New considerations on the threshold of ventricular

fibrillation for a.c.shocks at 50-60 HzG. Biegelmeier, Ph.D., C.Eng., F.I.E.E., and W. R. Lee, M.D., M.Sc, F.R.C.P.

Indexing terms: Hazards, Electric shocks

Abstract: The most common cause of death in electric shock is thought to be ventricular fibrillation, a condi-tion in which the circulation is arrested and death ensues very rapidly. A.C. shocks at 50-60 Hz are the mostfrequent cause of electrical accidents and therefore a careful analysis of available data must be the basis ofprotective measures. During the last few years valuable new results concerning the threshold of ventricularfibrillation have been obtained in experiments with animals, and progress in the field of physiology of theheart makes it possible to explain the influence of various parameters on the phenomena. For a givenshock duration, the distribution of the threshold of current to produce ventricular fibrillation is log-normal.Probability limits for 0-5% and 50% for ventricular fibrillation are determined for shock durations from8-3 ms to 5 s. When the 50% probabilities at different durations are examined a discontinuity is observed inthe vicinity of the period of one heart cycle. This divides the threshold into two levels, a high level atdurations shorter than one third of the period of the cardiac cycle and a level which is more than 20 timeslower at shock durations longer than about six heart cycles. At the higher level, ventricular fibrillation onlyoccurs if the shock falls within the vulnerable period of the cardiac cycle; at the lower level the initiation ofventricular fibrillation does not depend on the point of the cardiac cycle in which the shock starts.

1 Introduction

There is increasing evidence that the most common cause ofdeath from electric shock is ventricular fibrillation, and it istherefore of considerable practical importance to establishthe minimum values of commercial-frequency alternatingcurrents that are likely to cause fibrillation. As the cir-cumstances of an electrical accident only rarely allow areliable estimate of either the current or the period forwhich it flows, it is necessary to have recourse to animalexperiments to determine threshold currents. This methodintroduces difficulties of extrapolation of the findings tohuman beings. The two classic series of animal experimentsin this field have been those of Ferris and his colleagues in1936 at Columbia University, and the Bell Telephone Lab-oratories, N.Y.,1 and of Kouwenhoven and his associates atthe Johns Hopkins University, in 1959.2 The results ofthese studies were analysed by Dalziel in I9603 with theobject of establishing an acceptable relationship betweenthe three factors believed to be concerned, namely bodyweight, current magnitude and shock duration. Dalziel, whowas the first to apply statistical methods to measurementson ventricular fibrillation took as a basis for his work theassumption that the responses follow a normal distributionbelow percentile 50. He defined as the danger limit astraight line drawn by eye through the 0-5 percentile pointsplotted on log—log graph paper. The line had a slope of— 112, and was therefore represented by an equation of theform / = K/y/f

Since that time there have been further developments.First, Kiselev of the USSR Academy of Medical Sciencepublished, in 1963, a detailed experimental study of thres-hold currents in dogs.4 Lee and his colleagues determinedthe threshold for a.c. shocks of long duration in dogs withnormal acid base state.s The latter work showed that, forshocks of long durations compared with the cardiac cycle,

Paper 566A, first received 17th September, and in revised form7th December 1979Prof. Biegelmeier is President of the Technical Committee forcurrent-operated earth-leakage circuit-breakers in the C.E.E.Heiligenstadterstr., 187, A-1109 Wein XIX Austria. Dr Lee is Pro-fessor of Occupational Health, University of Manchester, StopfordBuilding, Oxford Rd, Manchester M13 9PT, England

IEEPROC. Vol. 127, No. 2, Pt. A, MARCH 1980

there existed a fairly constant low threshold for ventricularfibrillation. In Germany, Osypka published a study on thesubject6 and Jacobsen and his colleagues determined thethreshold of fibrillation for pigs.7 In all these experimentalresults a peculiar discontinuity of the threshold is apparentfor shock durations in the vicinity of one cardiac cycle.This had already been observed in 1960 and discussedbetween Dalziel and Biegelmeier.3'8

A further point emerged in the discussion on a paper byDalziel and Lee, when Smoot drew attention to the factthat when the results for any one shock duration are plot-ted on log-probability paper, the skewing visible onarithmetic probability paper disappears.9

The present paper reviews all experimental data to date.The fibrillation data of the experiments of Kouwenhovenwill first be plotted on log-probability paper and comparedwith the results of Kiselev, Ferris and Lee. Secondly, therelationship between fibrillating currents for 0-5% and 50%fibrillation probability, and shock duration will be studiedtaking special care to establish values for the high and lowlevels of the threshold, and the slope of the intermediateline in the vicinity of one cardiac cycle. Thirdly the inter-pretation of these results in the light of current physio-logical theory will be examined. Finally, the relevance ofthis knowledge to the prevention of electrocution will bediscussed.

2 Relationship between fibrillating current and shockduration

Kouwenhoven's experiments covered shock durations from8-3 ms to 5 s, and dogs, ranging in body weight from 8 to16 kg, served as the experimental animals. When the dis-tribution curves are drawn on log-probability paper, theresulting straight lines indicate that the current valuesfollow the logarithmic normal distribution. Figs. 1 to 5illustrate values at 5 of the $ time periods studied.Theoretically, the values / (50%) and Iav should be thesame; they are, however, only very close to each other.

At the shock duration of 0167 s (Fig. 3), which isapproximately the duration of half a cardiac cycle in thedog, the curve appears to be skewed and may be repre-sented by the two straight lines as indicated. This might

103

0143-702X/80/020103 + 8 $01-50/0

suggest that two different physiological mechanisms areinvolved, as follows: if the shock occurs during the vulner-able period of the heart cycle (see below), fibrillation willbe produced immediately and the threshold will be thesame as that found for any of the shorter duration shocksfalling during the vulnerable period. However, if the shockstarts during diastole (that is, during the period of relaxa-

tion and filling of the ventricles) it may initiate a prematureheartbeat and, continuing through that, fall in the vulner-able period of the second premature beat when the thres-hold will be considerably lower, as will be explained below.

The Ferris experiments which spanned shock durationsfrom 30 ms to 3 s were made on several larger animals includ-ing sheep, dogs, calves and pigs. The measurements also

99 59998

95

90

80

70

O 60° 50

4030

20

10

5

21

300 500 1000 2000I mA

5000

Fig. 1 Minimum fibrillating current distribution curve for dogs(Kou wenho ven)

8-3 ms shocks7(0-5%)= 890 mA7(50%) = 2000 mAIav — 2070 mA

99-5

99

98

95

90

80

60

50

40

30

20

10

5

21

G5300 500 1000 2000 5000

1 mA

Fig. 2 Minimum fibrillating current distribution curve for dogs(Kouwenhoven)

83-3 ms shocks7(0-5%) = 525 mA7(50%) = 1800 mAlav = 2040 mA

9959998

95

90

80

70

o 6 0

r 50

40

30

20

10

5

2

10-5

300 500 1000 2000 5000I.mA

Fig. 3 Minimum fibrillating current distribution curve for dogs(Kouwenhoven)

0-167 s shocks7(0-5%) = 650 mA7(50%) = 930 mA1(0,= 1240 mA

995

99

98

95

90

8 0

~s 7 0

60504 0

30

20

10

5

21

r

-

-

-

-

-

-

-

-

-

. Jw

- //

05300 500 1000 2000

I.mA5000

Fig 4 Minimum fibrillating current distribution curve for dogs(Kouwenhoven)

0-333 second shocks (period of cardiac cycle)7(0.5%) = 280 mA7(50%) = 720 mAlav = 7 7 S m A

104 IEEPROC. Vol. 127, No. 2, Pt. A, MARCH 1980

follow a log-normal distribution as shown in Fig. 6, whichgives the minimum fibrillating current distribution curve forsheep for 3Os shocks. Fig. 7 presents the results of Kiselevwith dogs, also for 30 s shocks. It can therefore be acceptedthat with the exception of shock durations of 0167 s, whichhave already been considered separately, the measurementsof fibrillating currents follow a log-normal distribution.

99 5

9998

95

90

80

70

_o 60^ 5 0

4030

20

10

5

21

0 5

Table 1: Fibrillating currents for long shock durations establishedby Lee and associates3

10 20 50 100I.mA

200 500

Fig. 5 Minimum fibrillation current distribution curve for dogs(Kouwenhoven)

5-0 second shocks7(0-5%)= 30 mA7 (50%) = 80 mAIfju = 83 mA

99 599

98

95

90

80

o 7 0

^ 6 05 0

4 0

3 0

20

10

5

2

1

05

r

-

-

-

-

-

-

----

-

-

-

-i i

10 20 50 100I.mA

200 500

Fig. 6 Minimum fibrillating current distribution curve for sheep(Ferris)3-0 second shocks/(0-5%) = 145mA7(50%) = 250 mAlav = 253 mA

Shock durations, s

Fibrillating current, mAMean ± s.d.

3

82-2 ± 11

10

78 ± 11

30

73 + 9

60

80 ± 7

For the thresholds for shocks of longer durations, i.e. upto 60s, the measurements of Lee and his associates5 areshown in Table 1. That series included 16 dogs ranging inbody weight from 10-4 kg to 18-6 kg.

Two views have been put forward concerning the shockparameters for the initiation of fibrillation. On the onehand Dalziel and Lee9 have suggested that the hazard froman electric shock from currents at power frequencies isrelated to the energy (I2t), and this concept appears toagree with the findings on impulse currents by Dalziel.10

On the other hand, Osypka,6 also working with power-frequencies, has maintained that it is the total charge (It)-which determines the hazard. It should be stressed thatboth of these concepts were based on an examination ofthe current and duration parameters of the shocks andneither proposed a physiological explanation.

From Figs. 1 to 5 the minimum fibrillating currents for0-5% and 50% probability may be taken for various shockdurations as a basis for calculating I2t and It values forcomparison. The values obtained are shown in Tables 2and 3.

It is interesting to note that in the vicinity of the shockduration of one cardiac cycle the I2t and the It values bothseem to be fairly constant. The It values for 50% fibrilla-tion probability are about 200 mAs. (mC) and for 0-5%fibrillation probability near 100 mAs (mC). The I2t valuessuggest that for shock durations somewhat longer than oneheart cycle, the energy necessary to cause fibrillation revertsto that required for very short durations.

Fig. 8 shows the threshold for the minimum fibrillatingcurrents for 50% fibrillation probability as a function of

99 5

99

98

95

90

80

70

30

20

10

5

2

1

0 510 20 50 100

1mA200 500

Fig. 7 Minimum fibrillating current distribution curve for dogs(Kiselev)

3-0 second shocks7(0-5%)= 34 mA/ (50%) = 70 mA/„„ = 74 mA

IEEPROC. Vol. 127, No. 2, Pt. A, MARCH 1980 105

Table 2: Minimum fibrillating currents for 0-5% and 50% probability as a function of shock duration after Kouwenhoven2

Shock duration

Minimum fibrillating current0-5% mA

Minimum fibrillating current50% mA

8-3 ms

890

2000

16 -7 ms

1000

1500

83-3 ms

525

1800

0-167s

650

930

0-33s

280

720

1 s

50

155

2s

50

170

5s

30

80

Table 3: I2 t and It — values for 0-5% and 50% fibrillation probability as a function of shock duration

Shock duration

/2 f (0-5%)A2s. 10"3

I1 t (50%)A2s. 10 ' 3

/ t (0-5%)As. 10"3

/1 (50%)As. 10"3

8-3 ms

6-6

33

7-4

17

16-7ms

16-7

37-5

16-7

25

83-3 ms

23

270

44

150

0-167s

70

144

108

155

0-33 s

26

173

93

240

1 s

2-5

24

50

155

2s

5

58

100

340

5s

4-5

32

150

400

shock duration for shock durations from 8-3ms. up to 60s.It consists of a fairly constant level of approximately 2 Afor shock durations below one third of the cardiac cycle,and a fairly constant level of approximately 80 mA forshock durations longer than six heart cycles. It should benoted that the high level is more than 20 times the value ofthe low level.

The slope of the line between the two levels has beencalculated from the Iav values which fall near the line, thatis from 0-083 s to 3 s inclusive (using the data of Lee andassociates5 for the 3 s. point). The slope is —0-89, which isvery nearly — 1 , suggesting that over that time range itfollows an equation of the form It = const., rather thanI2t = const.

3 Time factor in the production of ventricularfibrillation

It is accepted that ventricular fibrillation is caused when anelectric stimulus of sufficient strength strikes the heart inthe vulnerable period. This period, which is represented inthe electrocardiogram by the T-wave, is characterised by anonhomogenity or differences in the refractoriness of theheart fibres. Only then can fibrillation be initiated, and it is

r 06£ 0-4

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S 01006004

002

0 010004001002004 01 02 04 1 2

duration of shock s4 6 10 20 40 100

Fig. 8 Threshold of ventricular fibrillation current for dogs (50%probility) for shock durations from 8-33 ms to 60 s

(Average period of the cardiac cycle = 0-33 seconds)x S0% values, Kouwenhoven• average values, Leeo average for 3-0 second shock, Kisclev

106

self sustained by cyclic excitations. Fibrillation of theventricles (the main pumping chambers) is accompanied bya loss of co-ordinated muscular contraction and the heartmuscle quickly becomes exhausted and, if the condition isnot soon corrected, an irreversible standstill of the heartoccurs.

Although, as stated, ventricular fibrillation can only beinitiated during the vulnerable period, the threshold ofexcitation for ventricular fibrillation is, paradoxically,approximately twenty times higher than the threshold ofexcitation when the heart is at the end of the period ofrelaxation and filling of the ventricles (diastole). In diastolesuch comparatively weak shocks, if their duration is shorterthan the duration of one premature beat, simply cause onepremature ventricular beat and not fibrillation. If theirduration is longer, they cause several premature beats(extrasystoles) in the form of an acceleration rapidity ofheart beat so that these premature beats follow each otherwith increasing frequency. It is interesting to note thatFerris et al.1 invoked these extrasystoles, caused by anelectrical stimulus during diastole, in their explanation ofthe peculiar form of the threshold of ventricular fibrillation.They say in their paper;

'It is well known to physiologists that an electricalstimulus during diastole can cause a premature heartbeat, known as 'extrasystole'. This premature beat willhave a ventricular contraction and relaxation similar tothose of a normal heart beat, although smaller in magni-tude and duration. The relaxation corresponds to thepartial refractory phase of the normal cycle. This sug-gests that extrasystoles were caused in these cases withconsequent advancement in time of the partial refractoryphase, thereby bringing about fibrillation even thoughthe shock would not have been long enough to reach thenormal partial refractory phase.'

Too little attention was subsequently paid to these words,even when experimental results were made known bySugimoto and his colleagues,11 which could well have givenan answer to questions which then were already becomingpressing in the field of electrical safety. Sugimoto passed arelatively weak 60 Hz current directly to the exposedventricle of a dog using electrodes consisting of needleswhich delivered the desired pattern of stimuli to theventricle. They found that shocks of up to 1 secondproduced a series of premature responses. After eachpremature response, the fibrillation threshold fell and in the

IEEPROC. Vol. 127, No. 2, Pt. A, MARCH 1980

Table 4: Duration of premature beats and variation of threshold of ventricular fibrilla.tion depending on the number of premature beatsaccording to Sugimoto et at.

normal beat (f0)1. premature beat (t,)2. premature beat (f2)3. premature beat (f3)4. premature beat (f4)5. premature beat (f5)6. premature beat (r6)

Duration in %of normal beat

100322520-516-4 (x)13-1 (x)10-5 (x)

Threshold of fibrillation in %of threshold of normal beat

100100

3911.5

5-543-5

Total time to all premature beatsin % of normal beat

325777-594

107117.5

(x) extrapolated values from data given in Reference 11

end approached the normal threshold of excitation of theventricle in late diastole. These findings were in accord withthose of Han and his colleagues who (Fig. 10) had usedrectangular pulses at 10 ms intervals applied to the exposedventricle of a dog.12> 13

Sugimoto and his colleagues obtained this pattern ofresponse using three different patterns of stimulation, andAntoni14 has proposed a theory on the threshold ofventricular fibrillation based on the fibrillation threshold ofthe extrasystole.

4 Initiation of ventricular fibrillation

Basically the phenomenon of the Z-shaped threshold ofventricular fibrillation can be explained as follows:

The threshold, as seen before, consists of a high and alow level with an intermediate part when the shockduration is about one cardiac cycle. The high level cor-responds to the threshold of ventricular fibrillation whenthe shock falls in the vulnerable period (T wave) of thenormal heart beat. The low level is determined by thethreshold of excitation of the ventricle during diastole.

If an a.c.-shock of the order of this low level begins duringsystole (i.e. at a time in the cardiac cycle when the ventricleis refractory to stimulus) nothing happens until, if the shockis sufficiently long, diastole begins when a premature beatis initiated. If the shock starts during diastole, a prematurebeat is immediately initiated. However, the threshold offibrillation of this first premature beat is approximately thesame as that of the normal beat. Only if the shock lasts longenough to produce a second response is the threshold forventricular fibrillation lowered. It is interesting to note thatthe duration of the first extrasystole is approximately onethird that of one normal heart beat. If the a.c. shock con-tinues so as to cause more premature beats the threshold ofventricular fibrillation is progressively lowered until itapproaches the excitation threshold in diastole.

If the experimental results of Kouwenhoven andSugimoto are compared, the above mentioned theory forventricular fibrillation appears to be confirmed. The sets ofresults cannot be compared directly, firstly because theKouwenhoven results (see Table 2) use seconds to describeshock duration and milliamperes to describe shock mag-nitude, whereas the Sugimoto results use 'number of pre-mature ventricular contractions' to describe the shockdurations. Secondly, although the current magnitude isexpressed in milliamperes in both the studies, theKouwenhoven dogs received the shocks between a forelimband a hind limb and the Sugimoto dogs had the currentapplied directly to the heart. Hence, transformations mustalso be made on both the shock duration and the currentmagnitude.

With regard to the transformation of the number of

premature ventricular contractions into shock duration,Fig. Id of Sugimoto's paper was used as shown in Fig. 9.The durations of the first to third premature beat weredetermined with this method. The durations of the forth tosixth premature beat were extrapolated from the formervalues. The durations of the first to sixth premature beatare, respectively, 160, 125, 100, 80, 65 and 50ms. for aduration of the normal beat of 500 ms. The average ven-tricular fibrillation threshold in milliamps for the aboveshock durations have been calculated from Fig. 3 ofSugimotos paper which is reproduced as Fig. 10 in thispaper. These are shown in Table 4.

In order to compare these results with the experimentsof Kouwenhowen, the threshold of the normal heart beatof a live dog for a current path from fore leg to hind leg andof the first premature beat may be taken as 2000 mA for50% fibrillation probability (Table 2). As the threshold offibrillation in the partial refractory phase of the normalheart beat and of the first premature beat is the same, thetheoretical curve remains an a priori constant at the highvalue measured for the 10 ms. shocks up to a time whichcovers the first premature beat, i.e. approximately one thirdof the cardiac cycle. When the second, third etc. prematurebeat is covered the fibrillation threshold drops to the per-centages indicated in Table 4. For comparison with theKouwenhoven results it should be remembered that theaverage heart cycle for the Kouwenhoven dogs was 330 ms,whereas in Sugimotos paper 500 ms was mentioned. Withthe above mentioned data Table 5 gives the theoreticalthreshold for ventricular fibrillation for dogs for a heartperiod of 330 ms. These theoretical values are plottedagainst the results achieved directly during the experimentsof Kouwenhoven (Fig. 11) and considering the difference inthe experimental conditions they seem to give strongsupport to the explanation of the threshold of ventricularfibrillation by the theory of extrasystoles.

normal heart beatt = 500ms 1 2 3 Prernature

beat

1 2 3 4 ,

60 Hzurn

— 500ms —

Fig. 9 Method used for the determination of the relative length ofthe premature beats using the r.v.-signals of fig. 2d, of the Sugimotopaper

IEEPROC. Vol. 127, No. 2, Pt. A, MARCH 1980 107

Table 5: Theoretical threshold for ventricular fibrillation of dogsmeasured empirically by Kouwenhoven for 50% fibrillationprobability calculated according to the theory of prematureresponses with the values measured by Sugimoto (Duration of the

normal heart beat 330 ms)

Cardiac cycle

Normal beat andfirst prematurebeat d.p.b.)

1. + 2. p.b.

1. + 2. + 3.p.b.

1. + 2. + 3. +4.p.b.

1.+2. + 3.+4.+ 5 p.b.

1. + 2. + 3. +4+ 5. +6.p.b.

Shock duration

1/120 up to1/3 h.p. =110 ms

57% h.p. =188 ms

77, 5% h.p. =255 ms

94% h.p. =310ms

107% h.p =350 ms

11 7% h.p. =386 ms

Threshold

%100

39

11.5

5,5

4

3,5

Threshold

mA2000

780

230

110

80

70

h.p. = heart periodp.b. = premature beat

5 Relevance to the prevention of electrocution

As far as the results of experiments with various animals areconcerned, quantitative conclusions cannot be drawn fromvalues measured for dogs and for pigs unless both thecurrent pathway, and the body weights are taken intoaccount. Otherwise, only the qualitative similarity in shapecan be noted. This is the case for the experiments madeonly a few years ago by Jacob sen and his colleagues7 on thethreshold of ventricular fibrillation of pigs, the results beingshown in Fig. 12.

The points marked are the lowest currents having caused

30

25

20

^ 15a

10

0 -

1 2 3 A 5 6duration of 60c p s . no of responses

Fig. 10 Relationship between duration of 60 Hz shocks applieddirectly to the heart and ventricular fibrillation threshold (v.f.t.)after Sugimoto et al, data from 6 dogs

ventricular fibrillation and show the same Z-shaped thresholdalready noted in the results from dogs. The high and thelow levels discussed before are somewhat higher than fordogs, but on the whole it now seems justified to proposelimits which would safeguard against ventricular fibrillationfor men.

The values shown in Fig. 8 are those for the 50%probability of ventricular fibrillation for dogs. It is neces-sary to consider next the transformation to a maximumnonfibrillating threshold for humans. Dalziel and Lee,9

after considering the relationship of fibrillating current tobody weight for 3 second shocks, concluded that for a50 kg human the maximum non-fibrillating current was67 mA. It would seem prudent therefore, to set the lower

0008 0017 0 083 0167 0330 1 2 3

duration of shock,s

Fig. 11 Comparison of the threshold of ventricular fibrillation for50% probability (Kouwenhoven), with the calculated threshold(from Sugimoto)

a Values measured by Kouwenhoven et al.b Calculated values from Sugimoto

40

20

10

64

t 1I 0 6o 0 4

02

01001 002 004 01 02 04 06 1 2 4 6 10 20 40

duration of shock.periods of cardiac cycles

Fig. 12 Threshold of ventricular fibrillation for pigs

Shock durations from l/100s (half wave a.c. 50 Hz) to 10s (approx.15 heart cycles). Duration of the cardiac cycle average 0-54 s

108 IEEPROC. Vol. 127, No. 2, Pt. A, MARCH 1980

level of the Z somewhere about this value. The middle ofthe Z, as demonstrated earlier, has a slope of — 1 . Theupper level may then be calculated to be about 1 -96 A.Another possible factor in the transformation would be thedifference in heart rates between dogs and humans. Theresults in Fig. 8 are based on resting heart rate for the dogof about 180 beats per minute, which would be very fastfor humans whose resting heart rate is nearer to 70—80 perminute. Therefore in Fig. 13, which suggests the thresholdof ventricular fibrillation for men, a heart rate of 100 perminute was chosen for the standard duration of the cardiaccycle. This corresponds to the resting heart rate of a 3 yearold child or the heart rate of an adult after some physicalexercise.

Safety limits, although derived from physiological data,include other concepts; they must accept that absolutesafety remains an ideal, and that risk is an integral part oflife. Although it has been argued that it is not the functionof a scientist to decide the level of risk that others shouldaccept, he should endeavour to indicate the different levelsof risk and the practicability of achieving them.15 Anotherconcept in the setting of safety standards is simplification,without increasing risk, so that the standard may be morereadily applied in practice.

5000

10 20 50 100 200 500 XXX) 2000 5000 10000T, ms

Fig. 13 Threshold of ventricular fibrillation for men for 50%fibrillating probability and safety limit against ventricular fibrilla-tion for men

Standard duration of the cardiac cycle 0-6 sa threshold of fibrillation for men including children for 50%probability of fibrillation* nonfibrillating threshold for men including children. Below thisline there is usually no danger of fibrillation

IEE PROC. Vol. 127, No. 2, Pt. A, MARCH 1980

It is, therefore, proposed that for safety purposes theupper and lower levels as calculated above should be alteredfrom 67 mA for the lower and from 1 -96 A for the upper to50 mA and 500 mA, respectively. An incidental, althoughapparently not important, consequence of this is that,because the ratio between the upper and lower levels isreduced from 30 to 10, the time interval over which theshape of the Z occurs is reduced to 0-20 s to 2 s, so preserv-ing the slope at — 1 (Fig. 13. curve b).

The currents defined by the high level of the thresholdof ventricular fibrillation, in the order of amperes, are onlylikely in the case of high voltage accidents, of lightningstrokes, and only under extremely unfavourable conditionsin low voltage accidents. In low voltage accidents, up totouch voltages of 240 V, the body current remains normallyin the order of 100—200mA. For such currents ventricularfibrillation will only occur if the shock lasts longer thanapproximately half a second (Fig. 13). This is why current-operated earth-leakage circuit-breakers (ground fault inter-rupters) would serve to prevent death in low-voltage instal-lations. This has not only been demonstrated by experi-ments with ground fault interrupters on dogs as mentionedin the discussion of the paper by Dalziel and Lee,9 but hasalso subsequently been confirmed, up to 200 volts, byexperiments on human volunteers who were fully aware ofthe nature of the tests. The safety precautions and theresults are set out in detail in References 16 and 17.Fig. 14 shows the oscillograph of a body current of 118 mAr.m.s. which was caused by a touch voltage of 200 V, whenthe subject was grasping firmly large electrodes with theright and left hands.

One further point is that for short shocks of less thanone third of the heart beat, i.e. for the high level of fibrilla-tion, the probability of a fatal electrical accident is furtherreduced, as fibrillation only occurs if the shock startswithin the vulnerable period (T-wave) of the cardiac cycle.This phenomena might contribute to the explanation of thecurious fact that only about 30% of persons struck bylightning die.18

vvvvvvvvvvvvvvvvvvvvv1ms

Fig. 14 Oscillograph of body current and touch voltage UT of200 V (peak value 282 V), shock duration 10 ms

109

For protective measures in low voltage installations it istherefore suggested that the protective devices such as fuses,miniature circuit-breakers and earth-leakage circuit-breakersmust operate at least within 0-5 s in the case of an insula-tion fault to the body of an appliance. If this is ensured, amuch higher level of safety would be attained because, atpresent, the melting time of fuses in the case of an insula-tion fault is allowed to be in the order of many seconds.

With touch voltages up to 200 V, and a body impedanceof as low as 1000 f2, the current flowing through thehuman body might attain a value of 200 mA. If the operat-ing time of the safety devices remains below 0-5 s thecondition defined by curve b of Fig. 13 is fulfilled.

As it has now been shown by experiments on man thatprotection is given against direct contact, even under theadverse conditions of an accident, for touch voltages of upto 200 V, the use of current-operated earth-leakage circuit-breakers (ground fault interruptors) may be recommended.For branch circuits they should have tripping sensitivitiesnot higher than 30 mA with a tripping time of some 10milliseconds when the body current reaches dangerousvalues. In this way protection against ventricular fibrillationis ensured, but with tripping currents at this level noprotection would be afforded against hold on accidents. InIEC Specifications, a distinction is made between portableand fixed appliances; the former being considered as themain cause of fatal accidents. For the protection of fixedappliances and whole installations therefore a sensitivity ofpreferably 100 mA, or for special industrial applicationseven more, and in order to avoid nuisance tripping due toleakage currents a short time tripping delay up to 0-5 s,could be tolerated. However, a similar note of caution mustbe sounded about this recommendation and other means ofprotection should also be employed.

5 References

1 FERRIS, L.P., KING, B.G., SPENCE, P.W., and WILLIAMS,H.B.: 'Effects of electric shock on the heart.' Electr. Eng. 193655 pp. 498-503, Ref. ETZ, 1937, 58, p. 181

2 KOUWENHOVEN, W.B., KNICKERBOCKER, G.G., CHESNUT,R.W., MILNOR W.R., and SASS, D.J.: 'A.C.-shocks on varying

parameters affecting the heart.' Trans. Amer. Inst Electr Eng1959,78, pp. 163-169

3 DALZIEL C.F.: 'Threshold 60-cycle fibrillating currents.' AIEE-Conference Paper 60-40, 1960

4 KISELEV, A.P.: "Threshold values of safe current at main fre-quency.' Probl. of Elec. Equipment, Elec. Supply and Elec.Measurements (in Russian). Sb. MIIT, 1963, 171

5 SCOTT, J.R., LEE, W.R., and ZOLEDZIOWSKI, S.: 'Ventricularfibrillation threshold for a.c.-shocks of long duration, in dogswith normal acid-base state.' British Journal of IndustrialMedicine, 1973, 30, p. 155

6 OSYPKA, P.: 'MeBtechnische Untersuchungen Uber Stromstarke,Einwirkungsdauer und Stromweg bei elektrischen Wechsel-stromunfallen an Mensch und Tier, Bedeutung und Auswertungfur Starkstromanlagen.' Elektromedizin, 1963, 8, SonderdruckFachveilag Schiele und Schon, Berlin

7 JACOBSEN, J., BUNTENKOTTER, S., and REINHARD, H.J.:'Experimentelle Untersuchungen an Schweinen zue Frage derMortalitat durch sinustormige, phasenangeschnittene sowiegleichgerichtete elektrische Strflme.' Biomedizinische Technik,1975,20 pp. 99-107

8 BIEGELMEIER, G.: 'Ein Beitrag zur Problematik des Ber-Uhrungsspannungsschutzes in Niederspannungsanlagen. ETZ-B,1960, 12, p. 611

9 DALZIEL, C.F., and LEE, W.R.: 'Re-evaluation of lethal electriccurrents.' IEEE Trans. 1968, IGA-4, pp. 467-476 and 676-677

10 DALZIEL, C.F.: 'A study of the hazards of impulse currents,'Trans Am. Inst. Electr. Engs., 1953, 72, pp. 1032-1043

11 SUGIMOTO, T., SCHAAL, S.F., and WALLACE, A.G.: 'Factorsdetermining vulnerability to ventricular fibrillation induced by60-c.p.s. alternating current'. Circulation Research, 1967', XXI,pp. 601-608

12 HAN, J., DE JALON, P.D., and MOE, G.K.: 'Fibrillation thres-hold of premature ventricular responses.' ibid, 1966, 18, pp. 18—25

13 HAN,J.: 'Ventricular vulnerability during acute cardia occlusion'.American Journal of Cardiology, 1969 24, pp 857-864

14 ANTONI, H.: 'Ursachen der sogenannten vulnerablen Periodedes Herzens und ihre Beziehung zur elektrischen Flimmersch-welle.' Lecture given at the meeting of "Forschungs-furElektrounfalle", Freiburg i. Breisgau, October 1977

15 LEE, W.R.: The rule of reason.' (to be published) New YorkAcademy of Sciences

16 BIEGELMEIER, G.: 'Uber die Wirkungen des elektrischenStromes auf den Menschen.'^um* Af, 1977,94, pp. 107-118

17 BIEGELMEIER, G.: 'Uber die Korperimpedanzen lebenderMenschen bei Wechselstrom 50 Hz,' ETZ-Archiv, 1979, 1,pp. 145-150

18 BIEGELMEIER, G., BERGER, K., and KAROBATH, H.: 'Uberdie Wahrscheinlichkeit und den Mechanism us des Todes beiBlitzeinwirkungen,' Bulletin des SEV (in preparation)

William Rimmer Lee entered Guy'sHospital Medical School on an openscience scholarship and, after qualifyingand working in hospitals, he served fornearly seven years in the RAF MedicalBranch. Since then he has been amedical officer on British Rail andmedical adviser to the Cotton IndustryResearch Association and the elec-tricity board. For six years, up to1978 he was medical adviser to the

Electricity Council. He is Professor of occupational healthat the University of Manchester, consultant in occupationalmedicine to the Manchester Royal Infirmary and HopeHospital, Salford. He is also academic Registrar of theFaculty of Occupational Medicine at the Royal College ofPhysicians in London.

Gottfried Biegelmeier, Prof. Ing. Dr.phil., F.I.E.E., was born in Vienna,July 1924 and studied electricalengineering at the Politechnical Schoolat Vienna and physical and mathemat-ical sciences at the University ofVienna. 1949 he took his Doctorsdegree in Physics and started his careerat the Austrian testing station for thesafety of electrical equipment. In 1957he became a free-lance electrical

engineer and specialised in problems of electrical safety,especially in the field of earth-leakage circuit-breakers. In1960 he was appointed President of the Technical Commit-tee for current-operated earth-leakage circuit-breakers ofthe CEE, and in 1972 became a Fellow of the IEEFrom 1975 onward he undertook sensational self-experi-ments on the effects of electric shock on the human bodywhich clarified the limits of protection for current-operatede.l.c.b.'s. For these experiments he was given the GoldenMedal of the Republic of Austria, the Golden Cross of theArgentine Institute of Safety, and, in 1978, the title 'Pro-fessor' was bestowed on him by the President of theAustrian Republic.

110 IEE PROC. Vol. 127, No. 2, Pt. A, MARCH 1980