ECX4234 – Session 5 – Electric Shock
Lalith A. Samaliarachchi
Session 5
Electric shock
Contents
5.1 Electric shock
5.2 Time/current zones on the human body
5.3 Human body resistance
5.4 Direct and indirect contact
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Electric Shock: Understanding the Dangers and Time/Current Zones, Lecture notes of Electrical Engineering
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2012/2013
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Lalith A. Samaliarachchi
Session 5
Electric shock
Contents
5.1 Electric shock 5.2 Time/current zones on the human body 5.3 Human body resistance 5.4 Direct and indirect contact
Lalith A. Samaliarachchi
Aim
The aim of this session is to teach you about Electric Shock.
Lalith A. Samaliarachchi
Discussion
I am almost sure you would not have been able to answer that question. You don't have to worry. None of the others would have answered the question either. I would be happy if you had just tried to give it a thought. What we mean by ventricular fibrillation is a condition whereby the heart ceases to periodically contract/expand, but random twitching of the numerous muscular fibrils occurs. A normal contraction of the heart causes the blood circulation. Thus from the point of view of the blood circulation, a fibrillating heart means a cessation of the normal heart activity and instantaneous death. We have seen that it is the electrical energy absorbed by the body tissues that causes the electric shock. Now this energy is not an easy quantity for us to measure. How then do we know whether we could get a shock from an electrical installation or not? It would be better if we could express it in terms of either the voltage appearing across the body or the current through the body together with time. Obviously, all these are interrelated to some extent as energy is proportional to the product of voltage, current and time. Also the current is related to the voltage by the resistance of the human body. Question Is there any safe voltage or current?
Lalith A. Samaliarachchi
Discussion
You may have said that a power frequency voltage of 50V or less is safe or that a power frequency body current of 30mA or less is safe. You may even have said that a body current of less than l0mA or even 0.5mA is the safe current, or you may not have been able to guess at all. Now I would not say that any of the above answers are completely right or completely wrong. In the past it was thought that what produced an electric shock was voltage and that voltages less than 50V is inherently safe. Studies on the effects of electricity have shown that this is not really true and that it is the body current and the duration of flow that determines the extent of electric shock. Under normal and dry conditions a voltage of 50V or less would only cause a tingling feeling on most people. However there are recorded instances of people who have died at voltages much below 50V. You may have said 30mA (or 25mA) as the residual current circuit breakers used in normal domestic situations have this as the operating current. There rccbs would provide adequate safety under normal dry conditions, because they operate within a tenth of a second when this unbalance current is exceeded. However if body currents of 30mA are allowed to flow through for an indefinite period of time, respiratory trouble could eventually occur. You would perhaps have had a friend who had got a shock and who could not release the live part on his own. This is one of the things that occur when the current through the body exceeds a certain value. We call the minimum value at which this occurs as the threshold of let go. For an average person, this value is about l0mA. Upto about this value a person can withstand the current for an indefinite period without any permanent effect. A current of 0.5mA is thought to be the value below which there is hardly any sensation and thus is completely harmless. Even currents above 30mA could be safe provided the duration of the current is sufficiently small. Thus it is necessary to have a time/current characteristic on the human body showing the various degrees of shock.
5.2 Time/current zones on the human body
As we have discussed earlier, the degree of danger of electric shock for the victim is a function of the magnitude of the current, the parts of the body through which the current passes, and the duration of current flow. So a low current (above the threshold of let go) can be just as dangerous as a high current for a relatively short period. IEC publication 60479-1 updated in 2005 defines four zones of current-magnitude/time-duration, in each of which the pathophysiological effects are described (see figure 5.1). Any person coming into contact with live metal risks an electric shock. Curve C1 shows that when a current greater than 30 mA passes through a human being from one hand to feet, the person concerned is likely to be killed, unless the current is interrupted in a relatively short time. The point 500 ms/100 mA close to the curve C1 corresponds to a probability of heart fibrillation of the order of 0.14%.
Lalith A. Samaliarachchi 0.05 500 V 50 V 25 Ventricular interference, pain Respiratory difficulty 0.1-0.3 1000-3000 V 100-300 V 100-900 Ventricular fibrillation. Can be fatal 6 60000 V 6000 V 400, Sustained ventricular contraction followed by normal heart rhythm. Temporary respiratory paralysis and possibly burns.
5.3 Human body resistance
I mentioned to you earlier that voltages less than about 50V are generally regarded as safe (that is not falling onto zones 3 or 4). Now how does this compare with what we just learnt about the time/current characteristic for the human body? In order to find this relation, we need to know the relation between voltage and current, that is the body resistance. Question Do you have any idea of what sort of value the human body resistance has? And whether it is a constant?
Lalith A. Samaliarachchi
Discussion
I will give you the answer very briefly first, and then go on to expand on it, later. For an ordinary person, dressed and wearing shoes, and in a normal environment, the body resistance from hand to foot varies from about 1 kΩ to 2 kΩ so that an average value of about 1.5 kΩ may be taken. Thus a voltage of 50 V would correspond to 50/1.5 or 33mA, which is typically the value (30mA) that we considered earlier. However for a person sitting in a bath, it is generally agreed that the minimum resistance is around 500 to 600W. Under these conditions, 50V would correspond to a current of about 100mA, which is not safe. We will now look into the question of body resistance in more depth. There are two major components of body resistance. The first is the skin resistance and the second is that of the internal organs. The electrical resistance of the skin varies from one part of the body to the other, and also depends on conditions such as temperature, humidity, time of day and year, and so on. The least resistance, generally, is on the forehead, hands, soles and armpits. The order of resistivity for the skin is 200 W. The resistance of nerves and blood is much lower than the skin resistance. It is about 2Ω for the blood and 0.02W for the nerve trunk. In practice, we do not calculate the body resistances from the above but take a typical value, such as 1kΩ, as a standard value. The body resistance is also not a constant, but is highly non-linear. This nonlinearity is also to our disadvantage, as the resistance decreases with increase in voltage. For example, the human body resistance (under dry conditions) is in the region of 20 kΩ at less than 10 V, 4 kΩ at 50V, 1.5 kΩ at 100V, 1 kΩ at 200V and 0.5 kΩ at 500V. There is also a fair variation depending on the path of current. One Research by the name of P. Osypka has shown that, under the same conditions, the resistance of the hand-body-hand current path is about 1.36 kΩ, of the hand-body-legs path is about 0.97 kΩ, and of the both hands-body-legs path is about 0.67 kΩ Question By using the time/current diagram of figure 5.1, state the zones to which the following currents fall.
- a current of 100mA for l00ms
- a current of 100mA for 400
- a current of 100mA for 2 s
- a current of 30mA for 2 s
- a current of 10mA for2 s
Lalith A. Samaliarachchi
Discussion
Examination of the definitions shows that both relate to the contact of persons or livestock with certain parts which may result in an electric shock. However we see that direct contact relates to live parts whereas indirect contact relates to exposed parts, which are conductive but normally not live (such as a frame of an equipment), which are made live by a fault. The illustrations shown in figure 5.2(a) and (b) show you an example for direct contact and indirect contact. Figure 5. Question Look at figure 5.1 again. You will notice that unlike curves 'a' and 'b', curves 'c 1 ', 'c 2 ' and 'c 3 ' exhibit a sudden change in shape mid section. You will notice that these curves are almost vertical either above about 1000 ms or below about 200 ms. can you think of any possible reason for an abrupt change from about 50mA to 500mA in this region? Earth Earth LIVE NAUTRAL
(a) Direct Contact (b) Indirect Contact
Lalith A. Samaliarachchi
Discussion
It is basically due to the behaviour of the heart. The normal heart beats at about 72 times per minute or 1.2 times per second and, after exercise may be even doubles this. Thus under normal conditions the heart period is about 833 ms and going down even to 417 ms after exercise. There are also other reasons for changes in the heart beat and the heart period. The chances of ventricular fibrillation also increase dramatically when the shock duration exceeds one heart period. You can see that the region of abrupt change about 200 ms to 1000 ms) is enclosing the above range and slightly on either side. Self assessment question A circuit with a resistance 0.06 Ohm is protected by a 30 A semi enclosed ceramic fuse to BS3036. The external earth fault loop impedance to this circuit is 0.5 Ω and the source voltage is 240 V at 50 Hz. In case of a ground fault of effective resistance 1 Ω is occurred across the load, using suitable assumptions and calculations, determine:
- The fault current
- The time taken for the fuse to blow
- The prospective fault voltage across the fault
- The current that may flow through a typical human body if a person comes into contact with the fault at the load
- What form of a shock is likely to happen to the person
- If the above fuse is replaced by: 30 A cartridge fuse to BS A type B mcb to BS EN 60898. What would be the operating time in each case? BS3036 and BS1361 fuse characteristics could be obtained from IEE wiring Regulations 17th^ edition pages 244, 245 & 246. You may also use Time/Current zones characteristics (15 -100 Hz) on persons figure 5. Do not worry much about the term External earth fault loop impedance at this time of the course. We will discuss/understand it in details in latter part of this course