Effects of Current of Human Body

7/29/2019 Effects of Current of Human Body

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Effects of Current on the Human Body

Section 2

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Charles Dalziel[18,19] did much of the earlyresearch on the human bodys reaction tocurrent in the late 1940s and early 1950s.He used volunteers in his experiments and

found that the body reacts to different levelsof electrical current in different ways. For the

safety of the volunteers, this research wasconducted only at low levels of current, withmedical personnel present. Later, additionalresearch was carried out to determine thecorrectness of extrapolating Dalziels ndingsto higher current levels.

By monitoring the voltage applied, the re-

sulting current ow, and the reaction of the

volunteers, a great deal of information wasdeveloped. Calculations were made to developa value of resistance for the average humanbody. Voltages during some of the experimentswere measured at 21 volts hand to hand and10 volts from one hand to the feet. Calcula-tions of resistance using the measured valuesyielded 2,330 ohms hand-to-hand and 1,130

ohms hand-to-feet. This early low voltageresearch established an average safe let-gocurrent for an average man as 16 milliam-peres. It was also determined that the humanbody responds to current in an exponentialmanner. That is, the body responds to anincreasing current as the time shortens in asimilar manner as it responds to a decreasing

current and lengthening duration. This timecurrent relationship is shown in Figure 2-1.

Dalziels research culminated in Equation 1,which follows [15]. It relates current amplitude

and duration of ow through the heart to thethreshold of ventricular brillation. Statisti-cal studies have shown that 99.5% of all per-sons can withstand the passage of a current

magnitude (I) for the duration indicated (t) inthis equation without going into ventricularbrillation. The value k is an empirical con-stant, statistically determined, related to the

Effects of Current on the Human Body

electric shock energy tolerated by a certainpercentage of the population studied.

I = k/ t (Eq. 1)Where I = Current in milliampere

K= function of shock energy

= k50

is 116 for a 50 kg (110 lb.) body wt.= k

70is 157 for a 70 kg (155 lb.) body wt.

t = time in seconds

Using this formula, it can be determined thaton average a 110 lb. lineworker should with-

stand 67 milliamps for 3 seconds before goinginto heart brillation and a 155 lb. workerwould withstand 91 milliamps. Or the sameworkers would be susceptible to heart bril-lation after a 670 Amp. and 906 Amp. shockrespectively after only 0.03 seconds, or about2 cycles of 60 Hz. current ow through thechest cavity. However, at these current levelsother injuries may occur, such as burns if arc-

ing is present. Values presented in tables arecommonly rounded to even values of currentfor ease of presentation and remembering.

Dalziels research also formed the basis of thechart [18, 19] that is used throughout the indus-try today. The chart presents several levels ofcurrent and the average bodys response. Thetable for 60 Hz. is presented in Table 2-1.


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Effect Men Women

Nosensationonthehand 0.4 0.3

SlightTingling(PerceptionThreshold) 1.1 0.7

Shock,notpainful&musclecontrolnotlost 1.8 1.2

PainfulShock,painfulbutmusclecontrolnotlost 9.0 6.0

PainfulShock(LetGoThreshold) 16.0 10.5

Painful&SevereShock,musclescontract, 23.0 15.0breathingdifcult

PossibleVentricularFibrillation

Fromshortshocks(0.03Sec.) 1,000 1,000

Fromlongershocks(3.0Sec.) 100 100

VentricularFibrillation,CertainDeath Mustoccurduringsusceptible phaseofheartcycletobelethal.

Fromshortshocks(0.03Sec.) 2,750 2,750

Fromshortshocks(3.0Sec.) 275 275

AllvaluesaremilliampereRMSat60Hz.

Reaction to Various Currents By the Average BodyTable 2-1

Fig. 2-1

Current

milliampere

(mA)

Time (Sec.)

00.03 3.0


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The published literature typically presentsresistance values between extremities. Valuesare typically given from hand to hand, a hand

to both feet or from one foot to the other foot.Literature typically presents the body resis-tance as either 500 Ohms or 1,000 Ohms[1].Neither is truly representative of a specic,individual worker. Many other factors have

an affect upon the total lineworker resistance,such as: Are gloves being worn? What arethey made of? Are boots with insulating orconducting soles being worn? How callused

are the workers hands? The actual resistanceof an working individual may vary from the500 Ohms value to a few thousand Ohms.

Most literature of today assumes a body re-sistance of 1,000 Ohms but more and more

utilities are using a 500 Ohm value to err onthe side of safety. While this is an approximate

value, it allows calculations and comparisonsbetween safety equipment offerings to bemade. Resistance may be added to includethe wearing of protective leather gloves orshoes. The use of an alternate body resis-tance beyond those dened in standards, tomeet individual utility requirements, is leftup to the user.

If re-closing is not disabled, a second shockmay occur soon after the rst. If it occurs in

less than 0.5 sec. from the beginning of therst, the combined durations of the two shouldbe considered as one[1]. The short interval

without current does not provide sufcienttime for the person to recover from the rstshock before receiving the second.

It is agreed that the most serious current

path involves the chest cavity. That of hand-to-foot may be less dangerous but still maybe fatal. Keep in mind that while a shockmay be painful but not fatal, it may cause a

related accident. A shock reaction may causea loss of balance, a fall or the dropping ofequipment.

For voltages at or above 1,000 Volts (1 kV)and currents above 5 amperes, the body

resistance decreases because the outer skinis often punctured and the current travels

in the moist inner tissue, which has muchlower resistance. Burns of the bodys internalorgans can result from this type of currentpassage.

The protection methods discussed later aredesigned to ensure the body voltage is main-tained below a selected safe level. It mustbe reduced from the high current level that

results in burns or serious injury to a levelbelow that of heart brillation.

Notable Currents Are:

Perception Level (the least amount of current detectable by the ungloved hand) = 1.1 milliampere*

Painful Shock, painful but muscle control not lost = 9 milliampere*

Painful Shock (Let Go Threshold) = 16 milliampere*

Possible Ventricular Fibrillation:

With a duration of 0.030 Sec. > 1,000 milliampere*

With a duration of 3.000 Sec. > 100 milliampere*

*These are average levels for men, empirically developed from Charles Dalziels[18,19] research.

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Effects of Current of Human Body