Pilot measurements of ELF contact currents in some electric utility occupations.

Contact currents from touching objects with different voltages can produce electric fields within the body that produce neurological and other biological effects. To begin measuring these exposures among electric utility workers, a new contact current meter (CCM) was tested in a pilot study at Southern California Edison. The CCM was worn for 82 full-shift measurements by 76 volunteers from eight occupations who did not work directly with energized electrical equipment. The volunteers were exposed to an average of 285.8 contact current events above the meter's 1-microA threshold, but most of these were electrostatic spark discharges. Fourteen employees experienced an average of 135.1 contact currents events whose primary frequency was 60 Hz. Using a circuit model of the human body, the average contact currents going from arm to arm was 9.8 microA (maximum = 178.0 microA), and the average going down the torso was 25.5 microA (maximum = 662.0). The maximum exposures were experienced by a technical support employee working in a substation. All measurements in this pilot study were below the 3000 microA maximum permissible exposure for contact currents set by the Institute of Electrical and Electronic Engineers (IEEE). Combining these current measurements with the results of high-resolution dosimetry, the internal electric fields averaged an estimated 1.7 mV/m in the heart (maximum = 21.0 mV/m), and 1.9 mV/m in the hematopoietic bone marrow in the torso (maximum = 56.5 mV/m). These internal electric fields from contact currents are below the basic restriction of 943 mV/m in the IEEE exposure standards but are above 1 mV/m, a level where biological effects have been often reported in laboratory studies. Safety concerns limited the measurements to de-energized equipment, so we did not obtain data on work in energized high-voltage environments, the most likely sources of high contact currents. This pilot study identified other improvements to the contact current meter that would make it better able to measure exposures in future health studies.

Equipment grounding affects contact current exposure: a case study of sewing machines.

Exposure to contact current may occur when the body is in contact with two conductive surfaces with different electrical potentials. To date, no published data that describe such exposures or electrical conditions that may predispose to such exposures exist. Our investigation into contact current exposure included (i) a small sample of workers in a garment production facility with modern well-grounded equipment performing normal work tasks and (ii) a single individual simulating garment production tasks in a sewing machine repair facility with substandard equipment grounding. In both cases, we deployed a newly developed personal monitor that records contact current events at the power frequency of 60 Hz. The personal monitoring data suggested that more frequent exposure occurs in association with, and probably because of, poorer grounding practices. This preliminary conclusion was validated with controlled laboratory measurements of potentials to reference ground on specific locations of four sewing machines with different grounding characteristics. Propensity to exposure was greater in the two machines with inferior grounding characteristics, and increased in the other two when deprived of their grounding connections. Contact currents at or below threshold-of-perception levels can produce electric fields within tissues that may plausibly produce biological effects. On this basis, such exposures have been under-investigated relative to the far greater attention accorded to occupational electric and magnetic fields.