Electrical Safety

BiomedE / EECS 458Reading: Webster, Chapter 14

Electrical Currents in body

Vstim ~

Rskin Rskin

Rinternal

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/cm2)

Internal body resistance (100-200 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

sigsig

sigsig

HN

G1G1

G3H N G2

G2

HN

~G3

EEGECG

Pump

Erg’s bedside schematic

Outlet 1 Outlet

2

Outlet 3

sigsig

sigsig

HN

G1G1

G3 HN G2

G2

HN

~G3Vgl ~

Ground loop currents!!

EEGECG

Pump

Erg’s bedside schematic

Outlet 1 Outlet

1

Outlet 1

sigsig

sigsig

HN

G1G1

G1 HN G1

G1

HN

~G1

Solution: Single power distribution node with a common ground

EEGECG

Pump

Grounding System

Isolated Amplifier