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  • Define thermal hazard, shock hazard, and short circuit.
  • Explain what effects various levels of current have on the human body.

There are two known hazards of electricity—thermal and shock. A thermal hazard    is one where excessive electric power causes undesired thermal effects, such as starting a fire in the wall of a house. A shock hazard    occurs when electric current passes through a person. Shocks range in severity from painful, but otherwise harmless, to heart-stopping lethality. This section considers these hazards and the various factors affecting them in a quantitative manner. Electrical Safety: Systems and Devices will consider systems and devices for preventing electrical hazards.

Thermal hazards

Electric power causes undesired heating effects whenever electric energy is converted to thermal energy at a rate faster than it can be safely dissipated. A classic example of this is the short circuit    , a low-resistance path between terminals of a voltage source. An example of a short circuit is shown in [link] . Insulation on wires leading to an appliance has worn through, allowing the two wires to come into contact. Such an undesired contact with a high voltage is called a short . Since the resistance of the short, r size 12{r} {} , is very small, the power dissipated in the short, P = V 2 / r size 12{P = V rSup { size 8{2} } /r} {} , is very large. For example, if V size 12{V} {} is 120 V and r size 12{r} {} is 0 . 100 Ω size 12{0 "." "100" %OMEGA } {} , then the power is 144 kW, much greater than that used by a typical household appliance. Thermal energy delivered at this rate will very quickly raise the temperature of surrounding materials, melting or perhaps igniting them.

Part a shows an electric toaster of resistance capital R connected to an A C voltage source. The wires used to connect the toaster to the supply are worn out in one place, allowing them to come into contact with an undesired, lower resistance path, symbolized by lowercase r. Part b of the figure represents the circuit diagram for the electric connection described in part a. The voltage source is connected to two paths in parallel: the toaster with resistance capital R, and the undesired lower resistance path, symbolized by lowercase r.
A short circuit is an undesired low-resistance path across a voltage source. (a) Worn insulation on the wires of a toaster allow them to come into contact with a low resistance r size 12{r} {} . Since P = V 2 / r size 12{P = V rSup { size 8{2} } /r} {} , thermal power is created so rapidly that the cord melts or burns. (b) A schematic of the short circuit.

One particularly insidious aspect of a short circuit is that its resistance may actually be decreased due to the increase in temperature. This can happen if the short creates ionization. These charged atoms and molecules are free to move and, thus, lower the resistance r size 12{r} {} . Since P = V 2 / r size 12{P = V rSup { size 8{2} } /r} {} , the power dissipated in the short rises, possibly causing more ionization, more power, and so on. High voltages, such as the 480-V AC used in some industrial applications, lend themselves to this hazard, because higher voltages create higher initial power production in a short.

Another serious, but less dramatic, thermal hazard occurs when wires supplying power to a user are overloaded with too great a current. As discussed in the previous section, the power dissipated in the supply wires is P = I 2 R w size 12{P = I rSup { size 8{2} } R rSub { size 8{w} } } {} , where R w size 12{R rSub { size 8{w} } } {} is the resistance of the wires and I size 12{I} {} the current flowing through them. If either I size 12{I} {} or R w size 12{R rSub { size 8{w} } } {} is too large, the wires overheat. For example, a worn appliance cord (with some of its braided wires broken) may have R w = 2 . 00 Ω size 12{R rSub { size 8{w} } =2 "." "00"` %OMEGA } {} rather than the 0 . 100 Ω size 12{0 "." "100" %OMEGA } {} it should be. If 10.0 A of current passes through the cord, then P = I 2 R w = 200 W size 12{P = I rSup { size 8{2} } R rSub { size 8{w} } ="200"`W} {} is dissipated in the cord—much more than is safe. Similarly, if a wire with a 0 . 100 - Ω size 12{0 "." "100"- %OMEGA } {} resistance is meant to carry a few amps, but is instead carrying 100 A, it will severely overheat. The power dissipated in the wire will in that case be P = 1000 W size 12{P = "1000"`W} {} . Fuses and circuit breakers are used to limit excessive currents. (See [link] and [link] .) Each device opens the circuit automatically when a sustained current exceeds safe limits.

Questions & Answers

what is temperature
Adeleye Reply
temperature is the measure of degree of hotness or coldness of a body. measured in kelvin
a characteristic which tells hotness or coldness of a body
Average kinetic energy of an object
average kinetic energy of the particles in an object
A measure of the quantity of heat contained in an object which arises from the average kinetic energy of the constituent particles of that object. It can be measured using thermometers. It has a unit of kelvin in the thermodynamic scale and degree Celsius in the Celsius scale.
Mass of air bubble in material medium is negative. why?
Hrithik Reply
a car move 6m. what is the acceleration?
Umaru Reply
depends how long
What is the simplest explanation on the difference of principle, law and a theory
Kym Reply
how did the value of gravitational constant came give me the explanation
Varun Reply
how did the value of gravitational constant 6.67×10°-11Nm2kg-2
A steel ball is dropped onto a hard floor from a height of 1.50 m and rebounds to a height of 1.45 m. (a) Calculate its velocity just before it strikes the floor.
Kris Reply
0.5m* mate.
0.05 I meant.
Guess your solution is correct considering the ball fall from 1.5m height initially.
How can we compare different combinations of capacitors?
Prakash Reply
find the dimension of acceleration if it's unit is ms-2
Happiness Reply
b=-2 ,a =1
M^0 L^1T^-2
what is botany
it is a branch of science which deal with the study of plants animals and environment
what is work
Sunday Reply
a boy moving with an initial velocity of 2m\s and finally canes to rest with a velocity of 3m\s square at times 10se calculate it acceleration
6.6 lol 😁😁
show ur work
sorry..the answer is -10
your question is wrong
If the boy is coming to rest then how the hell will his final velocity be 3 it'll be zero
re-write the question
men i -10 isn't correct.
using v=u + at
ya..1/10 is very correct..
how did the value 6.67×10°-11Nm2kg2 came tell me please
Work is the product of force and distance
what is longitudinal wave
Badmus Reply
A longitudinal wave is wave which moves parallel or along the direction of propagation.
longitudinal wave in liquid is square root of bulk of modulus by density of liquid
Is British mathematical units the same as the United States units?(like inches, cm, ext.)
Nina Reply
We use SI units: kg, m etc but the US sometimes refer to inches etc as British units even though we no longer use them.
Thanks, just what I needed to know.
What is the advantage of a diffraction grating over a double slit in dispersing light into a spectrum?
Uditha Reply
can I ask questions?
Boniface Reply
hello guys
when you will ask the question
anybody can ask here
is free energy possible with magnets?
you could construct an aparatus that might have a slightly higher 'energy profit' than energy used, but you would havw to maintain the machine, and most likely keep it in a vacuum, for no air resistance, and cool it, so chances are quite slim.
calculate the force, p, required to just make a 6kg object move along the horizontal surface where the coefficient of friction is 0.25
Yes ask
if a man travel 7km 30degree east of North then 10km east find the resultant displacement
Ajali Reply
disagree. Displacement is the hypotenuse length of the final position to the starting position. Find x,y components of each leg of journey to determine final position, then use final components to calculate the displacement.
1.The giant star Betelgeuse emits radiant energy at a rate of 10exponent4 times greater than our sun, where as it surface temperature is only half (2900k) that of our sun. Estimate the radius of Betelgeuse assuming e=1, the sun's radius is s=7*10exponent8metres
James Reply
2. A ceramic teapot (e=0.20) and a shiny one (e=0.10), each hold 0.25 l of at 95degrees. A. Estimate the temperature rate of heat loss from each B. Estimate the temperature drop after 30mins for each. Consider only radiation and assume the surrounding at 20degrees
Practice Key Terms 4

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Source:  OpenStax, College physics. OpenStax CNX. Jul 27, 2015 Download for free at http://legacy.cnx.org/content/col11406/1.9
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