It is my intention to highlight, with this article, the great danger of electricity, today too often underestimated due to the lack of a proper and continuous training for both overconfidence and carelessness.
The following article is dedicated especially to young professionals and it is the result of ongoing professional training and, in particular, of personally experienced situations. This article is the result of several years of direct experience in electronic factories and on construction sites and not the result of a short training course.
In many years of electrical engineering and electronics I had the opportunity to attend many incidents and related first aid; for example, work tools such as scissors, screwdrivers, pliers, DIN rails that went inadvertently in contact with live parts especially inside electrical panels, or even tetanization and burns due to electrocution and electric arc.
So I have witnessed more or less serious accidents, most of which occurred unforeseeable circumstances such as what happened to me. At the beginning of the profession, I have had to deal with a discharge of about 1800 V dc (anode voltage of a high power amplifier, in a radio frequency cavity) not by direct contact but by proximity. This incident occurred in an excessively humid environment and as a result I had a numb arm and blocked for about 10 minutes due to muscle contraction. Fortunately, the contraction took place in order to remove my hand from the voltage and the result was a very small hole on the index finger of the hand and a hot arm, but this was enough to pay a constant attention from that moment on, especially in those cases of apparent security, where, wrongly, any form of precaution is not considered necessary.
Causes and effects
In our country (Italy), about 5% of injuries caused by electricity have a fatal outcome. Most of the home injuries takes place in the kitchen and/or bathroom where is more frequent a contact with water. Construction sites have a high percentage of accidents caused by electricity and about 20% of the fires are a result of electrical faults. Furthermore, many other injuries have electrical origin but are not included in the statistics because they are classified according to the agent who caused them such as: falls from electrical contact; death due to malfunction of industrial electrical equipment; explosions in places with explosive materials whose source of ignition has an electrical origin, and so on.
When you need to perform some operations in presence of electric current, you have to keep in mind that the injury is constantly lurking and substantially may occur at any moment:
- for reasons of force majeure, when the cause of the accident is unknown and not predictable;
- fortuitosly, it is practically the calculated risk accepted by the “workmanlike manner”and referred by the CEI as: “No rule, no matter how carefully designed, can absolutely ensure the immunity of persons and property from the dangers of electricity”.
The dangers of electricity for the people, can be classified into three types:
- effect of the electric arc (blindness, heat);
- indirect effects (falls, fires);
- electrocution (passage of current through the body).
Electric Arc
The electric arc is manifested as a source of heat very intense and concentrated, resulting in emission of toxic fumes, gases and a projection of incandescent particles. It manifests itself especially in the case of operations on machines for sectioning and control command or during a short circuit. The arcs from sectioning and control equipment in high voltage are particularly dangerous.
When the electric arc occurs on equipment for sectioning and control, is suitably dimensioned and channeled by paths in order to cool it and put it out. This type of arc is called controlled arc. This arc is similar to the arch used in metal welding by electrode; this type of arc can cause burns only due to inexperience. Much more dangerous and destructive it is the case of the electric arc as a result of a short circuit. This arc is generally underestimated because it is considered resolved through the application of the proper building techniques and dimensioning of cables and circuit breakers. In most cases, the short circuit is caused by events external to the system and outside of any forecast, such as a human error during maintenance, a cable sliced by an operating machine, wiring errors, etc..
For these reasons, the severity of a short circuit appears to be greater when the energy due to a fault causes an arc that does not developed in the components acts to interrupt it and then extinguish it, but it rather develops where the event is occurred accidentally, also to several tens of meters from the protections.
In this type of electric arc, three factors should be considered particularly hazardous:
- the heat that causes severe burns or, in the case of electrical panels, melting of metal parts such as bolts fixing and carpentry;
- strong currents that can destroy media and deform distribution bars, or even cause the death of an individual;
- the shock wave that can throw a person several feet away or warp panels and media in the case of electrical systems.
As a result of the foregoing, it is evident that a short circuit that develops an electrical arc in the air, turns out to be a dangerous event because the energy that radiates in fractions of a second, would not give a chance to the people around you.
Indirect effects
Indirect effects are those that do not depend exclusively on the current passing through the human body but on the reaction caused by this, such as falling from a scaffold as a result of accidental electrical contact, an explosion or fire due to electric sparks in environments where there are flammable materials, or accidents due to sudden power failure.
Electrocution
Electrocution or electric shock is the most common risk and is the most dangerous to the human body. In fact, when one or more parts of the body come into contact with live electrical components, an electrical discharge occurs, with consequent passage of a current through the whole body.
Electrocution occurs when the current has an entry point and an exit point in the body of the individual in contact. The entry point is the area of contact with the part in tension while the exit point is the area of the body that comes into contact with other conductors, thus allowing the circulation of the current within the body through a certain path. For example, if you accidentally touch a live part with a hand, but you’re isolated from ground by rubber shoes and there is no other contact with foreign matter, the conditions for the passage of current does not occur and therefore you shall not suffer electrocution. We have a similar example with birds that stay on the high-tension wires without suffering any damage.
Conversely, if the same circumstance occurs when you’re not isolated from ground, for example, if you are in bare feet, you will have electrocution with the current flowing from the hand as a point of entry, to the foot as the exit point. There are two modes in which the electrocution occurs:
direct contact. When a person touches directly a part ordinarily in voltage at the moment that it is made accessible. For example, electrical wires with the insulation damaged by abrasion, a holder, a terminal, a busbar, internal parts of an apparatus which has been removed from the protective cover, etc..
Direct contact may affect any two points of the human body, although the most common case is that which occurs between hands and feet with reclosing of the circuit through ground.
Those most at risk are usually the employees who work on construction sites where this type of contact is almost always due to excessive imprudence and almost always involves persons outside electrical work; a qualified electrician can indeed timely notice of danger and almost always able to limit the damage.
indirect contact. When a person comes into contact with metal parts, which, due to a fault in the isolation of certain components, are unexpectedly live parts. This type of contact is much more dangerous than direct because it involves equally all users of electrical equipment. it is virtually impossible to predict, especially when it concerns metal parts of appliances.
Damage caused
Normally, in the presence of an electrical accident, you are used to refer to the voltage as the cause of the damage. Actually, even if the process starts from the voltage, the electric current is the one that produces the damage directly through the human body and consequential damages are directly proportional to its value and durability of the contact.
The main pathophysiological effects that current flow can cause are:
– Tetanisation. It consists of the block involuntary of muscles when they are traversed by an electric current. The muscles do not obey the physiological electrical impulses sent from the brain and this causes an uncontrollable contraction. This contraction does not allow the person to detach from the power supply, the injured while being aware of the risk involved can not do anything to detach his body from the live part. In the case where a person is in contact with high currents, all muscles are affected by the phenomenon of tetanisation, not only those closest to the contact. Among them, also those of the lumbar bands and of the thighs are involved. This involves violent involuntary movements that can cause jumping even with falling several feet from the point of contact. The tetanisation generally manifests as tingling or painful shock with temporary paralysis of the muscles affected by the electrical discharge.
– Difficulties and / or stop breathing during the electrocution are caused by the same reasons that determine the tetanization. Depending on the current path, the muscles which allow the expansion of the rib cage contract preventing breathing. In these cases, you can have the phenomena of asphyxia with onset of cyanosis, so if assisted ventilation is not carried out immediately, the affected person is destined for death by asphyxiation; the phenomenon is reversible only if victim is rescued, even with artificial respiration, within 4 to 5 minutes.
– Ventricular fibrillation. It is the most dangerous effect of electrocution and is due to the overlap of the current contact with the internal physiological, which generates the uncoordinated heart contractions, causing loss to the heart rhythm. The muscles of the heart, called fibrils, contract and expand rhythmically at about 60/100 times per minute and are coordinated by an electrical pulse generator, the sinoatrial node. Ventricular fibrillation is reversible only if the heart is subjected to an electrical discharge very violent within five minutes. To achieve this purpose, the defibrillator is used, that is a medical device that applies an electrical pulse to the chest of the injured via two electrodes. The phenomenon of ventricular fibrillation is the major cause of most deaths by electrocution, and it is for this reason that every workplace, large or small, should be fitted with a special defibrillator with the presence of trained personnel;
– Burns are due to the strong heat developed by the current flow in the human body due to the Joule effect, the skin tissue is more sensitive to this phenomenon since it generally has a high electrical resistance (P =I2R). If the current intensity is very high, deep burns occur throughout the body and whole limbs (shoulders, arms, legs, etc..) san ne damaged. Burns can also be caused by arcs generated by electrical discharge produced by energized equipment. Arcs from high-voltage electrical equipment supplied are particularly damaging. Generally burns occur in the entry point and in the output point of the current from the body, precisely because the skin is the part that presents greater resistance and the consequences are directly proportional to the current intensity and the duration of the phenomenon.
Safety curves
The physiological effects of electricity on the human body does not depend on the intensity of contact voltage, but the resulting intensity of current through the body. The value of this intensity derived from the ratio of the contact voltage to the resistance of the human body to ground (I = V/R). The overall resistance of the human body takes changing values and there are many factors that contribute to define it, and among them there: age, sex, environmental conditions and physical propertie and the internal resistance which varies from individual to individual. The conditions of the skin such as sweat, abrasion, moisture, lead to a reduction of the resistance of the skin while the opposite occurs if in the area of contact the skin is hardened due to calluses. Other factors are the clothes worn, body mass, the surface of the electrical contact and the contact pressure.
In fact, a greater contact pressure corresponds a lower electrical resistance, this is the case when the work equipment is firmly supported and/or guided by the operator. Following tetanization, the muscles contract and not allow you to let go. For these reasons, the IEC standards require that such devices are class II double insulated. For example, at constant voltage of contact:
– A person walking barefoot on wet soil has a very low resistance so that the current value is very high resulting in grave danger;
– A person with insulated shoes on the floor partially insulating, presents a very high resistance for which the value of the current is low resulting in poor situation of danger.
Whereas each individual reacts differently to the passage of the current and that the path followed inside the body has a great influence, the physiological effects of electricity according to the intensity of current, for prolonged contact times and for alternating voltage the frequency of 50 Hz, are represented by the following safety curve current-time:
- Zone 1 up to 0.5 mA is not perceived and does not cause any effect whatever the duration of the contact. The value of 0.5 mA is considered as the “threshold of perception”;
- Zone 2 from 0.5 to 30 mA causes no harmful effects even if it is perceived, the value of 10 mA is considered as “danger threshold” and is defined as “release current”, ie the maximum current for which you are unable to detach from the grip due to the contraction of the muscles;
- Zone 2 from 30 to 200 mA harmful effects are closely related to the duration of contact, such as with current of 50 mA is a permissible contact period of not more than 0.1 s, the time of tolerability of the current decreases with increasing of the current;
- Zone 3 from 200 to 500 mA, the contact always causes harmful effects whatever the duration, you can have tetanisation, difficulty breathing, increased blood pressure, minor burns, light heart disease, especially if the current flows through the heart;
- Zone 4 over 500 mA, the contact always causes severe burns and ventricular fibrillation.
In practice, it refers to the limits of hazardous voltage, rather than the limits of hazardous current; these values are related by Ohm’s law by means of the resistance RB of the human body and the resistance REB of the person towards the ground. In obtaining the voltage-time curve of security we refer to the hand-foot path of a person who grabs an electric device with both hands and both feet on the ground. In series with the resistance of the human body, we assume a resistance of 1000 Ω REB in case the person is in ordinary conditions, for example inside buildings, and 200 Ω in the case the person is in particular conditions such as ooutside.
The curves obtained show a maximum voltage withstood by the human body without limit of time, a voltage of 50 V under normal environmental conditions and a voltage of 25 V in special circumstances. This value of voltage, 50 V and 25 V, takes the name of voltate of conventional contact limit UL. In the case of continuous current, the values of UL are 120 V for the ordinary conditions and 60 V for special conditions; these are the limits of tolerable voltage without the need for any form of protection, prescribed by the IEC 64-8 standard.
Regarding the current, its effect on the human body is considerably different because, unlike the alternating current, is not affected by the skin effect; this implies a greater tissue penetration, up to affect the vital ones. Overall, however, the human body suffers less damage equal intensity, with the passage of direct current instead of alternating current. This is because the frequency of 50 Hz causes the nervous contraction of muscles (tetanisation), while the DC current has mainly an effect of heating of the tissues affected by the electric discharge.
For the purposes of the danger of electrocution, the points of entry and exit of the electric current affect the possibility of crossing of the heart, and then the resulting probability of ventricular fibrillation. for this reason the IEC standards define a “factor of path F“, which indicates, for the same current intensity, the probability that the fibrillation is triggered as a function of the different paths followed by the current, considering as reference the path left hand-foot taken equal to 1.
- left hand – chest F = 1,5
- right hand – chest F = 1,3
- left hand – foot/feet F = 1
- right hand – foot/feet F = 0,8
- back – left hand F = 0,7
- buttocks – hand/s F = 0,7
- left hand – right hand F = 0,4
- back – right hand F = 0,3
The factor of path allows to obtain the value of the current Ih that have the same probability of triggering the fibrillation of a current Iref taken as reference, relative to the path left hand-foot with the relationship Ih = Iref/F (for example a current of 30 mA with reference to the path left hand-foot has the same effect of a current of 75 mA with path left hand-right hand, in fact Ih = 0.03/0.4 · 103 = 75 mA).
Another element that contributes to the danger of alternating electric current is the working frequency, the most commonly used is 50 Hz, but there are areas that use different values (eg electrotherapy, in aviation, welding, variable-speed motors, etc. ). In the moment of contact the human body acts as a conductor connecting two points, is crossed by an electric current. for the well-known phenomenon called skin effect (skin effect), the alternating electric current tends to concentrate on the surface of the conductor and its danger decreases with increasing frequency just because the current tends to pass only through the skin without affecting vital organs.
At constant voltage, increasing frequency another important phenomenon occurs, and is completely opposite to skin effect: the reduction of the impedance of the human body with a consequent increase of the current. However overall the danger of electric current decreases with increasing frequency, so that the voltage of 220V over the 500 Hz frequency of the danger of ventricular fibrillation is practically zero.
Precautions and recommendations
In case of shock, any rescuer must be very careful not to touch the injured person directly but must immediately cease to provide power at the main switch. Where it is not possible to interrupt the power to remove the injured from the source of electricity you can use wood or cloth, being careful that they are dry, and try to move it by holding the clothes.
After stopping contact with the electrical source, you should immediately check the condition of the person who may suffer from: burns; loss of consciousness; respiratory arrest and / or cardiac arrest. therefore it is appropriate to monitor the breathing and pulse, and if necessary artificial resuscitation. In case of injury from electrocution, keep in mind that if the person is not rescued within 4 or 5 minutes, may suffer irreparable consequences; in this case the timeliness is essential.
In case of burns, it is correct to behave in the following way:
a- if the person has no heart activity and not breathing, do not remove it and immediately begin artificial respiration and cardiac massage if necessary;
b- do not take off his clothes and especially not break blisters;
c- do not apply ointments or lotions;
d- coat the burned parts with dry sterile gauze, so as to prevent contact with the air;
e- keep the victim lying down with his feet raised with respect to the head;
f- in cases where the victim has not lost consciousness and is able to swallow, do not let him to drink any kind of alcohol, but only water with dissolved baking soda and salt content in the appropriate bags for burn victims present in the first aid equipment in places of work; in the case of domestic environment dissolve in a liter of water a tablespoon of baking soda and a tablespoon of salt;
g- Provide for the immediate transport at the nearest firt aid.
In all areas, but especially in the workplace, regarding the use of electrical equipment it is a good idea to adhere to the following recommendations:
a-to ensure the application of safety regulations;
b- be aware of the location of the main and/or area electrical panel;
c- often verify the proper operation of circuit breakers;
d- do not use electrical appliances near water or in the case of high humidity;
e- do not use equipment that does not comply with the rules;
f- do not use multiple plugs or multiple sockets maybe connected to other multiple sockets;
g- never use water to extinguish an electrical fire;
h- do not use equipment without elements that identify them with precision;
i- do not leave cables, extension cords, power tools abandoned on transit routes.
Conclusions
In case of utilization of electrical energy, you must keep in mind the following features that make it extremely dangerous, electricity is: INVISIBLE, ODORLESS, CLEAR, BUT ABOVE ALL QUIET AND FAST.