electrical safety

1. 504181416 Zeynep İrem Çakırca
BYM501E Fundamentals of Biomedical Engineering
Lecturer : İnci Çilesiz, PhD
11.11.2019
IMPORTANT SUSCEPTIBILTY
PARAMETERS

2. Outline
• Introduction
• Threshold and Let-Go Variability
• Frequency
• Shock (Stimulation) Duration
• Body Weight
• Points of Entry

3. Introduction
Figure 1. Physical effect vs. current level
• The current needed to generate
each effect depends on some
conditions such as variability of
let-go current and threshold of
perception, frequency, duration,
body weight, points of entry..

4. Threshold and Let-Go Variability
Figure 2. Distributions of perception thresholds and let-go currents
• Mean “threshold of perception”
1.1mA for men
0.7mA for women
• Minimum threshold of perception
500 µA
80 µA with gel electrodes (reduces skin
impedance)
• Mean “let-go current”
(let-go current = max current where
you can still release your grip)
• 16.5 mA for men
• 10.5 mA for women

5. Frequency
Figure 3. Let-go current vs frequency
Let-go current vs. frequency
• Minimal let-go current occurs at commercial
power-line frequencies of 50-60 Hz

6. Shock (Stimulation) Duration
Figure 4 shows that for short durations
the stimulation current threshold Id is
inversely related to the pulse duration
d by the well-known strenght-duration
equation :
Figure 4. Normalized analytical strength-duration curve for current I, charge Q and energy U.
• Fibrillation current is inversely
proportional to the shock pulse
duration
• longer pulses -> lower current
does damage

7. Shock (Stimulation) Duration
• The longer the duration, the smaller
the current at which ventricular
fibrillation occurs.
• Shock must occur long enough to
coincide with the most vulnerable
period occurring during the T wave.
Figure 5. Fibrillation current vs shock duration. Thresholds for VF in animals for 60 Hz AC current. Duration of
current(0.2 to 5 s) and weight of animal body were varied.

9. Body Weight
Fibrillation current increases with body weight
• 50 mA RMS for 6 Kg dogs
• 130 mA RMS for 24 Kg dogs

10. Points of Entry
Points of entry
• Skin impedance varies: 15 kΩ to 1 MΩ
(Resistive barrier that limits current flow)
• Tissue (beneath skin) has low impedance
Macroshock
• externally applied current
• spreads through the body so less
concentrated
Microshock
• applied current is concentrated at an
invasive point
• accepted safety limit is only 10 µA
• generally only dangerous if current flows
through the heart
Figure 6. Effect of entry points on current distribution. (a) Macroshock : Externally
applied current spreads throughout the body (b) Microshock : All the current applied
through an intracardiac catheter flows through the heart

11. Points of Entry
Figure 7.