Association of residential magnetic fields with contact voltage

Abstract

The US National Electrical Code's (NEC) requirement to ground a home's electrical service to the residential water line results in a voltage between the water line and earth, V W-E. The voltage may result from ground return current that flows into the earth via the water line or from inductive effects from other sources of magnetic fields, such as transmission lines. This voltage can, in turn, serve as a source for Vbath, the voltage between the water fixtures and conductive drain pipes sunk into the earth beneath a residence. Vbath can be a source of contact current exposure to a child touching a water fixture while bathing. Previous research has suggested that exposure to these currents could be the basis for the association between power-frequency magnetic fields and childhood leukemia. In this study, we assessed the association between measured Vbath and VW-E with the average spot-measured magnetic field, Bavg, in a sample of 191 single-family residences in the Denver metropolitan area. This area was the source of cases and controls for previous studies of electric and magnetic field (EMF) and childhood cancer. The association of both Vbath and VW-E with Bavg had upward trends across magnetic field strata (<0.1 microT (reference); 0.1-<0.3 microT; and > or = 0.3 microT). In addition, VW-E was associated with Vbath. Without further study, these results cannot be applied to multi-dwelling residences or to electrical systems prevalent in other nations. Nonetheless, when combined with the finding that contact current is a far more plausible candidate than the residential magnetic field for mediating biological effects on the basis of comparative dose to bone marrow, these associations indicate that contact current exposure deserves further study.

Recent advances in research relevant to electric and magnetic field exposure guidelines

“…Based on a limited set of measurements in two cities covering a total of 227 residences [Kavet and Zaffanella, 2002;Kavet et al, 2004], the source voltage for a bathing child is commonly no greater than a few hundred millivolts but could be as high as several volts; high levels of neighborhood ground currents, induction from nearby high voltage transmission lines or faulty grounding can be responsible for high levels [Kavet, 2005;Kavet et al, 2006]. For a source voltage of 500 mV, the contact current would range between 25 and 100 mA for subject body-to-bath resistances spanning 0.5-2 kO.…”

Residential Magnetic Fields and Measures of Neutral-to-Earth Voltage: Variability Within and Between Residences

“…In previous measurement studies, B avg has been reported as positively associated with V W-E and V bath across residences (Kavet and Zaffanella 2002;Kavet et al 2004). Nonetheless, our understanding of the potential role of V bath in explaining the epidemiologic results would be incomplete without a further understanding of how B avg , V W-E , and V bath are interrelated within residences, which is the issue addressed in the measurements reported in this study.…”

“…This voltage, represented in Fig. 1 (left), is also a source of contact current exposure to a bathing child while touching a metal plumbing fixture contiguous with the water line, provided the drain into the earth is conductive (Kavet and Zaffanella 2002;Kavet et al 2004). The voltage across the child is referred to as V bath , which is represented in Fig.…”

Contact current hypothesis: Summary of results to date

“…A low prevalence of the highest current, 200 A, does not necessarily lead to a decrement of the association; rather, the balance of weights across all feeder loads plays a role. The diversity of feeder currents together with unbalanced currents across large regional swaths is quite likely a factor largely responsible for the positive relationships between B avg and V W-E that have been measured [Kavet and Zaffanella, 2002;Kavet et al, 2004].…”