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Electric shock danger sign

Edvard

Do You Understand What Is Electric Shock?

Introduction

Electric current passing through the body, particularly alternating currentat power f requencies o f 50 Hz and

60 Hz, may disrupt t he nervous system, causing muscular reaction and the painful sensation o f electric sho ck

The most common reaction is to be thrown of f the conductor as a result o f the muscular contraction.

However, in a small number of instances, the consequence is death f rom cardiac arrest, o r f rom ventricular

f ibrillation (where the heart muscle beats in a spasmodic and irregular fashion) or f rom respirato ry arrest .

The psychological ef f ectsare largely determined by the magnitude and f requency of the current, the waveform

(for example, continuous sine wave, or half wave rectif ied sine wave, or pulsed waveform), its duration, and

the path it t akes t hrough the body.

An authoritat ive guide o n the topic is published in IEC 60479. The following text concentrates on the most

common s ituation of a shock from a continuous power f requency ac wavef orm.

The magnitude of the current is the applied voltage divided by the impedance of the body. The overall circuit

impedance will comprise the body of the casualty and the other components in the shock circuit, including tha

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Figure 16.1 - Dep iction of a typical indire ct contact electric shock

of the power source and the interconnecting cables. For t his reason, the voltage applied to the body, which is

commonly known as the touch voltage, will of ten be lower than the source voltage.

The impedance of the body is determined by the magnitude of the touch voltage (there being an inverse

relationship between impedanceand voltage) and other facto rs, such as the wetness o f the skin, the cross-

sectional area of contact with the conductors, and whether or not the skin is broken or penetrated by the

conductors.

As a general rule o f thumb, at an applied voltage o f 230 V at 50 Hz, the total body impedance fo r a hand-to-

f eet path will be in the range 1000 to 2500 f or most of the population, f alling to around 750 at voltage

in excess o f about 1000 V.

The path that the current takes through the body

has a signif icant ef f ect on the impedance. For

example, the impedance for a hand-to- chest path

is in the order of 50 per cent of the impedance

f or a hand-to-f oo t path. Moreover, the currents

path through the body is a significant

determinant of the eff ect on the heart.

Table 16.1 summarizes the physiological ef f ects

of current pass ing through the body.

The effects relate to a hand-to-hand shock

exceeding 1 s f or a person in good health. If the

duration wereless than 1 s, greater currents

could be tolerated without such adverse

reactions.

Electric shock accidents are mos t common o n

low-voltage systemsand are usually subdivided into two categories of direct contact and indirect contact

shocks. A direct contact shock occurs when conductors that are meant to be live, such as bare wires o r

terminals, are to uched. An indirect contact shock occurs when an exposed conductive part t hat has become liv

under f ault conditions is touched, as depicted in Fig. 16.1.

Examples of an exposed conductive part are the metal casing of a washing machine and the metal casing of

switchgear. This type of accident, which requires two f aults to occur (the loss of the earth connection followed

by a phase-to-earth fault), is quite common.

Physiological Eff ects

Table 16.1 The effect of passing alternating current (50 Hz) through the body from hand-to-hand

Current(mA)

Physiological e ff ect

0.52 Threshold of percept ion

210 Painf ul sensation, increasing with current. Muscular contraction may occur, leading to being thrownof f

1025 Threshold of let go, meaning that gripped electrodes cannot be released once the current is

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First aid - Emerge ncy defibril lator

f lowing. Cramp-like muscular contractions. May be dif f iculty in breathing leading to danger ofasphyxiation f rom respirato ry muscular contraction

2580 Severe muscular contraction, sometimes severe enough to cause bone dislocation and f racture.Increased likelihood o f respiratory f ailure. Increased blood pressure. Increasing likelihood o fventricular f ibrillation (unco-ordinated contractions of the heart muscles so that it ceases to pumpef f ectively). Poss ible cardiac arrest

Over80

Burns at point o f contact and in internal tissues. Death f rom ventricular f ibrillation, cardiac arrest ,or o ther consequential injuries

First Aid with Emergency Defibrillator

When providing f irst aid to an electric

shock casualty, the f irst action should be

to remove the cause by switching-of f the

supply or o therwise breaking contact

between the casualty and the live

conductor. Cardiopulmonaryresuscitat ion may be required.

If the casualty is suffering from ventricular

f ibrillation, the only ef f ective way to

restore normal heart rhythm is by the use

of a def ibrillator.

Where a def ibrillato r is not immediately

available, the f irst aider should carry out

cardiopulmonary resuscitation unt il either

the casualty recovers o r prof essionalassistance arrives.

SOURCE:J.M. Madden

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