Effect of overcurrent passing through human body
When
selecting the rated sensitivity current of ELCB used to prevent injury from
electric shocks, it is necessary to understand the physiological symptoms of
the human body in reception to electricity. According to Biegelmeier’s report, the
human’s characteristics to electricity can be classified as shown in Table 6.
1. When the passing current increases, the heart chamber (heart) starts
fibrillating, the pulse is distributed, and the circulation of blood to supply fresh
blood throughout the body stops. This is an extremely dangerous current that
can lead to death. This current value requires medical experimentation, which was
carried out (including animal experiments) in the United States and Germany,
etc. The results were documented, and were usually several 10mA. When intended
to prevent electric shocks accidents, it is best to provide protective measures
that limit the involuntary current (limit at which separation is possible).
However, when the continuity of the power fed, etc., is considered in relation
with the circuit’s leakage current, providing protective measures for the
ventricular fibrillation electrical current is unavoidable.
The
human’s physiological symptoms are greatly affected by the current square time
product. On the other hand, Koeppen found that even if the current value
exceeds 50mA, human life can be saved if the energizing time is extremely
short. The limit is current time product 50mA • s. The relation of these is shown in Fig. 6. 2.
When
both are compared based on the characteristics in this figure, it can be seen
that Koeppen’s limits are less than Dalziel’s limits, so using the safer 50mA • s as a reference is suitable. Koeppen also writes
about this, but using the reference of 50mA • s, 30mA • s is set
as the safety factor in Europe. While there are no reports on the effect if a current
of 50mA is continuously passed to the human body, typically if a current of
50mA or less is passed, the human would reflectively let go of the conductor.
The size of the c urrent that passes through the human body is determined by
the human body’s resistance and the contact voltage. The human body’s
resistance varies by individual. It can be affected by race, dryness of skin,
state of contact with electrode (contact area or contact pressure, etc.), and the
size of the contact voltage. Freiberger (Germany) has reported that when the
hand to foot, the most typical current path is looked at, the human resistance
is within the range shown in Fig. 6.3. However, in adverse conditions where the
skin surface resistance is ignored, the resistance drops to 500W. Thus, this value should be adopted when laying importance on
safety.
Based
on the above, when considering measures to prevent electric shock accidents,
the human resistance in respect to contact voltage must be obtained from Fig.
6.3, and the size of the current passing through the human body must be estimated.
In respect to the safe voltage, it is difficult to definitively set the danger
voltage in the relation of the human resistance. If the environmental
conditions or electrical conditions are poor, the contact voltage must be low.
In the IEC Standards the voltage is a safe special low voltage. Within preset conditions,
the maximum voltage must be 25VAC or less.
Electric shock current and physiological reaction on human body
Electric shock protection, rated current sensitivity and operating time
As explained above, there are various theories on the physiological symptoms that occur to a human
when currents
pass through. If the safety standards were set
following the IEC curve given in Fig. 6. 5, the
following areas could be
could be considered.
l.
In areas were secondary accidents could result
because of electric
shocks, the area of curve b and below
II. Curve c1 and below where secondary accidents would not result because of electric shocks.
(1) Taking measures using curve b as protection standard
As
shown in Fig. 6. 4, usually there is no hazard to the human body if the passing
current is 5mA or less. The 5Ma current is a level at which a person generally
feels tingling. The person could “let go” at his current, so normally the person
can provide his/her own protection. If a person inadvertently and directly
touches a 200mA voltage to ground liver wire, a 200mA (human resistance 1000W) current will flow through the body. In this case, based on curve b
the operating time must be within 0.01 sec.
(2) Taking measures using curve C1 as protection standard
In
levels with a small current value, curve C1 shows the drop from 50mA in one second and the drop from 40mA after three
seconds. In levels with a high current level, curve c1 shows the drop from
500mA at 10ms or less, and from 400mA at 100ms. If the current passing to the
human body exceeds 40mA, the risk of a serious physiological effect occurs as
the current value and time increase.
(3) Current passing through human body, time product 30mA • s
Measures
are often using the protection standard of 30mA • s based on Koeppen’s ventricular fibrillation limit. However, even in this case the electric device must be
grounded as a rule. If
the electric device is improperly grounded (portable or movable device that easily generates a
ground), the following
two conditions must be satisfied to suppress the current to within the ventricular fibrillation limit even the human touches the high voltage. l Rated current
sensitivity 30mA or less l Current/time product to ELCB operation Within 30mA • s Note that grounding work is usually performed, so if the selection maintains the relation of (rated
current sensitivity) X (grounding resistance value) <
(tolerable contact voltage), then the electric shock protection can be provided at 200mA or 500mA even if the rated current
sensitivity is not 30mA.
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| Fig. 6. 5 Effect of AC current (15Hz to 100Hz) on human body (IEC/TS60479-1) |
The physiological effect in each zone is as follows
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