Page 101 - Electrical Safety of Low Voltage Systems

84 Chapter Five

F Left Hand Right Hand Both Hands Right Foot
Left hand n/a 0.4 n/a 1
Right hand 0.4 n/a n/a 0.8
Both hands 1 n/a n/a 1
Left foot 1 0.8 1 0.04
Right foot 1 0.8 1 n/a
Both feet 1 (reference) 0.8 1 n/a
Chest 1.5 1.3 n/a n/a
Back 0.7 0.3 n/a n/a
Glutei 0.7 0.7 0.7 n/a
TABLE 5.1 Heart-Current Factor F Applicable to Current Not Flowing Through
the Reference Path

For other paths, then, the following corrective heart-current factor
F must be considered:

I LH-2F
F = (5.4)
I F
where I LH-2F is the fibrillating current for a left-hand-to-feet pathway,
assumed as reference, while I F is the current flowing through a differ-
ent route that has the same probability to cause ventricular fibrillation
as I LH-2F , but not necessarily the same intensity.
Values for F for various paths as the result of possible combina-
tions are reported in Table 5.1.

Example 5.1 A chest-to-left-hand current (F = 1.5) equal to 50 mA has the same
probability to cause ventricular fibrillation as a 75-mA left-hand-to-feet current
(i.e., reference current); a left-foot-to-right-foot current (F = 0.04) must be 25
times larger than a left-hand-to-feet current in order to cause ventricular fibril-
lation with equal probability; a left-foot-to-right-foot current I F LF-RF (F = 0.04)
must be 20 times larger than a right-hand-to-feet current (F = 0.8) in order to
cause ventricular fibrillation with the same probability, in fact:

I LH-2F = 0.8I F RH-2F ,

0.8I RH-2F 0.8I RH-2F
I LH-2F F LF-RF F RH-2F
0.04 = LF-RF = LF-RF ⇒ I F = = 20I F
I F I F 0.04
5.7 Human Body Impedance
The human body impedance Z B consists of resistive and capacitive
elements. The skin, by acting as an insulating dielectric between the
conductive tissue underneath it and live parts, can be thought as a