Biomedical Engineering Reference
In-Depth Information
Above about 45
C, the time duration becomes more and
more critical. In high voltage accidents the heat effect
may be very important, and patients are treated as
thermal burn patients. In particular, special attention is
paid to the fluid balance, because electrical burn patients
tend to go into renal failure more readily than thermal
burns of equal severity. As electric current disposes
thermal energy directly into the tissue, the electric burn
is often deeper than a thermal burn caused by thermal
energy penetrating from the surface. The general expe-
rience is therefore that an electrical burn is more severe
than it may look like the first hours after the injury.
60
50%
99%
40
20
1%
5
50
500
5000
f
(Hz)
Electrolytic effects
Electrolytic effects are related to DC, applied or rectified
by non-linear effects at the electrodes or in the tissue.
Also with very low frequency AC (e.g.
<
10 Hz), each
half period may last so long as to cause considerable non-
reversible electrolytic effects. With large quantities of
electricity (
Q¼ It
) passed, the
electrolytic
effects may be
systemic and dangerous (lightning and high voltage ac-
cidents). The risk of skin chemical burns is greater under
the cathode (alkali formation) than the anode (acid for-
mation), the natural skin pH is on the acidic side
(pH
<
5.5).
Nerve damage
is often reported in high voltage
accidents.
Figure 4.1-31 Frequency dependence of let-go currents. Statis-
tic for 134 men and 28 women. Source: From Dalziel (1972).
Let-go current
Let-go current threshold (15 mA @ 50/60 Hz) is the
current level when the current density in muscles and
nerves is so large that the external current controls the
muscles. As the grip muscles are stronger than the
opening muscles of the hand, a grip around the current
carrying conductor cannot be loosened by the person
himself. Let-go current levels are therefore the most im-
portant data for safety analysis. The result in
Fig. 4.1-31
shows that 1% of the population have a let-go threshold as
low as 9 mA at power line frequencies.
Fatal levels
are reached at current levels
>
50mA@50/
60 Hz if the current path is through vital organs: heart,
lung or brain stem (cf. the electric chair,
Table 4.1-2
).
Current limiting body resistance
The most important current limiting resistance of the
human body is the dry skin. This may be impaired by
high-field electrical breakdown, skin moisturizing or
a skin wound. Skin breakdown may occur under 10 VAC
50/60 Hz due to electro-osmotic breakdown (Grimnes,
1983b).
Without the protective action of the skin, the
internal
body resistance
may be divided into a
constrictional zone
resistance
with increased current density near an elec-
trode, and
segmental resistances
of each body segment
with rather uniform current density. With small area
Heat effects
Jouleian heat is dependent on the in-phase components
of potential difference and current density. The resulting
temperature rise is dependent on the power density, the
specific heat of the tissue and the cooling effect of the
blood perfusion.
The tissue damage is very dependent on exposure
time, cells can tolerate long time exposure of 43
C.
Table 4.1-2 50/60 Hz threshold levels of perception and hazard
Current threshold
Voltage threshold
(very approximate)
Organs affected
Type
Comments
0.3 mA
20 V
Skin
Perception threshold
Electrovibration, mechanical
10 mA
20 mV
Heart
Microshock hazard
Myocard excitation
1 mA
10 V
Skin
Perception threshold
Nerve excitation
15 mA
50 V
Muscles
Let-go
Loss of muscle control
50 mA
250 V
Heart, lung, brain stem
Macroshock hazard
Nerve excitation
