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Human Thresholds of Electric Shock at Power Transmission Frequencies

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John C. Keesey, Frank S. Letcher · 1970

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Humans can perceive electrical current as low as 0.5 milliamps at power frequencies, with safety thresholds much lower than commonly assumed.

Plain English Summary

Summary written for general audiences

This 1970 study established human safety thresholds for electrical shock from power transmission frequencies (50-60 Hz). Researchers found some people can perceive as little as 0.5 milliamps, while 9 milliamps represents the maximum current most adult men can still release their grip from. The study recommended 5 milliamps as a safety threshold for the general population, including children.

Why This Matters

This foundational research reveals how surprisingly low our tolerance is for power frequency electrical current. What makes this particularly relevant today is that we're surrounded by 50-60 Hz fields from power lines, household wiring, and appliances. While this study focused on direct electrical contact rather than field exposure, it demonstrates the body's extreme sensitivity to power frequency electricity. The finding that some individuals can perceive half a milliamp shows remarkable biological sensitivity to these frequencies. The science demonstrates that safety margins for electrical exposure have been known for decades, yet regulatory standards often don't account for the most sensitive individuals in the population, including children who may have even lower thresholds.

Exposure Information

Specific exposure levels were not quantified in this study.

Cite This Study
John C. Keesey, Frank S. Letcher (1970). Human Thresholds of Electric Shock at Power Transmission Frequencies.
Show BibTeX
@article{human_thresholds_of_electric_shock_at_power_transmission_frequencies_g7080,
  author = {John C. Keesey and Frank S. Letcher},
  title = {Human Thresholds of Electric Shock at Power Transmission Frequencies},
  year = {1970},
  
  
}

Quick Questions About This Study

Some humans can perceive less than 0.5 milliamps of 50-60 Hz current, depending on hand contact type with electrically energized circuits. This extremely low threshold demonstrates remarkable human sensitivity to power frequency electricity.
Most adult male workers can release their grip from 9 milliamps of 50-60 Hz current. However, the study recommends a much lower 5 milliamp safety threshold for the general population including children.
The 5 milliamp recommendation accounts for children and sensitive individuals in the general population. Current exceeding release threshold is considered hazardous and potentially lethal, requiring conservative safety margins.
Body size, current pathway through the body, contact duration, and total electrical resistance all influence shock thresholds. These variables explain why safety standards must account for worst-case scenarios.
Yes, voltages calculated from reliable experimental current data and expected body resistances are lower than voltages generally recommended as safe, suggesting current safety standards may be inadequate.