Any form of energy, when not properly controlled or harnessed, can result in serious danger to those who use it. The dangers inherent with electricity can generally be divided into two categories: direct and indirect. The direct danger is the damage that the electricity itself can do to the human body, such as stoppage of breathing or regular heartbeats, or burns. The indirect dangers include the damages that can result to the human body as a result of something caused by electricity, such as a fall, an explosion, or a fire. 

INTRODUCTION

The main purposes of this article are to provide the reader with an awareness of the overall dangers of electricity, and to provide some insight into the physiological effects of electrical shock on the human body. The dangers inherent with electricity can generally be classified as either indirect dangers or direct dangers. Although indirect dangers will be discussed, the main part of the article will focus on the physiological effects of electrical shock on the human body.

INDIRECT DANGERS

The storage and use of electricity is often associated with sparks--electrical or electrostatic discharges. For example, motors with brushes have short arcs between the brush tips and the moving rotor constantly during operation; relays and switches open and close with a small arc between the contacts; plugging in an appliance which is already turned on causes a small spark; connecting a battery to a device which is turned on will also cause a small spark. And finally, there is the common discharge of static electricity that occurs when a charged body comes into contact or near-contact with a body at a different potential. In most circumstances, such sparks are not a problem. However, if the atmosphere in which the spark occurs is laden with fine particulates, or a flammable gas, one spark can set off an explosion and/or a fire. There are numerous examples of such incidents (1, 2, 3); reference #3 tells the all-too-common story of an explosion caused when a man was using volatile chemicals in a basement and a spark ignited the fumes.

Spark plugs are probably the most common example of an explosion ignited by an electric discharge. In this case, the explosion is intended and occurs under carefully controlled conditions. However, electric or electrostatic discharges can be very dangerous in many less-controlled environments. These environments include grain elevators, paint-spraying booths, explosives and fireworks facilities, fuel storage facilities, coal mines, and many others. If there are fine airborne particulates of combustible material, or vapors of volatile compounds, the conditions are ripe for ignition and subsequent explosion.

Another indirect danger associated with electricity, and directly connected with the above danger of explosions, is the danger of fire. Not all explosions ignited by an electric spark result in a subsequent fire, but many do. So, even if a person survives the initial blast, unless they are removed from the area, they could be injured or killed by a subsequent fire.

A final indirect danger associated with electricity is associated with one of the physiological responses of the human body to electric shock, and is the hazard of being involuntarily moved by the electric shock. The body’s muscles contract when they receive a small electrical signal from the brain through the nerve system. External electrical signals, such as those resulting from an electric shock, can also cause muscle contraction, as demonstrated long ago (1791) by Luigi Galvani and his experiments with frogs and electricity (4). These externally caused muscle contractions are involuntary, and in many cases can be violent. For example, a worker near Bristol, England was digging near a buried  electric cable, and was thrown over three meters when he accidentally sliced through the cable (5). Fortunately, he survived. But if he had been thrown against moving machinery or against some sharp object, he would not have been so fortunate. The force that threw him over three meters was not the electricity itself, but the reaction of his muscles to the electrical stimulus, involuntarily contracting and throwing his body.

DIRECT DANGERS

The direct dangers associated with electricity are primarily divided into burns and cardiac effects. The former danger can readily be modeled when considering the body as a conductor of electricity. The latter danger is much more complex, and involves an understanding of the normal operation of the heart and the interfering action electricity can have on it.

Burns

The flow of current through a resistive material is always associated with the production of heat. This heat is proportional to the resistance of the material and the square of the current, according to the equation P=I²R, where P is the power in watts, I is the current in amperes, and R is the resistance in ohms.

The primary resistive material of the human body is the epidermis, the layer of dead skin cells that lies on top of the dermis. Normally, this layer of skin is relatively dry, and the cells themselves are also dry, having died and released their moisture. Thus, this layer of skin provides a reasonable amount of resistance to the flow of electricity, generally from about 1 kS up to about 100 kS. The wide variation is a function of the ambient humidity, the individual’s production of body oils, and exterior emollients that may have been added to the skin, such as lotion. It also depends on the degree to which the skin is compressed; greater compression forces the dead skin cells into contact with each other and reduces the resistance of the skin.

Once electricity enters the body through the skin, it encounters very little resistance due to the electrolytes (conductive fluids) contained within the body. Most of the fluids within the body are electrolytes, and vary in resistance from milli-ohms to only a few ohms.

 If the body comes into contact with a source of electricity, the amount of current that flows is proportional to the voltage, and inversely proportional to the resistance, according to the well-known Ohm’s Law: I=E/R, where I is the current, E is the voltage, and R is the resistance. Due to the large difference between the resistance of the skin compared to that of the inner body with all its electrolytes, the major limitation to the flow of current is the skin. This would also be where the greatest heating occurs, since the heat is proportional to the resistance. In applications where electricity is purposely applied to the body, such as in defibrillation, electrosurgery, electromuscular stimulation, or electrocutions, great care is taken to reduce the resistance of the skin by applying conductive gels to both the skin and the electrodes. Without such gels, properly applied, significant skin burns often result. For example, reference #6 describes the care that must be taken to prevent such burns during electrosurgery, even when the currents are as low as 200 mA. Reference #7 describes a “liquid and potash solution” used to lower the resistance of the skin for the electrocution of death-row inmates.

The nature of this heating is such that if the amount of heat produced by the flow of electrical current exceeds the heat escape paths, a significant temperature rise occurs at the interface between the body and the electrode. Heat escape occurs primarily through the conductive and convective action of body fluids moving in the body, particularly the blood. A minimal amount of heat escapes by convection to the air; heat escaping by radiation is negligible. The ability of the body fluids to remove heat is relatively low, partly due to their relatively low thermal conductivity, and partly due to the relatively low flow rate (convection) of these fluids. This allows a rapid buildup of heat on the epidermis, which is thermally conducted to the lower layers of live skin cells, causing the skin burns. At prolonged higher current, burns in the inner tissue result, particularly the muscle tissue.

As serious as these burns are, they are generally not the primary cause of death in victims of accidental electrocution. But they can be the cause of serious subsequent infections which can occasionally prove fatal.

Cardiac Effects                       

The effects of electricity on the human heart are generally the most serious considerations when dealing with electricity, because this is how most victims of accidental electrocution die. Table 1 (8) summarizes the effects of electric current on the human body; note that currents less than 5 mA are generally imperceptible, and that currents above 100 mA are the lethal currents. Currents in excess of 6 A can cause severe burns and associated trauma; currents above 20 A can physically dismember the body. Currents between 100 mA and 1 A are the most dangerous to the heart, and voltages between 50 V and 240 V are those that can readily produce these currents, if the skin is wet. According to Ohm’s Law and the low 1 kS value of the resistance of the skin, a 50 V source produces only 50 mA through the body, which is painful but generally not deadly. However, if the voltage is 120 V, the current becomes 120 mA; at 240 V, the current is 240 mA, both of which are right in the range of the currents most dangerous to the heart.

Table 1: Physiological Effects of Various Current Intensities

The danger is that these electrical currents will interrupt the normal electrical signals of the body that cause the rhythmic contractions of the heart muscle. When this happens, the heart enters a state of fibrillation, which is essentially the ineffective random quivering contractions of the heart, rather than the rhythmic full contractions that pump blood. If fibrillation is not overcome within a matter of 3-5 minutes, the victim will die.

 A frequent question that arises is why some people are relatively unaffected by currents between 100 mA and 1 A. Research on the heart has shown that ventricular fibrillation as a result of electric shock is also a matter of timing (9). The contraction cycle of the heart proceeds through various phases, each of which occupy a different amount of time. Although it is possible to induce fibrillation during each phase, the difficulty of doing so is dramatically lower during the reset portion of the systole phase. If a lethal amount of current enters the heart during this phase, there is a very high likelihood that the heart will go into fibrillation. The odds of being shocked during this phase of the heart cycle are approximately 20%. Thus, the majority of people shocked by a lethal amount of current will live through it, but it is truly a game of Russian roulette.

Current Path Effects

Finally, there are the effects of the path of the current through the body. Some people have been struck by lightning and the main current path stayed on the outside of the skin; they were fortunate and were not killed. People who work around 120 V or 240 V are instructed to keep one hand behind their back; this prevents them from accidentally putting one hand on a live wire while the other hand is grounded. If they were to make contact in this way, the path for the electricity would pass through the heart, the most dangerous path possible. If one hand is kept behind the back, accidentally contacting a live wire will not pass current through the heart, and the current is then much less dangerous.

COMMON SAFETY QUESTIONS

Given the understanding of the previous sections, some common questions arise about safety with electricity. Some of these, along with their answers, follow.

1. Q: If it is current that is dangerous, why do the warning signs say, “Danger! High Voltage!”? A: The answer is Ohm’s Law; if the voltage is low (generally <50V), it is incapable of producing sufficient current in the human body under normal conditions, and therefore is safe. However, if the voltage is high (generally considered >600 V), it is always very dangerous, and often fatal, because the resulting currents are so high. The high voltage is what makes the high (lethal) currents possible.

2. Q: The 12 V of an automobile battery is greater than the 9 V of a small 9-V battery, yet I can feel the 9 V on my tongue, while I cannot feel the 12 V in my hands if I hold the bare jumper cables. Why is that?  A: The answer is again Ohm’s Law. The resistance of the skin on the hands is much higher than that of the tongue, plus the tongue is wet, which further lowers the resistance. Looking over Table 1, we can see that the sensations experienced on the tongue with 9-V batteries mean that the current is in the range of 30 mA for fresh batteries, down to <5 mA for deader batteries. This means that the resistance of the tongue is about 300 S, resulting in 30 mA at 9 Volts, and only 10 mA at a relatively dead voltage of 3 V.

3. Q: At the county fair, I saw a person sit in an electric chair. When the voltage was turned on, they were able to touch and light up a flourescent tube held by an assistant. They were unaffected by the experience. How can this be?  A: Flourescent tubes are lit up by high voltages and low currents; for example, a 20-Watt higher-voltage flourescent tubes is lit up by 4 kV; according to the power formula, I=P/E, the current required is 20 W / 4 kV = 5 mA. This low of a current is barely perceptible to the person sitting in the electric chair.

4. Q: While using an arc welder one time, I felt a mild shock as I knelt on the workpiece. These arc welders are powered by 240 V and are capable of delivering well over 100 A; how could I have survived?  A: Arc welders DO deliver very high amperages, but they don’t need the high voltages for the welding, since there is a short circuit and thus very low resistance. The welder steps the voltage down to roughly 40 Volts, which is why you felt a tingling; however, 40 Volts is insufficient to cause bodily injury under most circumstances.

5. Q: Pure water is a relatively good insulator. Why, then, is water such a concern when discussing safety with electricity?  A: Although it is true that pure water is a reasonably good insulator, water picks up and dissolves impurities quite readily, and these impurities impart to the water a dramatically lower resistance. Also, when skin is exposed to water, especially for any extended period of time, the outer layer of dead skin cells becomes saturated with the water and the normally high resistance of this skin layer drops significantly. Water also increases the surface area over which the electricity can contact the skin, since the water fills all the air gaps normally present between the cells. The increase in surface area also lowers resistance to electricity.

CONCLUSION

The indirect dangers of electricity, although very significant, are generally much less hazardous than the direct dangers. This is because the exacerbating conditions necessary for these indirect dangers are much less common than the simple conditions necessary for simple exposure to electricity.

            The primary danger of direct exposure to electricity is coronary fibrillation, a condition which is quickly fatal if not reversed. Since we can do little to control the voltages in our homes and businesses, the only way to lower the current that enters our body if we accidentally are exposed to electricity is to keep our resistance high. This we can do by always wearing shoes and by staying dry while working on or near electricity.