Electrical Safety

1
Electrical Safety

• BiomedE / EECS 458
• Reading Webster, Chapter 14

2
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 ? -- limb,
trunk)
Skin resistance (15 k ? to 1 M?/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
Ergs bedside schematic
sig
sig
H
EEG
Outlet 1
N
H
sig
ECG
Outlet 2
N
sig
G1
G1
G2
G3
Pump
G3
H
N
G2
Outlet 3
15
Ergs bedside schematic
Ground loop currents!!
sig
sig
H
EEG
Outlet 1
N
H
sig
ECG
Outlet 2
N
sig
G1
G1
G2
G3
Pump
G3
H
N
G2
Outlet 3
16
Ergs 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
H
N
G1
Outlet 1
17
Grounding System
18
Isolated Amplifier