manual and automatic switching devices, such as those associated with lights and appliances, or (2) repetitive, originating from electronic controllers, such as dimmer switches, that produce transients twice per power-frequency cycle. They also found that the rate of occurrence of transients increases from residential locations to business and industrial locations. Transients might propagate from a residence or business through secondary distribution wiring to neighboring residences and along ground pathways (conductive plumbing pipes). Switching operations in the primary distribution lines can also generate transients within the residences.
Personal exposure to transient fields is extremely complex because of their strong spatial dependence. Exposure to high-frequency fields is typically due to sources within the residence, but exposure to low-frequency fields can also be due to external sources in the neighborhood.
The authors suggest that "higher current configurations may correlate locally with higher demand indicating either increased population density or individual customers with large current usage such as industries or businesses. Thus, a residence classified as being in a 'higher' current configuration would more likely be in a locale where the rate of transient occurrence is higher, because of the likelihood of having more neighbors or being closer to businesses and industries. Furthermore, such a residence would be physically closer to neighboring transient sources, implying shorter propagation distances and an increased probability of 'receiving' a transient generated at a neighboring residence" (EPRI 1994a).
On the other hand, current flows are generally more variable at points in the power-delivery system that serve fewer customers; therefore, the most uniform power flows should be found on the multiple or thick three-phase primary lines and high-tension lines near high current configuration (HCC) homes. So, if the causal exposure is related to long-period variability, it would probably not correlate with the average field strength or with wire codes.