ELECTRIC SHOCK AND SENSITIVE EARTH-LEAKAGE PROTECTION (Part 1)
By H. P. SMITH, B.Sc. (Eng.), A.M.(S.A.)I.E.E.
A paper presented to the Institution of Certificated Mechanical and Electrical Engineers, South Africa (Orange Free State Branch), on 3rd June 1965, and to the Association of Supervising Electrical Engineers, Johannesburg, on 15th June 1965.
Generally speaking, the factors which determine the seriousness of electric shock are:
(1) Body electrical resistance.
(2) Path of current flow through the body.
(3) Amount of current and duration of time current flows through the body.
Body Electrical Resistance
As a conductor, the human body is made up of two parallel paths. One of these, the outer skin, is of relatively high resistance, whereas the other, composed of the bloodstream and the tissues, is of relatively low resistance.
The results of work done by Freiberger in Berlin in 1933 show the great importance of the epidermis (outer layer of skin) as a barrier against severe electric shock.
Finding that experiments on living persons were limited by a maximum safe current of 20 milliamperes, Freiberger conducted 2 600 tests on 60 corpses in order to determine the electrical resistance of the human body. The tests were made with alternating currents up to 5 200 volts at 50 cycles/second.
When the epidermis was intact, the resistance between hand and hand, or hand and foot, was of the order of 100 000 ohms for voltages up to 30 volts A.C. As the voltage was increased, the resistance fell to 40 000 ohms at 100 volts and then rapidly to 2 000 ohms at 200 volts. The curve then flattened out again and at 1 000 volts, the resistance was 600 ohms.
If, however, contact was made beneath the epidermis, the resistance was found to be constant at about 600 ohms at all voltages.
By observing the variations in resistance of a particular subject with the time after death and extrapolating to zero, Freiberger obtained approximate values for the external and internal resistances of the body (i.e. with and without the epidermis) at the instant of death, and so that of a living body. He concluded that the internal resistance of a living body is not generally greater than 1 000 ohms.
The minimum value of body resistance for a current-path hand to hand or hand to foot on A.C. systems is generally accepted in Continental practice ohms; moreover, if contact is made with open wounds or the body is immersed in a conducting liquid, an authority on the subject states that the resistance may be as low as 200 ohms.
Path of Current Flow Through Body
The injurious effect of electric current flowing through the body depends largely on the path followed by the current. Currents flowing through the lower part of the brain affect the breathing nerve centre, and currents through the heart affect the operation of the heart. Therefore, current paths from head to leg, arm to arm, arm to leg, the chest, chest to the arm are the most dangerous, and the most likely where electrocution occurs.
Most electrocutions in the home and in mining and industry are due to "earth-fault" currents, i.e. current flowing from a "live conductor or apparatus to ground, the current pathway being, in the main, between one hand and the feet.
Amount of Current and Duration of Time Current Flows Through Body
An electric shock is only dangerous when the current through the body exceeds a certain minimum value, which is dependent not only on the current but also on the time for which it flows; a low current for a long time could prove equally as dangerous as a high current for a relatively brief period.
The applied voltage is in itself only important in producing this minimum current through the body resistance.
The current passing through the body is, from ohm's law, equal to the applied voltage divided by the resistance of the path taken by the current.
Therefore, for a 220 volt supply, the maximum current with a body resistance of 500 ohms would be 440 milliamperes. Considering the voltage for power purposes on the mines, viz. 500/550 volts between phases or ± 300 volts to earth, the maximum current flow with a minimum body resistance of 500 ohms between phases would be ± 1 ampere, and line to earth ± 600 milliamperes.
Much smaller currents than these can, however, be lethal, as is shown below.
The effect of passing current for an unlimited length of time through the human body is, in the present state of knowledge, as follows:
A current of 1 milliamp.- Threshold of perception.
A current of 10-15 milliamps.-Tightening of muscles and difficulty in releasing any object gripped. Acute discomfort.
A current of 25-30 milliamps.-Extension of muscular tightening to the thoracic muscles; dangerous if not quickly stopped.
A current of over 50 milliamps.-Fibrillation of the heart which is generally lethal.
Perception Level
Tests were conducted on human subjects by Dalziel (4) to establish the threshold of perception, i.e. that value of current at which one first perceives the tingling sensation due to passage of an electric current. He found that for subjects holding a small copper wire, this value was of the order of 1 milli-ampere. When contacting the tongue the perception level was found to be about 45 microamperes.
At first sight, it would appear that there is no danger constituted y currents of this low order. If, however, one considers the case of an artisan working on structural steelwork at some height above the ground when using an electric drilling machine, a leakage of current just above the perception level could cause him to lose his balance and fall to the ground.
Death or serious injury, in this case, would be due to the fall and not as a result of the primary effects of electric shock.
'Let-go Current’
With increasing alternating current the sensations of tingling give way to contractions of the muscles. The muscular contractions increase as the current is increased. Sensations of pain develop, and voluntary control of the muscles that lie in the current pathway becomes increasingly difficult. Finally, a value of current is reached for which the subject cannot release his grasp of the conductor. At this point, he is said to 'freeze' to the circuit. The maximum current a person can tolerate when holding a conductor in one hand and still let go of the conductor by using the muscles directly stimulated by that current is called his 'let-go' current. Experience has shown that an individual can withstand, with no ill-effects, except possibly sore muscles, repeated exposure to his let-go current for at least the time required for him to release the conductor.
Let-go currents were determined by Dalziel (4) for 34 men and 28 women. In these tests, the subjects held and then released a test electrode consisting of copper wire. The circuit was completed by placing the other hand or barefoot on a flat brass plate, or by clamping a conductive band lined with saline-soaked gauze on the upper arm.
The average value, or let-go threshold, was established at 15.87 and 10.5 milliamperes for men and women respectively. Dalziel calculated that 99.5 per cent of men would be able to release at 8.8 milli-amperes.
Muscular Contraction
At higher values of current, the attendant increase in muscular contractions causes these to progress up the arm to the chest until they become so severe that the victim is unable to breathe. Obviously, if the current flows for more than a few minutes, death may result from asphyxiation. However, if the circuit is interrupted in a reasonable time, breathing resumes automatically, and no serious after-effects result.
The uninterrupted value of current necessary to cause this condition is about 25-30 milliamperes.
Ventricular Fibrillation
As fibrillation of the heart is the major cause of death in electric shock fatalities, we shall consider this condition in detail.
Ventricular fibrillation is an unco-ordinated asynchronous contraction of the ventricular muscle fibres of the heart in contrast to their normal co-ordinated and rhythmic contraction. It results from an abnormal stimulation rather than from damage to the heart. In the fibrillating condition, the heart seems to quiver rather than to beat; after a short period of time the pumping action of the heart ceases; failure of circulation results in an asphyxial death within a few minutes.
Let us consider a normal electrocardiogram (Fig. 1).
The normal heart cycle lasts for around 0.75 seconds of which the critical or "T phase" during which ventricular fibrillation occurs lasts for about 0.15 second towards the latter end of the cycle.
For electric shocks lasting more than one complete heart-cycle and of a value of current above the fibrillation threshold, ventricular fibrillation is certain to occur. For shocks lasting less than one heart cycle, fibrillation will occur if the shock coincides with the critical 0.15 second 'T phase.'
Work on human subjects in order to ascertain the value of current at which ventricular fibrillation occurs is obviously impossible and the resort had to be made to experiments on animals. This work was conducted by:
Ferris, King, Spence and Williams in the United States of America in 1936 (1).
Kouwenhoven, also in the United States of America, in 1959 (2).
Kayser, Raule and Zink in Germany (3).
The Ferris Group specialized in tests on sheep and the Kouwenhoven and Kayser groups experimented with dogs.
Charles Dalziel (4) analysed the results of these researchers, statistically, to produce a probability curve such that one individual in a group of 200 subjects, that is t per cent of a large group, might suffer ventricular fibrillation if subjected to shocks below the limit of this curve. In other words, 99.5 per cent of the subjects would not suffer ventricular fibrillation.
Figure 2 shows the 0.5 per cent curves of current/ time derived from the experiments as well as Dalziel's predicted 0.5 per cent limit curves for the animals in question. The duration of the cardiac cycle for dogs and sheep, viz. 0.31 and 0.47 respectively, are also shown in the curves.
From his analysis of the experimental results of tests on animals, Dalziel states 'fibrillating currents at least should be roughly proportional to the size and weight' and, working on the bodyweight of a man as being 70 kilograms (+ 150 lb) he arrives at a formula for the threshold of ventricular fibrillation for 99.5 per cent of human beings as:
where i is fibrillating current for a man (milliamperes), t is shock duration in seconds.
The curve obtained from applying Dalziel's formula is shown as (1) in Fig. 3.
In a report of experts emanating from a meeting in Geneva during October 1961, the following conservative formula was proposed:
This is plotted as (2) in Fig. 3. In their publication Electric Shock as it Pertains to the Electric Fence, Underwriters Laboratories Inc. in the United States publish a derived curve of contact time versus minimum fibrillating current for a two-year-old child. This is derived from an analysis of the results of the work done by the Ferris Group and relating the heart and body weights of a two-year-old child as well as allowing for the difference in the cardiac cycle of the child and that of the animals on which the tests were conducted.
This is shown as (3) in Fig. 3.
The Ferris Group applied 913-30 millisecond shocks at 60 cycles/second frequency to 132 sheep, the current path being between the right fore-limb and left hind-limb.
Figure 4 shows the results of these tests for current applied during the partial refractory phase (T phase) of the cardiac cycle.
As is seen from the figure, for short-duration shocks the susceptibility of the heart to fibrillate increases with the increasing current until a most dangerous current is reached-then the susceptibility decreases. The explanation of the phenomenon is that very high currents paralyze the nerve centres in the heart, the heart is contracted and silenced and fibrillation is prevented. Death is inevitable if the shock is of appreciable duration; however, if the shock is of short duration, and if the heart has not been damaged, interruption of the current may be followed by a spontaneous resumption of its normal rhythmic contractions. This is offered, by Dalziel, as an explanation for frequent accident cases in which victims apparently withstood relatively high currents.
This phenomenon is the basis for countershock, or defibrillation shock treatment to arrest ventricular fibrillation.