Transcript Electric shock - University of British Columbia

Human
Body
Electric Shock
Lecture Notes
Physics and Astronomy Outreach Program at the University of British Columbia
Goals
Electric Shock
• Physics: To show the fundamental
laws applied in a ground fault circuit
interrupter (GFCI).
• Biology: To understand the effect of
electrical current on the human body.
Physics and Astronomy Outreach Program at the University of British Columbia
Big Ideas
Electric Shock
• Physics: Ohms law determines
current flow through a resistor.
Electromagnetic induction can be
used to prevent electrical shocks.
• Biology: Human body acts like a
resistor. Electrical currents interfere
with nerve impulses. This affects
muscle contraction, heart functioning
and breathing. Burns can also occur.
Physics and Astronomy Outreach Program at the University of British Columbia
Human body
Electric Shock
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Human body acts like a resistor. V =IR
Dry skin1 : R = 10-100 kΩ
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Rubber shoes2: R = 20MΩ
Wet skin1 : R ~ 1 kΩ
Inside body1: Water and electrolytes. Each
limb has R = ~ 500Ω
Leather shoes, dry2, R = 100-500 kΩ
Leather shoes, wet2, R = 5 – 20 kΩ
Total resistance depends on path of
current.
Physics and Astronomy Outreach Program at the University of British Columbia
Electric shock
Electric Shock
• Effect of current on body (60 Hz AC)1:
0-1 mA
1-3 mA
3-5 mA
6-9 mA
30 mA
80 mA
5-10 A
Imperceptible
Perceptible, mild
Annoyance
Muscles contract and cannot let-go
Asphyxiation
Ventricular fibrillation
Cardiac arrest, burns
• Duration of exposure important as well.
Physics and Astronomy Outreach Program at the University of British Columbia
GFCIs
Electric Shock
• Ground fault circuit interrupters (GFCIs) are
installed in power outlets in wet environments
such as bathrooms, kitchens, outside, pools etc.
• They sense the development of an imbalance
in the current going out of the outlet and the
current returning to the outlet.
• They will shut off the current flow if an
imbalance is sensed.
Physics and Astronomy Outreach Program at the University of British Columbia
GFCIs cont’d
Electric Shock
• The development of an
imbalance in current
will cause a change in the
B- field inside the sense coil.
• A change in magnetic
flux

 per turn

 B  A  BA
induces a voltage
N
d per turn
dt
•  activates electronics
which stops the current flow.
GFCIs cont’d
Electric Shock
• Designed to shut off in 7.5 s for a 5
mA imbalance and within 25 ms for
greater than a 264 mA imbalance3.
• The duration of shock is limited, not
the amount of current.
• GFCIs do not protect against
touching the hot and neutral wires.
Physics and Astronomy Outreach Program at the University of British Columbia
References
Electric Shock
[1] Magison, E.C., Electrical instruments
in hazardous locations 3rd ed.,
Instrument Society of America (1972)
[2]http://www.allaboutcircuits.com/vol_1/c
hpt_3/3.html
[3] Quick, R. C., IEEE Transactions on
Industry Applications, Vol IA-11, No. 1
January/February 1975
Physics and Astronomy Outreach Program at the University of British Columbia