Page 364 - Handbook of Electrical Engineering

352 HANDBOOK OF ELECTRICAL ENGINEERING

Figure 13.1 Intensity of electric shock for human beings.

Where K h = 0.116 for a human body weighing 50 kg.
I h = non-fibrillating current tolerable by a human body, in amperes.
t n = non-fibrillating time duration of tolerance, in seconds.

Figure 13.1 shows the form of the characteristic as the line A-B, and the approximate regions of
increasing danger. IEC60479 part 1 shows a similar figure with its Figure 14 providing numerical data.

In a practical situation the value of t n will be equal to the fault current clearance time of the
device protecting the circuit. The current I h maybeassumedtobelimited bythe resistance of the
human body as it makes contact with two different potentials. In Reference 2 Ryder recommended
in 1949 that a resistance of 500 ohms could be used to represent the resistance between both hands.
In more recent times the IEEE80, in its Chapter 5 uses 1000 ohms generally in relation to the design
of substation grounding grid and rod systems. The assumption used by Ryder was that the hands
were thoroughly wet, which is still a reasonable assumption. The IEC60479 (1994 edition), part 1,
clause 2.6 also uses 500 ohms as the appropriate value for hand-to-hand and hand-to-foot when the
contact area is large, and notes that it does not vary significantly with the contact area unless it is
very small, i.e. a few square millimetres. Ryder also recommends a limiting current that a human
should be subjected to without fatal consequences as 100 mA, at a power frequency of 50 to 60 Hz.
IEC60479, part 1, and Reference 3 describe the impedances of different parts of the human body and
how they form a complete electrical circuit.
It is on the basis of a body resistance of 500 ohms and a current of 100 mA that the hand-to-
hand maximum voltage limit of 50 volts (root-mean-square alternating voltage) has been established
and used in the international literature, e.g. IEC60364. The corresponding hand-to-hand direct voltage