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Electrical Safety

Electrical Safety. BiomedE / EECS 458 Reading: Webster, Chapter 14. R internal. R skin. R skin. ~. Vstim. Electrical Currents in body. Three phenomena are possible through electrical currents Electrical stimulation of excitable tissue (nerve & muscle) Resistive heating of tissue

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Electrical Safety

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  1. Electrical Safety BiomedE / EECS 458 Reading: Webster, Chapter 14

  2. Rinternal Rskin Rskin ~ Vstim Electrical Currents in body • Three phenomena are possible through electrical currents • Electrical stimulation of excitable tissue (nerve & muscle) • Resistive heating of tissue • Electrochemical burns and tissue damage

  3. Physiological effects of electricity Conditions: 70-kg human, moistened hands, AGW #8 copper wire

  4. Variability of Perception Thresholds and Let-go Currents Physiological effects of electrical shocks depend on body size ( among other factors)

  5. Let-go Current vs. Frequency 60 Hz

  6. Fibrillation Current vs. Shock Duration

  7. Effects of Entry Points on Current Distribution Microshocks -- currents as low as 20 uA can induce cardiac fibrillation

  8. Electric-power Distribution

  9. Macroshock Hazards Most instruments have grounded or non-conductive cases

  10. Leakage-current Pathways

  11. Ground-fault Currents

  12. “Equivalent Resistance Guy” Internal body resistance (100-200 Ω -- limb, trunk) Skin resistance (15 k Ω to 1 MΩ/cm2) Skin surface Sub-dermal surface

  13. Erg in intensive care • ECG monitor (lead I) • EEG monitor (single lead) • Catheter in right arm for IV drug infusion through automated system • Each system is plugged into a separate wall outlet

  14. Erg’s bedside schematic sig sig H EEG Outlet 1 N H sig ECG Outlet 2 N G1 sig G1 G2 ~G3 Pump H N G3 G2 Outlet 3

  15. ~ Vgl Erg’s bedside schematic Ground loop currents!! sig sig H EEG Outlet 1 N H sig ECG Outlet 2 N G1 sig G1 G2 ~G3 Pump G3 N H G2 Outlet 3

  16. Erg’s bedside schematic Solution: Single power distribution node with a common ground sig sig H EEG Outlet 1 N H sig ECG Outlet 1 N G1 sig G1 G1 ~G1 Pump G1 N H G1 Outlet 1

  17. Grounding System

  18. Isolated Amplifier

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