Electrical Safety Biomed E EECS 458 Reading Webster

Electrical Safety Biomed. E / EECS 458 Reading: Webster, Chapter 14

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

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

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

Let-go Current vs. Frequency 60 Hz

Fibrillation Current vs. Shock Duration

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

Electric-power Distribution

Macroshock Hazards Most instruments have grounded or nonconductive cases

Leakage-current Pathways

Ground-fault Currents

“Equivalent Resistance Guy” Internal body resistance (100 -200 Ω -- limb, trunk) Skin resistance (15 k Ω to 1 MΩ/cm 2) Skin surface Sub-dermal surface

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

Erg’s bedside schematic sig H Outlet N 1 G 1 sig EEG G 1 sig ~G 3 Pump H N Outlet 3 G 2 ECG H N Outlet 2 G 2

Erg’s bedside schematic Ground loop currents!! sig H Outlet N 1 G 1 Vgl sig EEG G 1 ~ sig ~G 3 Pump G 3 N H Outlet 3 G 2 ECG H N Outlet 2 G 2

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

Grounding System

Isolated Amplifier