The Hazards of Electric Shock (for Study)

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© ABB Group March 27, 2009 | Slide 1

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F550e E-learning course

Residual current devices basicsThe hazards of electric shock

Welcome to the training course about the basics of the residual current devices.

This first training module will introduce you to the hazards of electric shock.

Please, if you need help navigating this module, click the Help button in the top right corner.

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Attention: in the Attachment button, in the top corner near the Exit button, this e-learning presentation can be downloaded in two different formats, with or without notes.

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After completing this module, you will be able to

Objectives

� gain a deeper knowledge of the general effects of current on human beings;

� understand how the human body behaves in caseof an electrical contact;

� have a clear idea regarding the dangerous current thresholds and their effects;

� continue the basic RCDs course with the other modules.

� have a solid base of definitions and concepts;

After completing this module, you will be able to:

gain a deeper knowledge of the general effects of current on human beings;

understand how the human body behaves in case of an electrical contact;

have a clear idea regarding the dangerous current thresholds and their effects;

have a solid base of definitions and concepts; which will permit you to continue the basic residual current devices course with the other modules.

Remember that, after completing the module, you will be asked tocomplete a short test module to check your own learning.

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IEC 60364 – Low voltage electrical installations Part 4-41: Protection for safety – Protection against electric shock

� Reference Standards panorama

IEC/TS 60479 - Effects of currenton human beings and livestockPart 1: general aspectsPart 2: special aspectsPart 3: effects of currents passing through the body of livestock Part 4: effects of lightning strokes on human beings and livestock Part 5: touch voltage threshold values for physiological effects

Standards overview

Here is the reference Standards panorama concerning the hazards of electric shock on human beings. Precisely, the Technical Specification IEC/TS 60479 - Effects of current on human beings and livestock provides guidance on the effects of shock currents on body, for use in the establishment of electrical safety requirements.

The Technical Specification is divided into 5 parts, dealing respectively:

part 1: the general aspects,

part 2: the special aspects,

part 3: the effects of currents passing through the body of livestock,

part 4: the effects of lightning strokes on human beings and livestock,

and part 5: the touch voltage threshold values for physiological effects.

The contents of all these Technical Specification parts are being used as a basis for fixing requirements for protection against shock, typically in the Standard IEC 60364 – Low voltage electrical installations.

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Introduction

Zi internal impedance

Zs1, Zs2 impedances of the skin

ZT total impedance

The danger depends mainly on the magnitude and duration of the current flow

The human body can be considered an impedance

� The hazards of electric shock

This value of impedance depends on many factors

Let’s see now a brief introduction regarding the hazards of electricshock. For a given current path through the human body, the danger to people depends mainly on the magnitude and the duration of the current. From the electrical point of view, the human body can be considered an impedance, because the different parts of the human body (such as the skin, blood, muscles, other tissues and joints) present to the electric current a certain impedance composed of resistive and capacitive components.

This value of impedance depends on many factors and, in particular, on current path, touch voltage, duration of current flow, frequency, degree of moisture of the skin, surface area and pressure of the contact and temperature.

A schematic diagram for the impedance of the human body is shownhere, where the internal impedance of the human body Zi can be considered as mostly resistive, the two impedances of the skin ZS can be considered as a network of resistances and capacitances and their sum ZT is the total impedance of the human body.

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Zip

Zip = internal partial impedance of one extremity

The impedance of the human body

ZipZip

ZipZip

Impedance hand to hand is 2 Zip

Impedance hand to foot is 2 Zip

Impedance from one hand to both feet is 3/2 Zip

Impedance from both hands to both feet is Zip

� The human body

In this figure there is a simplified schematic diagram for the internal impedances of the human body. In order to simplify the circuit diagram, the impedance of the trunk of the body is considered neglected and the Zip is the internal partial impedance of one extremity (arm or leg), assumed to have the same value of impedance.

And the following can be a scheme of the electrical system (the head is not here considered). Obviously the kinds of the contacts can bedifferent and as a consequence also the offered internal impedances change: for example the internal impedance for a contact hand to hand is 2 Zip, the internal impedance from one hand to foot is again 2 Zip, the internal impedance for a contact hand to both feet is 3/2 Zip and the internal impedance from both hands to both feet is Zip.

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� Partial impedances

Internal partial impedances

The sum of the internal partial impedances Zip as well as the impedances of the skin give as a result the total body impedance ZTfor the given current path.

Also the internal partial impedances of the human body Zip are given by the Standard IEC 60479-1: the numbers express the percentage of the internal impedance of the human body for the part of the body concerned, in relation to the path hand to foot.

The sum of the internal partial impedances Zip for all parts of the body which are interested by the current path, as well as the impedances of the skin in the surface areas of contact give as a result the total body impedance ZT for the given current path. The numbers outside the body show internal portions of the impedance to be added to the total, when the current enters at that point.

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� Values of total body impedances (IEC 60479-1)

Again, in the Standard IEC 60479-1, there are many tables showing the values of total body impedances ZT for a current path hand to hand, for a value of the alternate current at 50/60Hz, in function of the touch voltage. In particular this table is valid by considering a large surface areas of contact, in dry conditions, but other tables are present with different conditions, small or medium surface areas of contact and water-wet or saltwater-wet conditions.

The table shows the values of the total impedance of the human body ZT that are not exceeded for a percentage of 5%, 50% and 95% of the population, in dependence on a touch voltage from 25V to 1000V.

To give a reference value, it is possible to say that, for a voltage of about 200V, almost all people offer a value of impedance of about 2000�.

For lower touch voltages, there are considerable variations in the impedance of the skin and the total impedance of the human body similarly varies widely. For higher touch voltages, the total impedance depends less and less on the impedance of the skin and its valueapproaches that of the internal impedance Zi.

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Time - current diagram

� Time - current diagram

Sinusoidal alternating currentFrequency between 15 - 100HzCurrent pathway from one hand to feet

A: Perception thresholdB: Tetanization thresholdC: Ventricular fibrillation thresholds.

But in the reality our real interest is surely in the behaviour of human beings towards the currents. This is the reason why the Standard IEC 60479-1 illustrates a diagram, with the time duration on the y-axis and the current magnitude on the x-axis, to which the physiological effects of a sinusoidal alternating current, with a frequency between 15 and 100Hz, passing through the human body, have been related. This time-current diagram is intended with a current pathway considered from one hand to feet.

The thresholds here shown depend on several parameters, such as the area of the body in contact with an electrode, the conditions of the contact (dry, wet, pressure, temperature) and also on the physiological characteristics of the people (adults, males and females).

Basically, four zones can be represented, AC-1, AC-2, AC-3 and AC-4, divided by 3 main lines. The first line A is defined Perception threshold, the second line B is defined Tetanization threshold and the last one C, or better the last family of curves C, is related to the Ventricular fibrillation thresholds.

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AC-4 zone: possibility of irreversible effects

AC-4.1 zone: up to 5% probability of heart fibrillation

AC-4.2 zone: up to 50% probability of heart fibrillation

AC-4.3 zone: more than 50% probability of heart fibrillation

No reaction

No physiological effects

Tetanization


� Time - current diagram

The first zone, AC-1, is also called no reaction area: a value of 0.5mA, independent on time, is assumed for the threshold of no reaction when touching a conductive surface; usually currents below this threshold are not perceived by people.

In zone AC-2, we find shock perceived but not dangerous and harmful physiological effects: here usually no organic damage has to be expected, only perception and likely involuntary muscular contractions. In particular, a value of about 5mA can be assumed as the threshold of let-go, the maximum value of touch current at which a person holdingelectrodes can let-go of them.

In zone AC-3, there is the tetanization area, practically a strong involuntary muscular contractions can cause difficulty in breathing and immobilization; other effects increasing with current magnitude are disturbances of heart function, due to the overlapping of electrical impulses. Usually no organic irreversible damage are expected in this zone.

The AC-4 is the most dangerous area, with the possibility of irreversible effects: here are an high risk of cardiac arrest, breathing arrest, severeburns due to heat dissipations and at last the death. In particular, between the curves c1 and c2, delimiting the area AC-4.1, the probability of ventricular fibrillation increases up to about 5%, between the curves c2 and c3, delimiting the area AC-4.2, the probability of ventricular fibrillation is up to about 50% and beyond curve c3,delimiting area AC-4.3, the probability of ventricular fibrillation is above 50%.

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30mA


� 30mA threshold

Well, summarizing it is possible to state that when a current higher than 30mA passes through a part of a human body, there is serious danger for safety if the current is not interrupted in a very short time. This is the reason why all the international Standards and local regulations make always reference to this value of residual current, 30mA, for the protection of people.

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� General effects of current

Effects of current on the human body

Principally the risk of ventricular fibrillation is considered to be the main mechanism of death of fatal electrical accidents. Ventricular fibrillation is fatal because it denies blood flow which transports required oxygen.

Other effects may affect respiration and might prevent the person from shouting for help. These related mechanisms include functional disturbance of respiratory control, paralysis of respiratory muscles, damage to the neural activation pathways for these muscles, and damage to the respiratory control mechanism within the brainstem. These effects, if permanent, lead inevitably to death. If a person is to recover from a reversible respiratory effect, prompt artificial respiration is mandatory. Nonetheless, the person may still die. If current flows through critical parts such as the spinal cord or the respiratory control centre, death can occur.

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� Ventricular fibrillation

FI

Iref

h =4.0mA90

mA225 =

Effects of current on the human body

For other current paths, the probability of ventricular fibrillation can be different

As said before, the time-current diagram is stated for an hand to both feet current pathway. For other current paths, the probability of ventricular fibrillation can be different, according to the following heart-current factors.

The heart-current factor F permits the calculation of a current Ih, through paths other than left hand to feet, which represents the same danger of ventricular fibrillation as that corresponding Iref given in the time-current diagram, according to this formula comprising the F value as the heart-current factor given in the table.

For example, a current of 225mA, left hand to right hand, has the same likelihood of producing ventricular fibrillation as a current of 90mA, left hand to feet, due to an heart-current factor of 0.4..

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Influence of current frequency

� The influence of frequencyTotal impedance of the human body decreases in a way inversely proportional to the frequency.

The physiological effects of current decrease when the frequency increases.

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As regards the influence of frequency, generally the total impedance of the human body decreases approximately in a way inversely proportional to the frequency, mainly for touch voltages in the order of some tens of volts. In the graph, the total body impedance ZT, for the 50% of the population, is shown for a frequency range from 50Hz to 2kHz, by considering touch voltages from 10V to 1000V (current path hand to hand, large surface areas of contact in dry conditions).

If the total impedance of the human body decreases it means that more current passes through the human body, at constant voltage; but, on the contrary, the physiological effects of current decrease when the frequency increases, mainly concerning the risk of ventricular fibrillation. The figure shows the variation of the threshold of ventricular fibrillation within the frequency range 50/60Hz to 1000Hz (shock durations islonger than one heart period and current paths are longitudinal through the trunk of the body); the frequency factor, on the y-axis, is ratio of the threshold current for the relevant physiological effects at the frequency f, in comparison to the threshold current at 50/60Hz. Practically, the risk of ventricular fibrillation decreases 14 times at 1000Hz in comparison to 50/60Hz: therefore higher frequency currents are less dangerous.Shock perception ultimately disappears at frequencies above 15/20kHz, because nerve receptors does not respond to high frequencies and also due to the well known skin effect (in a uniform conductor, the current density near the surface of the conductor is greater than that at its core). However, the human body is not a uniform conductor and the current may take any number of paths through the body, also causing damage without any discomfort.

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Direct current

� Direct current effectsElectrical accidents with direct current are much less fatal: the let go of parts gripped is less difficult; the threshold of ventricular fibrillation is higher than for alternating current;the total resistance RT is higher for direct current than for alternate current.

Electrical accidents with direct current are much less fatal, occurring only under very unfavourable conditions, for example, in mines. This is partly due to the fact that with direct current, the let go of parts gripped is less difficult and that for shock durations longer than the period of the cardiac cycle, the threshold of ventricular fibrillation is considerably and several times higher than for alternating current.

Furthermore, the total impedance, or better the total resistance RT of the human body, is higher for direct current than for alternating current (due to the blocking effect of the capacitances of the human skin).

A time-current diagram for direct current exists, concerning an hand tofeet current pathway; as an example, under conditions comparable to those applied in studies with alternate current, the threshold of reaction was found to be about 2mA, in comparison to 0.5mA in alternatingcurrent.

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Time - voltage safety curve

It results that:

for all the voltage values below 50V, the tolerance time is indefinite;

at 50V the tolerance time is 5s.

� Time - voltage safety curve

The voltage of 50V is reduced to 25V in special locations.

In direct current, the reference is 120V.

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At the end of this presentation, after all these explications, it is possible to draw a time-voltage safety curve.

For alternating current, this time-voltage safety curve can be generally considered, as a result of many simplifications always in favour of the safety of people: this diagram puts in relation the tolerance time, on the y-axis, with the touch voltage, on the x-axis.

It results that for all voltage values below 50V, the tolerance time of a human being is indefinite; at a voltage of 50V, the tolerance time is 5s.

The touch voltage of 50V is reduced to 25V for electrical installations in special locations or where an increased shock risk is present, because of humidity, critical external condition or restricted movements of people; such as places containing a bath tub or shower basin, swimming pools, rooms and cabins containing sauna heaters, installations of agricultural and horticultural premises, restrictive conducting locations and so on.

In case of direct current, the tolerance time a human being is indefinite for a touch voltage below 120V.

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Thank you for your attention. You may now go ahead and move on to the next unit.

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The Hazards of Electric Shock (for Study)