1.3.

SHOCK
1.3.1. Description
Electric shock is the physical stimulation that occurs when electric current flows through the human body. The distribution of
current flow through the body is a function of the resistance of the various paths through which the current flows. The final
trauma associated with the electric shock is usually determined by the most critical path called the shock circuit. The
symptoms may include a mild tingling sensation, violent muscle contractions, heart arrhythmia, or tissue damage. Detailed
descriptions of electric current trauma are included in Chap. 9. For the purposes of this chapter, tissue damage may be
attributed to at least two major causes.

Burning. Burns caused by electric current are almost always third degree because the burning occurs from the inside of the
body. This means that the growth centers are destroyed. Electric-current burns can be especially severe when they involve
vital internal organs.

Cell Wall Damage. Research funded by the Electric Power Research Institute (EPRI) has shown that cell death can result from
the enlargement of cellular pores due to high-intensity electric fields. 1 This research has been performed primarily by Dr.
Raphael C. Lee and his colleagues at the University of Chicago. This trauma, called electroporation, allows ions to flow freely
through the cell membranes, causing cell death.

1.3.2. Influencing Factors

Several factors influence the severity of electrical shock. These factors include the physical condition and responses of the
victim, the path of the current flow, the duration of the current flow, the magnitude of the current, the frequency of the current,
and the voltage magnitude causing the shock.

Physical Condition and Physical Response. The physical condition of the individual greatly influences the effects of current
flow. A given amount of current flow will often cause less trauma to a person in good physical condition. Moreover, if the
victim of the shock has any specific medical problems such as heart or lung ailments, these parts of the body will be severely
affected by relatively low currents. A diseased heart, for example, is more likely to suffer ventricular fibrillation than a healthy
heart.

Current Duration. The amount of energy delivered to the body is directly proportional to the length of time that the current
flows; consequently, the degree of trauma is also directly proportional to the duration of the current. Three examples illustrate
this concept:

1. Current flow through body tissues delivers energy in the form of heat. The magnitude of energy may be approximated by

J = I 2Rt

(1.1)

where J = energy, joules

 I = current, amperes

 R = resistance of the current path through the body, ohms

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  t = time of current flow, seconds

If sufficient heat is delivered, tissue burning and/or organ shutdown can occur. Note that the amount of heat that is
delivered is directly proportional to the duration of the current (t).

2. Some portion of the externally caused current flow will tend to follow the current paths used by the body's central nervous
system. Since the external current is much larger than the normal nervous system current flow, damage can occur to the
nervous system. Note that nervous system damage can be fatal even with relatively short durations of current; however,
increased duration heightens the chance that damage will occur.

3. Generally, a longer duration of current through the heart is more likely to cause ventricular fibrillation. Fibrillation seems to
occur when the externally applied electric field overlaps with the body's cardiac cycle. The likelihood of this event
increases with time.

Frequency. Table 1.1 lists the broad relationships between frequency and the harmful effects of current flow through the
body. Note that at higher frequencies, the effects of Joule (I 2t) heating become less significant. This decrease is related to the
increased capacitive current flow at higher frequencies.

Table 1.1 Important Frequency Ranges of Electrical Injury

Frequency Regimen Applications Harmful effects

DC–10 kHz Low Commercial electrical power; soft tissue healing; transcutaneous Joule heating; destructive cell
frequency electrical stimulation membrane potentials

100 kHz–100 Radio Diathermy; electrocautery Joule heating; dielectric heating of

MHz frequency proteins

100 MHz– Microwave Microwave ovens Dielectric heating of water

100 GHz

10 13–10 14 Hz Infrared Heating; CO 2 lasers Dielectric heating of water

10 14–10 15 Hz Visible light Optical lasers Retinal injury; photochemical reactions

10 15 Hz and Ionizing Radiotherapy; x-ray imaging; UV therapy Generation of free radicals

higher radiation

It should be noted that some differences are apparent even between DC (zero Hz) and standard power line frequencies (50 to
60 Hz). When equal current magnitudes are compared (DC to AC rms), anecdotally, DC seems to exhibit two significant
behavioral differences:

1. Victims of DC shock have indicated that they feel greater heating from DC than from AC. The reason for this phenomenon
is not totally understood; however, it has been reported on many occasions.

2. The DC current "let-go" threshold seems to be higher than the AC "let-go" threshold.

Despite the slight differences, personnel should work on or near DC power supplies with the same level of respect that they
use when working on or near AC power supplies. This includes the use of appropriate protective equipment and procedures.

Note: Unless otherwise specifically noted, the equipment and procedures suggested in this handbook should be used for all
power frequencies up to and including 400 Hz.

Voltage Magnitude. The magnitude of the voltage affects electric shock in one or more of the following three ways:

1. At voltages above 400 volts (V), the electrical pressure (voltage) may be sufficient to puncture the epidermis. Since the

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 epidermis provides the major part of the resistance of the human body, the current magnitude will increase dramatically
and lethally when this puncture occurs.

2. The degree of electroporation is higher for greater voltage gradients. That is, the higher voltages cause more intense
fields, which in turn increase the severity of the electroporation.

3. Higher voltages are more likely to cause electrical arcing. While this is not a shock hazard per se, it is related to the shock
hazard since arcing may occur at the point of contact with the electrical conductor.

Although current regulatory and consensus standards use 50 V as the lower limit for the shock hazard, recent research has
shown that harmful or even fatal shocks can result from contact with circuits as low as 30 V.2

Current Magnitude. The magnitude of the current that flows through the body obeys Ohm's law, that is,

E
I=
R

(1.2)

where I = current magnitude, amperes (A)

 E = applied voltage, volts (V)

 R = resistance of path through which current flows, ohms (Ω)

In Fig. 1.1 the worker has contacted a 120-V circuit when an electric drill short-circuits internally. The internal short circuit
impresses 120 V across the body of the worker from the hand to the feet. This creates a current flow through the worker to
the ground and back to the source. The total current flow in this case is given by the formula

E
I=
R1 + R2

(1.3)

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 Figure 1.1 Electric shock current path.

Variable R2 is the resistance of the earth and for the purposes of this analysis may be ignored. Variable R1 is the resistance of
the worker's body and includes the skin resistance, the internal body resistance, and the resistance of the shoes where they
contact the earth.

Typical values for the various components can be found in Tables 1.2 and 1.3. Assume, for example, that the worker shown in
Fig. 1.1 is wearing leather shoes and is standing in wet soil. This person is perspiring heavily and has an internal resistance of
200 Ω. From Tables 1.2 and 1.3 the total resistance can be calculated as

500 Ω (drillhandle) + 200 Ω (internal) + 5000 Ω (wet shoes) = 5700 Ω

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Table 1.2 Nominal Resistance Values for Various Parts of the Human Body

Resistance

Condition (area to suit) Dry Wet

Finger touch 40 kΩ–1 MΩ 4–15 kΩ

Hand holding wire 10–50 kΩ 3–6 kΩ

Finger-thumb grasp* 10–30 kΩ 2–5 kΩ

Hand holding pliers 5–10 kΩ 1–3 kΩ

Palm touch 3–8 kΩ 1–2 kΩ

Hand around 1 1–3 kΩ 0.5–1.5 kΩ

1
2 -inch (in) pipe (or drill handle)

Two hands around 1 0.5–1.5 kΩ 250–750 Ω

1
2 -in pipe

Hand immersed — 200–500 Ω

Foot immersed — 100–300 Ω

Human body, internal, excluding skin — 200–1000 Ω

*Data interpolated.

Source: This table was compiled from Kouwenhoven and Milner. Permission obtained from estate of Ralph Lee.

Table 1.3 Nominal Resistance Values for Various Materials

Material Resistance*

Rubber gloves or soles >20 MΩ

Dry concrete above grade 1–5 MΩ

Dry concrete on grade 0.2–1 MΩ

Leather sole, dry, including foot 0.1–0.5 MΩ

Leather sole, damp, including foot 5–20 kΩ

Wet concrete on grade 1–5 kΩ

*Resistances shown are for 130-cm 2 areas.

Source: Courtesy Ralph Lee.

From this information the total current flow through the body for a 120-V circuit is calculated as

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 120
I= = 21.1 milliamperes (mA)
5700

(1.4)

Table 1.4 lists the approximate effects that various currents will have on a 68-kilogram (kg) human being. The current flow of
21.1 mA is sufficient to cause the worker to go into an "electrical hold." This is a condition wherein the muscles are contracted
and held by the passage of the electric current—the worker cannot let go. Under these circumstances, the electric shock would
continue until the current was interrupted or until someone intervened and freed the worker from the contact. Unless the
worker is freed quickly, tissue and material heating will cause the resistances to drop, resulting in an increase in the current.
Such cases are frequently fatal.

Table 1.4 Nominal Human Response to Current Magnitudes

Current Physiological phenomena Feeling or lethal incidence

(60 Hz)

<1 mA None Imperceptible

1 mA Perception threshold Mild sensation

1–3 mA Painful sensation

3–10 mA

10 mA Paralysis threshold of arms Cannot release hand grip; if no grip, victim may be thrown clear (may progress to higher current
and be fatal)

30 mA Respiratory paralysis Stoppage of breathing (frequently fatal)

75 mA Fibrillation threshold 0.5% Heart action discoordinated (probably fatal)

250 mA Fibrillation threshold 99.5%

(≥5-s exposure)

4A Heart paralysis threshold (no Heart stops for duration of current passage; for short shocks, may restart on interruption of
fibrillation) current (usually not fatal from heart dysfunction)

≥5 A Tissue burning Not fatal unless vital organs are burned

Notes: (1) This data is approximate and based on a 68-kg (150-lb) person. (2) Information for higher current levels is obtained from data derived
from accident victims. (3) Responses are nominal and will vary widely by individual.

Source: Courtesy Ralph Lee.

The reader should note that the values given in this example are for illustration only. Much lower values can and do occur, and
many workers have been electrocuted in exactly this same scenario.

Parts of the Body. Current flow affects the various bodily organs in different manners. For example, the heart can be caused
to fibrillate with as little as 75 mA. The diaphragm and the breathing system can be paralyzed, which possibly may be fatal
without outside intervention, with less than 30 mA of current flow. The specific responses of the various body parts to current
flow are covered in later sections.

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