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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 circut
hasiya Reply
newtons law of motion
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in other words, the radioactive atom or elements have unequal number of protons to neutrons.
state the laws of refraction
state laws of reflection
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Atomic transmutation
Basirat Reply
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mosco Reply
what is an atomic
Awene Reply
reference on periodic table
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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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