4.6 PRACTICAL APPLICATION: A CASE STUDY 217TABLE 4.6.1Resistance Estimates for Various Skin-Contact Conditions
Resistance
Condition Dry WetFinger touch 40 k–1000 k 4–15 k
Hand holding wire 15–50 k 3–6 k
Finger–thumb grasp 10–30 k 2–5 k
Hand holding pliers 5–10 k 1–3 k
Palm touch 3–8 k 1–2 k
Hand around 1^12 -in pipe (or drill handle) 1–3 k 0.1–1.5 k
Two hands around 1^12 -in pipe 0.5–1.5 k 250–750
Hand immersed 200–500
Foot immersed 100–300
Human body, internal
excluding skin, 200–1000
Adapted from R. Lee, “Electrical Safety in Industrial Plants,”IEEE Spectrum(June 1971).
TABLE 4.6.2Resistance Estimates for Equal Areas (130 cm^2 )
of Various Materials
Material ResistanceRubber gloves or soles >20 M
Dry concrete above grade 1–5 M
Dry concrete on grade 0.2–1 M
Leather sole, dry, including foot 0.1–0.5 M
Leather sole, damp, including foot 5–20 k
Wet concrete on grade 1–5 k
Adapted from R. Lee, “Electrical Safety in Industrial Plants,”IEEE Spectrum(June 1971).
resistance but also the resistance of the shoes and the resistance between the shoes and ground.
Tables 4.6.1 and 4.6.2 give basic information for estimating the total resistance to ground.
Let us estimate the current caused when a person is standing on moist ground with leather-
soled damp shoes and unwillingly grabs hold of 240-V wire with a wet palm. Referring to Tables
4.6.1 and 4.6.2 and taking the lowest values, one has 1 kfor the grasp, 200for the body, and
5kfor the feet and shoes. Thus the largest current the victim might carry would be 240V/6.2
k, or about 40 mA. From Figure 4.6.1 one can see that there is a good chance that the victim
would be unable to release the grasp and unable to breathe. Thus a dangerous situation may exist.
When a person is experiencing electric shock, because the damaging effects are progressive,
time becomes a critical factor. First, the source of electric energy should be removed from the
shock victim, even before an emergency medical service is called in. CPR (cardiopulmonary
resuscitation) may have to be administered by a trained person. The condition of ventricular
fibrillation occurs, the heart loses its synchronized pumping action, and the blood circulation
ceases. Sophisticated medical equipment may be needed to restore coherent heart pumping.
When working around electric power, the following general precautions are to be taken:
- Make sure that the power is off before working on any electric wiring or electric equipment.
- Wear rubber gloves and rubber-soled shoes, if possible.
- Avoid standing on a wet surface or on moist ground.
- Avoid working alone, as far as possible, around exposed electric power.