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

Electrical Currents in body Vstim ~ R skin R internal 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 uA can induce cardiac fibrillation

Electric-power Distribution

Macroshock Hazards Most instruments have grounded or non- conductive cases

Leakage-current Pathways

Ground-fault Currents

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

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 Outlet 1 Outlet 2 Outlet 3 sig H N G1 G3HNG2 H N ~G3 EEG ECG Pump

Erg’s bedside schematic Outlet 1 Outlet 2 Outlet 3 sig H N G1 G3HNG2 H N ~G3 Vgl ~ Ground loop currents!! EEG ECG Pump

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

Grounding System

Isolated Amplifier