Aligning Exposure Limits for Contact Currents with Exposure Limits for Electric Fields

Abstract

Abstract The Institute for Electrical and Electronic Engineers (IEEE) and the International Commission on Non-ionizing Radiation Protection (ICNIRP) have established limits for exposures to electromagnetic fields across the 0–300 GHz (non-ionizing) spectrum, including limits on contact currents (CC) specified by IEEE for 0–110 MHz (ICNIRP issued a CC “guidance level”). Both sets of limits seek to protect against potentially adverse effects, including aversive electrostimulation at frequencies <100 kHz and excessive heating of tissue at frequencies >100 kHz. For the most part, CC is linked to electric field (E-field) exposures for an ungrounded person contacting a grounded object, with the short-circuit current (ISC ) through the contact point (usually the hand) equivalent to the current through the grounded feet of a free-standing person exposed to a vertically polarized E-field. The physical linkage between these two quantities dictates that their respective exposure limits align with one another, which is presently not the case, especially with respect to frequencies from100 kHz to 110 MHz. Here we focus specifically on recommendations for revisions to the IEEE standard, IEEE Std C95.1™-2019 (“IEEE C95.1”), in which the E-field exposure limit (E-field exposure reference levels, ERLs) >100 kHz induces substantially greater currents than the CC ERLs currently prescribed. The most important scenario deserving of attention concerns finger contact through a 1-cm2 cross-sectional interface between the skin and a grounded conductor in which the rate of temperature rise in the presence of an E-field ERL can be rapid enough to cause a burn injury. This rate is highly dependent on the moistness/dryness of the skin at the contact point (i.e., its impedance)—a highly variable value—with temperature increasing more rapidly with increasing dryness (greater contact impedance). The two main remedies to alleviate the possibility of injury in this “touch” scenario are to (a) limit the time of finger contact to 1 s in all cases and (b) revise the E-field ERL between 100 kHz and 30 MHz from a “hockey-stick-shaped” curve vs. frequency to a “ramp” across this frequency range. These measures factored in with the real-world prevalence of potentially hazardous scenarios should afford greater protection against adverse outcomes than is presently the case. IEEE C95.1 also specifies limits for grasp contact (15 cm2 in the palm) and associated wrist heating, plus heating in the ankles from free-standing induction. However, these scenarios are more manageable compared to finger touch due mainly to the comparatively lower rates of tissue heating attributable to the wrist’s and ankle’s relatively greater cross-sectional area. Recommendations for grasp can thus be dealt with separately. Two identified but unaddressed issues in IEEE C95.1 deserving of further attention are first, the circumstance in which a grounded person contacts an ungrounded object situated in an electric field for which there are countless numbers of scenarios that are not amenable to a single ERL. Second, arcing between an extended limb and E-field-exposed object is perhaps the most hazardous of all scenarios. Both of these scenarios cannot be stereotyped and must be dealt with on a case-by-case basis. Future revisions of IEEE Std C95.1-2019 (and the ICNIRP guidelines) will benefit from improved insight into strategies of affording protection from potentially adverse effects in these circumstances.