20.6 Electric hazards and the human body

 Page 1 / 9
• 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, $r$ , is very small, the power dissipated in the short, $P={V}^{2}/r$ , is very large. For example, if $V$ is 120 V and $r$ is $0\text{.}\text{100}\phantom{\rule{0.25em}{0ex}}\Omega$ , 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. 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 $r$ . Since $P={V}^{2}/r$ , 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 $P={I}^{2}{R}_{\text{w}}$ , where ${R}_{\text{w}}$ is the resistance of the wires and $I$ the current flowing through them. If either $I$ or ${R}_{\text{w}}$ is too large, the wires overheat. For example, a worn appliance cord (with some of its braided wires broken) may have ${R}_{\text{w}}=2\text{.}\text{00}\phantom{\rule{0.25em}{0ex}}\Omega$ rather than the $0\text{.}\text{100}\phantom{\rule{0.25em}{0ex}}\Omega$ it should be. If 10.0 A of current passes through the cord, then $P={I}^{2}{R}_{\text{w}}=\text{200 W}$ is dissipated in the cord—much more than is safe. Similarly, if a wire with a $0\text{.}\text{100}\phantom{\rule{0.25em}{0ex}}\text{-}\phantom{\rule{0.25em}{0ex}}\Omega$ 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 $P=\text{1000 W}$ . 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.

what is circut
newtons law of motion
hasiya
First law:In an inertial frame of reference, an object either remains at rest or continues to move at a constant velocity, unless acted upon by a force.
Manan
is the ability to do work
what is energy
energy is ability of the capacity to doing work
shafiu
what is vector
A quantity that has both magnitude and direction
Donaldo
can a body with out mass float in space
mosco
Is the quantity that has both magnitude and direction
Amoah
Yes it can float in space,e.g.polyethene has no mass that's why it can float in space
Amoah
that's my suggestion,any other explanation can be given also,thanks
Amoah
A charge of 1.6*10^-6C is placed in a uniform electric field in a density 2*5^10Nc^-1, what is the magnitude of the electric force exerted on the charge?
what's phenomena
Phenomena is an observable fact or event.
Love
Prove that 1/d+1/v=1/f
What interference
What is a polarized light called?
Moyinoluwa
what is a half life
the time taken for a radioactive element to decay by half of its original mass
ken
what is radioactive element
mohammed
Half of the total time required by a radioactive nuclear atom to totally disintegrate
Justice
radioactive elements are those with unstable nuclei(ie have protons more than neutrons, or neutrons more than protons
Justice
in other words, the radioactive atom or elements have unequal number of protons to neutrons.
Justice
state the laws of refraction
Fabian
state laws of reflection
Fabian
Why does a bicycle rider bends towards the corner when is turning?
Mac
When do we say that the stone thrown vertically up wards accelerate negatively?
Mac
Give two importance of insulator placed between plates of a capacitor.
Mac
Macho had a shoe with a big sole moving in mudy Road, shanitah had a shoe with a small sole. Give reasons for those two cases.
Mac
when was the name taken from
retardation of a car
Biola
when was the name retardation taken
Biola
did you mean a motion with velocity decreases uniformly by the time? then, the vector acceleration is opposite direction with vector velocity
Sphere
what's velocity
mosco
Velocity is the rate of change of displacement
Divya
Atomic transmutation
An atom is the smallest indivisible particular of an element
what is an atomic
reference on periodic table
what Is resonance?
phenomena of increasing amplitude from normal position of a substance due to some external source.
akif By  By    By By  By 