Shock current

Ground or crushed rock coverings, about 80-150 mm thick, are useful to slow the evaporation of soil moisture and hence retain the moisture of the topsoil layers. It will also diminish the intensity of shock currents due to higher contact resistance between the feet and the soil. Typical values may vary from 1000 to 5000 Qm. 

Reynolds H, Hochman J. Cardiogenic shock current concepts and improving outcomes. Circulation 2000 Feb 5 117(5) 686-697... 

Although there are reports of spontaneous reduction of intussusception (Swischuck etal. 1994 Navarro and Daneman 2004a), chronic and recurrent intussusceptions, and rare cases of spontaneous sloughing of the gangrenous intussusceptum through the rectum (Ravitch 1986), the usual course of an untreated intussusception is bowel obstruction followed by bowel perforation with peritonitis and septic shock. Currently, the overall perforation rate in developed countries is low (0%-3%). 

If the body becomes part of a more powerful external circuit, such as the electrical mains, and current flows through it, the body s normal electrical operations are disrupted. The shock current causes unnatural operation of... 

If the body becomes part of a more powerful external circuit, such as the electrical mains, and current flows through it, the body s normal electrical operations are disrupted. The shock current causes unnatural operation of the muscles and the result may be that the person is unable to release the live conductor causing the shock, or the person may be thrown across the room. The current which flows through the body is determined by the resistance of the human body and the surface resistance of the skin on the hands and feet. 

The severity of an electric shock is the product of the current value and the time for which it flows through the body. Thus a 50 mA shock current (i.e. 0.05 A) could probably flow through a body, without much danger, for up to 4 seconds whereas a 500 mA current (0.5 A) flowing for only 50 ms (0.05 s) could be fatal. 

Personal sensitivity to electric shock varies somewhat with age, sex, heart condition, etc., but for an average person the relationship between shock current, and time for which the body can accommodate it, is given by a formula of the following kind ... 

Kuhl, A.L., Chien, K.-Y., Ferguson, R.E., Collins, J.P., Glaz, H.M., and Colella, P., Simulation of a Turbulent, Dusty Boundary Layer behind a Shock, Current Topics in Shock Waves, edited by Y. Kim, American Institute of Physics, New York, NY, 1990 (in press). 

TABLE 22.1 Electric Shock Current Levels and Physiological Effects (AC 60 Hz)... 

Ironically, the treatment for cardiac arrest induced by an electric shock is a massive countershock, which causes the entire heart muscle to contract. The random and uncoordinated ventricular fibrillation contractions (if present) are thus stilled. Under ideal conditions, normal heart rhythm is restored once the shock current ceases. The countershock is generated by a cardiac defibrillator, various portable models of which are available for use by emergency medical technicians and other trained personnel. Although portable defibrillators may be available at industrial sites where there is a high risk of electrical shock to plant personnel, they should be used only by trained personnel. AppHcation of a defibrillator to an unconscious subject whose heart is beating can induce cardiac standstill or ventricular fibrillation, just the conditions that the defibrillator was designed to correct. 

The severity of an electrical shock depends on a number of factors, the most significant of which are the combination of the magnitude and the duration of the flow of shock current through the body. 

Thus an RCD will not prevent electric shock because shock current must flow through the body to cause sufficient out-of-balance between the conductors to be detected by the device. However, their sensitivity and speed of operation will limit the current flow to a few fractions of a second, thus making the shock more survivable. A typical current level that will result in operation of the device (tripping current) is 30 mA (0.03 A), with operation taking place within 40 ms (0.04 s). The design and construction parameters for RCDs are specified in BSs. ... 

The phase voltage at the substation transformer will be a little higher than 230 V to allow for the inevitable voltage drop in the distribution cables. In urban areas, the line/neutral and the line/earth loop impedances will be comparable and will probably be only a small fraction of an ohm, whereas the victim s hand-to-hand impedance will be in the order of 2000 ohms. Under these circumstances the effects of the circuit impedances can be ignored. The victim s touch voltage will be about 230 V and, for a total body impedance of 2000 ohms, the shock current would be 230/2000 = 0.11 A. This is high enough to cause ventricular fibrillation in many people should the current flow for about 0.5 s. 

The nature of the fault described means that the touch voltage is in the order of 230 V. However, many indirect contact shock accidents occur at less than mains voltage. This can be quite fortunate for the injured person because the shock current will be lower, thereby reducing the adverse effects and improving their chances of being able to let go of the conductors and survive the incident. 

Although the scaffold pole was a poor earth electrode, having a considerable earth resistance, it allowed sufficient current to flow through the labourer to cause fatal electrical injuries. Typical resistances would be 400 ohms for the earth connection and 2000 ohms for the labourer s body, resulting in a shock current of 230/(400 + 2000) = 0.096 A. A hand-to-hand current of 96 mA is sufficient to cause ventricular fibrillation. 

If it is not possible to establish an equipotential zone because, for example, there is a conducting floor (such as a concrete floor) or because equipment is being used outdoors, supplementary or alternative measures must be taken. The use of residual current circuit breakers to provide sensitive earth leakage protection, in addition to the overcurrent protection, is one acceptable option the RCD would detect earth fault currents, including shock currents flowing to earth, and rapidly interrupt the circuit. Indeed, it is general practice to ensure that socket outlets that will foreseeably be used to supply external equipment should have RCD-protection fitted. 

Protection by limitation of energy is in subsections 411-04 and 471-03. The intention is not to prevent the shock sensation but to limit the shock current and/or its duration so as to avoid injury to persons and animals. Common examples of equipment complying with this requirement are electric fence energisers, electrostatic paint and powder sprayers, and tungsten inert gas (TIG) welding electrodes. 

Ventricular Fibrillation Because only a small amount of current is required to disrupt the natural rhythm of the heart, ventricular fibrillation is considered the most dangerous electric shock hazard. The shock current needs to pass through the heart during the phase when the ventricles are starting to relax after a contraction (Lee 1966). When fibrillation occurs, the effective pumping action of the heart ceases, the pulse disappears, and death usually occurs within minutes. The lower boundary of the threshold of ventricular fibrillation is generally considered to be 50 mA. 

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