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Part a of the figure shows an electric fuse with metal having low melting point enclosed in a case with wires leading to the circuit and voltage source. There is a viewing window in the fuse casing. Part b shows a circuit breaker. There is a movable metal strip at one end from which a connector to the circuit is attached at a fixed contact point. There is a compressed spring and switch gear attached adjacent to each other at the other end of the movable metal strip. The movable metallic strip has a bimetallic strip attached perpendicular to it at its center. At the opposite end of the bimetallic strip, there is a connector to the voltage source.
(a) A fuse has a metal strip with a low melting point that, when overheated by an excessive current, permanently breaks the connection of a circuit to a voltage source. (b) A circuit breaker is an automatic but restorable electric switch. The one shown here has a bimetallic strip that bends to the right and into the notch if overheated. The spring then forces the metal strip downward, breaking the electrical connection at the points.
The diagram shows an electric circuit with an A C voltage source, a fuse or circuit breaker, and a resistance R all connected in series to form a closed circuit.
Schematic of a circuit with a fuse or circuit breaker in it. Fuses and circuit breakers act like automatic switches that open when sustained current exceeds desired limits.

Fuses and circuit breakers for typical household voltages and currents are relatively simple to produce, but those for large voltages and currents experience special problems. For example, when a circuit breaker tries to interrupt the flow of high-voltage electricity, a spark can jump across its points that ionizes the air in the gap and allows the current to continue flowing. Large circuit breakers found in power-distribution systems employ insulating gas and even use jets of gas to blow out such sparks. Here AC is safer than DC, since AC current goes through zero 120 times per second, giving a quick opportunity to extinguish these arcs.

Shock hazards

Electrical currents through people produce tremendously varied effects. An electrical current can be used to block back pain. The possibility of using electrical current to stimulate muscle action in paralyzed limbs, perhaps allowing paraplegics to walk, is under study. TV dramatizations in which electrical shocks are used to bring a heart attack victim out of ventricular fibrillation (a massively irregular, often fatal, beating of the heart) are more than common. Yet most electrical shock fatalities occur because a current put the heart into fibrillation. A pacemaker uses electrical shocks to stimulate the heart to beat properly. Some fatal shocks do not produce burns, but warts can be safely burned off with electric current (though freezing using liquid nitrogen is now more common). Of course, there are consistent explanations for these disparate effects. The major factors upon which the effects of electrical shock depend are

  1. The amount of current I size 12{I} {}
  2. The path taken by the current
  3. The duration of the shock
  4. The frequency f size 12{f} {} of the current ( f = 0 size 12{f=0} {} for DC)

[link] gives the effects of electrical shocks as a function of current for a typical accidental shock. The effects are for a shock that passes through the trunk of the body, has a duration of 1 s, and is caused by 60-Hz power.

Part a of the diagram shows a person working on an electrically hot wire with a metal tool. The next step shows that he is a victim of electric shock and is thrown backward with his arms and legs stretched. The metal tool also falls off his hand. Part b of the diagram shows a person holding the electrically hot wire with his hands. The person is not thrown away. He cannot let go of the wire because the muscles that close the fingers are stronger than those that open them.
An electric current can cause muscular contractions with varying effects. (a) The victim is “thrown” backward by involuntary muscle contractions that extend the legs and torso. (b) The victim can’t let go of the wire that is stimulating all the muscles in the hand. Those that close the fingers are stronger than those that open them.
Effects of electrical shock as a function of current For an average male shocked through trunk of body for 1 s by 60-Hz AC. Values for females are 60–80% of those listed.
Current (mA) Effect
1 Threshold of sensation
5 Maximum harmless current
10–20 Onset of sustained muscular contraction; cannot let go for duration of shock; contraction of chest muscles may stop breathing during shock
50 Onset of pain
100–300+ Ventricular fibrillation possible; often fatal
300 Onset of burns depending on concentration of current
6000 (6 A) Onset of sustained ventricular contraction and respiratory paralysis; both cease when shock ends; heartbeat may return to normal; used to defibrillate the heart
Practice Key Terms 4

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Source:  OpenStax, Physics of the world around us. OpenStax CNX. May 21, 2015 Download for free at http://legacy.cnx.org/content/col11797/1.1
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