1. Electrical currents in the body can cause three phenomena: electrical stimulation of excitable tissue, resistive heating of tissue, and electrochemical burns and tissue damage. 2. The physiological effects of electricity depend on factors like body size, and even microshocks of 20 microamperes can induce cardiac fibrillation. 3. In a patient case, connecting multiple medical devices like an ECG monitor, EEG monitor, and IV pump to different electrical outlets caused ground loop currents, risking electrical shock hazards. Connecting the devices to a single power distribution node with a common ground solved this issue.

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

2. 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
• Electrochemicalburns 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”
Skin surface
Sub-dermal surface
Skin resistance (15 k Ω to 1 MΩ/cm2)
Internal body resistance (100-
200 Ω -- limb, trunk)

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
Outlet
1
Outlet
2
Outlet
3
sig
sig
sig
sig
H
N
G1
G1
G3
H N G2
G2
H
N
~G3
EEG
ECG
Pump

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

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

17. Grounding System

18. Isolated Amplifier