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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

some questions on vector
Rose Reply
where are mcq?
wald Reply
Please what is the meaning of mcq
Appiah
what is static on its own
Aniedi Reply
where is the position of distance
AHMED Reply
what is physics
Mbaba Reply
calculate the angular displacement for an object which completes five revolutions
Bisalom Reply
I need someone to explain how white light disperses to form the "ROYGBIV".
Dera Reply
when it pass through a glass prism through a process called dispersion of light
Mahmud
What is an atom
Sulaimon Reply
An atom is the smallest indivisible particle of an element
Dera
When a toilet is flushed or a sink is drained, the water (and other material) begins to rotate about the drain on the way down. Assuming no initial rotation and a flow initially directly straight toward the drain, explain what causes the rotation and which direction it has in the northern hemisphere.
Collin Reply
find the change in entropy of a 2.00 kg block of gold at 1063^0C when it meets to become liquid gold at 1063^0C
precious Reply
if you are asked to make a very sensitive thermometer which of the following fluids would you choose
precious
between mercy and gasoline
precious
it good to use mercury because mercury does not wet glass and it does not evaporate easily
Desmond
0
firdaus
SFAR Sifar SIFAT -<SIFST
firdaus
how many particles are in 2 moles of chromium
Mario Reply
if so use the normal formula number of atom= number of particle/Avogadro's number
Aki
n= np/avogadtos constant. therefore n= 24/ 6.022×10²³
albert
24÷6.022×10²³
albert
@Albert is wrong
Aki
when you cross multiple it should give you Number of particles= mole*Avogadro's number X=2m*6.022*10^²³ X=1.20*10²⁴g
Aki
1.204×10^-22
Maame
please what is final velocity and initial velocity
Nonso Reply
don't know
Ekene
what do you want to become in future
Ekene
Y ar u asking pls
Nonso
I think initial velocity is the velocity that the mobile starts with at the start time (t=0s) but I don't think I heard abt final velocity
Malak
Malak where are you now I need to learn more from you
Ekene
initial velocity is the velocity an object possess at it intial position or is the starting velocity, while a final velocity is the velocity an object or body possess at it final stage or at the end of it motion
Mubarak
Bohr is kimia, of toksid, cloud, tree have cloud, tree, river but small from toksid fish or another.
firdaus Reply
Heavy, heavy kehidupan susah, kekayaan, berlambak, bergumpul. Dikenali.
firdaus Reply
Gravitional, Gravitional mean kehidup seseorang. Kehidupan bumi, kehidupan muka bumi, kehidupan dalam longitude, kehidupan dalam momentom, kehidupan dalam mongitude. Kehidupan dalam Pelajaran, mean Pelajar kolej.
firdaus
Nonconservative. Sains belajar
firdaus
hello
LFX
what are the types of kinetics
pawi Reply
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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