), the home in question is usually near a power line and the main field source is usually the balanced part of the power-line load current.
The strongest magnetic fields in homes are generally caused by appliances. However, the fields usually decrease rapidly with distance. When the main source of a magnetic field in an appliance is a coil of wire, the field decreases approximately as the inverse cube of the distance. Some of the magnetic-field values measured near household and other appliances are shown in Tables 2-3 and 2-4. The values show the range of all the measurements made (e.g., 95% of the color television sets measured emitted magnetic fields less than 0.33 µT at a distance of 56 cm). The values are based on measurements of the rms fields averaged over time from about 1 sec or more for the spot measurements to 24 hr for long-term and personal exposure measurements. Different appliances of the same type can produce different magnetic fields because of differences in their design. Important differences are the amount of current they use, the size and shape of conducting parts, the number of turns of wire in coils, and whether shielding or field-canceling technology was used.