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Fundamental Concepts of Electrical Safety Engineering

CONTENTS

1.1 Introduction

1.2 Electric Shock

1.2.1 Ventricular Fibrillation

1.2.2 The Heart-current Factor

1.3 The Electrical Impedance of the Human Body

1.3.1 The Internal Resistance of the Human Body

1.4 Thermal Shock

1.5 Heated Surfaces of Electrical Equipment and Contact Burn Injuries

1.6 Ground-Potential and Ground-Resistance

1.6.1 Area of Influence of a Ground-electrode

1.7 Hemispherical Electrodes in Parallel

1.8 Hemispherical Electrodes in Series

1.9 Person's Body Resistance-to-ground and Touch Voltages

Example 1.1

1.10 Identification of Extraneous-Conductive-Parts

1.11 Measuring Touch Voltages

References

I shall be telling this with a sigh
Somewhere ages and ages hence:
Two roads diverged in a wood, and I-
I took the one less traveled by,
And that has made all the difference.

Robert Frost

1.1 Introduction

The renewable energy sector has been rapidly growing in the past decade [1-3], and so has been the number of accidents involving workers in "green" projects. Statistics in the United States reveal that injuries and death are caused by lack of safety training and safety procedures [4]. The Electric hazard, but also Falls, Struck by and Caught in between hazards, are always present during all photovoltaic, solar thermal, and wind tower construction projects, regardless of the magnitude of the job.

The culture of the safety-by-design [5, 6] seems to be the appropriate response to the increased risk offered by renewable energy systems (RES). RES may challenge the safety of workers because they are generally always live, and the system voltage may exceed 500 V d.c.1

In addition to safety training and procedures, electrical safety may be conveyed through engineering measures that reduce the risk of electric shock below a threshold that is conventionally deemed acceptable by applicable standards. In fault-free conditions, the basic protection ensures that persons cannot come into contact with parts normally live (i.e., proper insulation of electrical components). In the case of failure of the basic insulation of components, the fault protection ensures defense against electric shock by automatic interruption of the fault current. In some scenarios, the fault protection may be obtained with alternative methods to the fault current interruption.

In general, the safety-by-design of RES [7] is achieved if hazardous energized parts are never accessible, and that equipment/appliances, also referred to as exposed-conductive-parts (ECPs), are never hazardous either under normal operations or in the event of single-faults. In essence, touch voltages and contact durations must be within the magnitudes deemed safe by applicable technical standards and codes.

1.2 Electric Shock

External electrical stimuli applied to the human body can prevent operational skeletal and cardiac muscles from properly operating, as well as destroy bodily tissues by thermal shock.[8]

External a.c. currents with frequency ranging from 50 to 100 Hz of magnitude around 10 mA for adult males and 15 mA for adult females, can override the internal electrical signals from the brain controlling the body muscles, render the person unable to "let go" of an energized part and cause painful muscle contractions.

For d.c. currents, thresholds of let-go cannot be positively defined. The circulation of d.c. current through the body only causes a sensation of warmth, and the person is subjected to painful muscle contractions only during making and breaking of the d.c. current.

Stevens' Law [9, 10] describes the perceived strength of a physical stimulus as a function of its intensity, expressed in its physical units. According to Stevens' Law, the perception of electric shock is superlinear with the stimulus, as varies as the 3.5 power of the a.c. voltage applied. The coefficient 3.5 relates the magnitude of the applied voltage to the perceived magnitude of the shock as current through the fingers; thus, a small increase in the applied voltage is perceived as a larger increase in the electric shock.

A 30 mA-current, if interrupted within 300 ms, can cause involuntary muscular contractions but usually no harmful electrical physiological effects. Longer disconnection time, up to 5 s, can cause muscular contractions, difficulty in breathing, reversible disturbances of heart function, but usually no organic damage.

Higher body currents inhibit internal muscle control, prejudicing the function of the muscles involved in the breathing process, thus causing asphyxia.

1.2.1 Ventricular Fibrillation

The ventricular fibrillation (V-fib) [11] is the loss of the normal heart rhythm. The V-fib causes the ventricles to quiver, or fibrillate, instead of contracting normally, preventing the heart from pumping blood and causing cardiac arrest. The ventricular fibrillation is the main cause of death in electric shock accidents.

The cardiac muscle, whose fibers have high contractile strength, specializes in pumping blood throughout the person's body. The contractions of the heart are stimulated by the sinoatrial node (SA), situated in the right atrium, which generates electrical impulses. The impulses propagate through the conductive tissues named Bundle of His, and Purkinje fibers, and reach the atrioventricular node (AV), situated in the center of the heart (Figure 1.1).

Figure 1.1 Electric conduction of the heart.

The Bundle of His, which departs from the AV, conducts the stimuli to the ventricles, which, after filling with blood, contract and push the blood through the arteries during the systole. After the contraction, the heart relaxes and fills up with blood again, awaiting further stimuli to contract again.

The net charge of the heart is zero; however, positive and negative charges are dynamically separated during each cardiac cycle and form an electric dipole vector that rotates and varies in magnitude with time. Thus, electric potential differences at different places along the person's body also change with time during each cardiac cycle, and this can be observed in an electrocardiogram (i.e., EKG or ECG) (Figure 1.2). Usually, scalar EKG measurements are performed: vector EKGs, which may more deeply describe the heart dipole rotation, are rarely performed. Typical potential differences showed in the EKG range between 30 and 500 µV.

Figure 1.2 A normal electrocardiogram (EKG).

During the P wave, the right and left atria contract; during the Q-R-S complex, the right and left ventricles contract (systole). The last event of the cycle, the S-T-U interval, is the repolarization of the ventricles: they return to the resting state; their walls relax and await the next signal. This complex procedure continues as the atria refill with blood and more electrical signals are sent by the SA; the heart-period duration is around 400 ms.

The superposition of external currents of larger magnitude to the normal bodily currents will override the control signals from the brain to the skeletal and cardiac muscles, which can no longer operate as intended, exposing persons to the risk of death.

The last half of the T wave is referred to as the relative refractory period, or the vulnerable period, which is known as the crucial time interval during which external electrical stimuli (i.e., electric shock) may induce the ventricular fibrillation.

For shock durations shorter than the cardiac cycle, the ventricular fibrillation may not occur, based on the lower probability that the external stimulus occurs during the vulnerable period of the heart.

For d.c. currents, experiments on animals and data derived from electrical accidents demonstrate that the threshold of V-fib for a downward current is about twice as high as for an upward current; therefore, downward currents are less hazardous than upward currents.

For shock durations longer than the cardiac cycle (e.g., 0.5-1 s), the threshold of fibrillation for d.c. is several times higher than for a.c. However, for shock durations shorter than 200 ms, the threshold of d.c. fibrillation is approximately the same as for a.c. (measured in r.m.s. values).

1.2.2 The Heart-current Factor

The probability that the V-fib is induced is dependent upon the pathway of the body current. To compare the danger of different current paths through the body, standard IEC 60479-12 defines the heart-current factor F [12] (Eq. 1.1).

Iref is the body current that determines V-fib for the path left hand to feet, and Ih is the fibrillation current for different body paths, as shown in Table 1.1.

Table 1.1 Heart-current factor F for different current...