Electric Shock

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What effect does electricity have on the human body?

Big Ideas: 
  • The human body acts like a resistor, obeying Ohm's law V=IR (voltage (V) = current (I) * resistance (R))
  • Currents can cause physiological problems such as unwanted muscle contractions, interference with the heart and breathing, and burns.
  • Ground-fault circuit interrupters use electromagnetic induction to limit the duration of time someone is shocked due to contact with high voltage from a power outlet.

When a potential difference, V, exists across the human body, the body acts like a resistor with resistance R depending on the path the current follows in the body.  Current, I, passes through the body as given by Ohm's law, V=IR. This could happen for example if you touch a high voltage wire in a cable allowing current to pass from the wire, through you, to the ground.  Many people believe it is voltage that is dangerous when in fact it is the amount of current that determines injury.  A high voltage can lead to a large current for a given resistance.  Signs warn us of 'high voltage' rather than 'high current' because there is always high voltage present, but not a high current (unless you touch something you shouldn't!).

If current, I, passes through your body along a path with resistance R, energy is deposited per unit time according to the power P, where P = I2R. The electrical nerve impulses in the human body which enable muscle contractions, breathing and the beating of the heart can also be interfered with. This occurs because the permeabilty of the nerve cell membrane to ion flow changes depending on the potential difference between the outside and inside of the cell. When the potential difference is affected by electrical current passing through the body this causes pores in the nerve cells to open which stimulates muscle contraction1,2.  In addition to the amount of current, the duration of exposure is also a critical factor in determining injury.  A situation where a person's hand muscles contract so that they can not let go of a wire that is shocking them is especially dangerous 3.

<br />
\begin{tabular}{ c c }<br />
\hline<br />
  Current& Physiological response \\ \hline<br />
  0 - 1 mA & imperceptible\\<br />
  1 - 3 mA & perceptible, mild  \\<br />
3 - 5 mA & annoyance \\<br />
6 - 9 mA & muscles contract and cannot let go \\<br />
30 mA & asphyxiation\\<br />
80 mA & ventricular fibrillation\\<br />
5 - 10 A & cardiac arrest, burns \\<br />
\hline<br />
\end{tabular}<br />

Table 1. Typical responses of the body to current (60 Hz AC) 4

The resistance of the body to current depends on many factors such as surface area of contact, voltage applied and whether the skin is wet or open 5.  The resistance of the skin decreases with increasing voltage so that lower voltages are much safer to work with 6.

The voltage dependence of skin resistance is due to electroporosis where pores in the skin cell membranes open due to the applied voltage and increase in diameter as the applied voltage increases 7.

Wet skin has a much lower resistance than dry skin at around 1 kΩ compared to 10-100 kΩ for dry skin at low voltages 4.  The inside of the body is basically water with electrolytes which provides per limb around 500 Ω resistance.  Wearing insulating shoes increases the total resistance.  Rubber shoes have a resistance of around 20 MΩ, whereas dry leather soles provide a resistance of 100 - 500 kΩ. Wet leather soles only provide 5 - 20 kΩ 8.

Ground fault circuit interrupters (GFCIs) are used to provide extra protection against injury from shock in bathrooms, garages, crawlspaces, above kitchen countertops, by indoor swimming pools or hot tubs, and anywhere outside 9. See Figure 1, for an example of what an outlet with a GFCI installed looks like.

Fig 1.  Picture of an outlet with GFCI installed.

GFCIs work by sensing the development of a difference in current flowing in the 'hot' (higher voltage) wire in an electrical outlet and the 'neutral' (grounded) return wire. If an appliance is working normally, it forms a closed loop and the current must be the same everywhere in the loop, it has nowhere else to go. If you touch a blank wire and current is flowing through you to some other ground, the current being sent back to the outlet in the neutral wire would be less than the current going out by the hot wire.  Figure 2 shows a schematic of a GFCI.

Fig 2.  Schematic of a GFCI.  The GFCI is designed to interrupt current flow when a difference occurs in the amount of current flowing in the hot and neutral wires 3

A magnetic core surrounds the hot and neutral wires and a sense coil is wrapped around the magnetic core.  According to Ampere's law, when the current flowing through the hot wire is the same as the current flowing through the neutral wire, the magnetic field is zero inside the sense coil.

If an imbalance develops, the magnetic field in the sense coil changes from zero to a non zero value in a certain amount of time, which corresponds to a change in the magnetic flux through the sense coil.  According to Faraday's law, this change in magnetic flux through the sense coil induces a voltage in the sense coil and is used by other electronics to open a switch (interrupter) so that current can no longer flow.

GFCIs are designed to shut off in 7.25 s for a 5 mA imbalance and 25 ms for a 264 mA or greater imbalance 10.  Note that GFCIs do not limit the amount of current that goes through a person being shocked, only the duration of the shock.  A person will still get a painful shock and so proper precaution must always be taken. Also note that if a person touches both the 'hot' and the 'neutral' wire a GFCI would NOT shut off the current. (However, this is quite unlikely).  This is because there is no current imbalance.  GFCIs can also fail so as mentioned they are not a replacement for proper safety practices.

Other safety devices such as fuses and circuit breakers exist, but are designed to protect your home against larger than normal currents flowing through a circuit. Fires can be caused when large currents heat up wiring. These devices are not sensitive to the relatively small amount of currents that can harm us.

  • 1. Lee, R.C., Dougherty, W., " Electrical Injury: Mechanisms, Manifestations, and Therapy", IEEE Transactions on Dielectrics and Electrical Insulation, Vol. 10, No. 5, October 2003
  • 2. Bryan. B.C, Andrews, C.J., Hurley, R.A., Taber, K. H., " Electrical Injury, Part I: Mechanisms", J Neuropsychiatry Clin Neurosci 21:3, Summer 2009
  • 3. a. b. Greenwald, E.K., Electrical Hazards and Accidents, p. 24-49, Van Nostrand Reinhold, (1991)
  • 4. a. b. Magison, E.C., Electrical Instruments in Hazardous Locations, p. 353-361, Instrument Society of America ( 1972)
  • 5. W.F. Cooper, Electrical Safety Engineering, p. 44, ButterWorth & Co. (Publishers) Ltd (1978)
  • 6. Adams, J. M., Electrical Safety a guide to the causes and prevention of electrical hazards, p.3, The institution of electrical engineers, London, United Kingdom ( 1994)
  • 7. Lochner, G.P., The voltage-current characteristic of the human skin,p.7, Masters of engineering dissertation, University of Pretoria
  • 8. All about circuits, Shock current path http://www.allaboutcircuits.com/vol_1/chpt_3/3.html [August 17, 2010]
  • 9. BC Hydro, Ground fault circuit interrupter FACT SHEET [August 17, 2010] http://www.bchydro.com/etc/medialib/internet/documents/Power_Smart_FACT_...
  • 10. Quick, R.C., "Ground Fault Circuit Interrupter - Design and Operating Characteristics", IEEE Transactions on Industry Aplications, Vol. IA -11, No. 1., January/Feburary 1975

Comments

Skin does not behave like an

Skin does not behave like an ohmic resistor. So current is not proportional to the applied voltage, or in other words, the resistance depends on the applied voltage.

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