(E9) Fuses and Electric ShockFuses and Circuit BreakersA fuse or a circuit breaker automatically disconnects a circuit from the electric power the moment more current is drawn than its design allows. Electrical codes require one or the other to be installed at the point where power first enters a building. The electric supply to a home is usually divided into several circuits, each with its own fuse or breaker, of a rating appropriate for the thickness of the circuit's wires. That way, if the power is interrupted in part of the home, the rest of the home stays connected. A fuse is an insert plugged into the circuit, containing a piece of a special wire, able to carry a regular current for a long time without wearing out, but melting quickly if the current, say, doubles in intensity. The fuses one buys either screw into sockets like those of light bulbs, or are (like car fuses) short insulated tubes with metal ends, fitting into clips that hold them. The old home where I grew up had fuses of a simpler kind, rectangular ceramic plugs with brass contacts at their ends (drawing). Usually a plug was held by a pair of clips (shielded from the user) inside a ceramic box: one clip was connected to the input power and held the contact at one end of the plug (drawing). The other clip was connected to the home's circuit and held the brass contact at the other end. The ceramic block had a narrow hole, and the brass contacts had screws attached, between which a piece of "fuse wire" (spools of which were sold in stores) was threaded through the hole. If the circuit got overloaded, the wire would melt somewhere inside the hole (where it could not cause any fire!) and the fuse would be "blown", stopping the current. 
To replace the fuse one pulled out the plug, unwound the ends of the broken wire from the screws and discarded them. One now threaded a fresh piece of "fuse wire" through the hole, wound its ends around the two screws and tightened the screws to hold the wire firmly. Then the plug would be replaced. Before replacing it, of course, one needed to find why the fuse blew--otherwise, it was likely to "blow" again, often, at once Nowadays fuses have been largely replaced by circuit breakers, automatic switches with compressed springs that try to open them up and thus break the connection. A wire holds them closed, but it is part of the circuit: if the current in the circuit is too large, the wire heats up and expands, its grip on the switch loosens and the spring pushes the switch open. Some circuit breakers are screwed into sockets like fuses and have "reset" buttons (allowing the homeowner to replace fuses with them), but most modern ones are plastic and rectangular, and plug into fitting electrical contacts in the "entrance box" where electric power enters the home. Before doing any work or repair on a circuit in the home, find its connections in the entrance box, the metal box where power enters the house and where all circuit breakers are located. Then disconnect that part of the circuit by resetting its switch from "ON" to "OFF", or unscrew the circuit breaker from its socket. Then check and make sure that the circuit you interrupted was indeed the one with the problem and is now "dead." For this you can use an electricians small "neon light tester" described further below, or an electric meter. When a fuse or circuit breaker "blows," that is a sign its circuit was drawing a current larger than it was designed to handle. That could be caused by a short circuit somewhere--but not necessarily: an overload will also occur if too many appliances are plugged into the same circuit. If a kitchen circuit is disrupted when a toaster, coffee maker and microwave oven operate simultaneously, it may be a smart idea to delay using at least one of them: overloaded circuits have also been known to start fires. If an extension cord feels unusually warm, that too is a warning sign. In any case, wait some minutes before resetting a tripped circuit breaker: because it was released by some wire heating up and expanding, it will not allow itself to be reset until that wire cools down again. 
If no such obvious reason exists, check for a short circuit--e.g. a frayed cord or a short inside an appliance, often announcing its presence by the smell of scorched insulation. Never simply reset the breaker or replace the fuse--if you do, it is likely that it will blow again, putting you back to where you were before. At the very least, unplug or shut off the electric appliance or light which you suspect causes the problem and then reset. If the light goes out again, you have at least cleared one suspect.
To remember:
Electric ShockOur bodies operate by chemistry, not by electricity. Being saturated in salty fluids such as blood, they conduct electric current well and generally do not channel them into well-defined paths, the way electric and electronic appliances do. Still, small electric impulses--spikes of voltage--are associated with the beating heart, and are used by a medical instrument--the electrocardiograph (EKG)--to monitor heartbeats and detect irregularities. Electrodes leading to the instrument are attached to specific locations on the body, and the tiny voltages are amplified, recorded and interpreted. In addition, individuals with irregular heartbeat can be provided with electronic pacemakers. A wire (insulated but with a bare end) is advanced through a blood vessel until its end reaches its assigned position inside the heart, as can be verified by x-rays. It is then connected to a small electric circuit which produces pulses of voltage at selected intervals--often waiting and doing so only if the body has failed to initiate a heartbeat. The device and its battery--even radio controls for checking out its operation!--are contained inside a plastic capsule, which is connected to the wire and inserted under the patient's skin in the chest area. It uses so little electric current that it may last for 10 years before further surgery is needed to replace it. Small voltage waves are also associated with nerve action, and the pattern of such "brain waves" picked up from electrical contacts attached to the skull (EEG or Electric Encephalogram) also has medical uses, e.g. in the diagnosis of epilepsy. However, the association of heart action with electricity points to a possible problem: a strong "electric shock" to the body may on affect the rhythm of the heart, and the results can be fatal, especially if the flow pattern of the electric current passes the heart. The damage depends not on the voltage but on the size of the current which flows through the body: currents over 20 milliamperes (20 thousandths of an ampere or 0.02 ampere = 1/50 ampere) can be dangerous. Strong electric currents that do not pass the heart can still inflict bad burns. As noted, human tissue is a good conductor of electricity. The main obstacle to the flow of current through the body is the skin. Dry skin offers some resistance, and these days when most shoes have soles of rubber or plastic, these also add protection, preventing the current from flowing to the ground (which, as was seen, can complete the circuit). However, handling live wires with wet or sweaty hands increases the danger, as does handling electricity while standing with bare feet on the ground. In any electrical work, of course, the first rule is to disconnect the power or separate the victim from the source of voltage, using plastic etc. to shield oneself from "hot" voltage. The second is to check that power indeed no longer reaches the work area. A small neon lamp connected through a large electrical resistance to two connecting insulating wires ("leads") is often used for such check. Touch one lead to the suspected object, hold the other in your hand: if the object is "hot" a current will flow through the lamp and light it, but because of the enormous resistance, it will be far too weak for you to feel. Electric fuses and circuit breakers do not offer sufficient protection, because to set them off typically takes 15-20 amperes, while to harm a person takes much less. A recent device offers better protection--the so-called "ground fault interrupt" device often installed in power outlets, in particular in bathrooms, where wet hands and bare feet are most likely to be encountered.
In the usual electric circuit at home, the current flowing in along the black "hot" wire is exactly equal to the one flowing back on the white "return" wire (usually grounded at the entrance to the house). The ground fault interrupt device has an electronic circuit which constantly compares these two currents. As long as they are equal, no action is taken. However, if even a little of the incoming current leaks away to the ground--through leaky insulation or, just possibly, through the body of a careless user--the return current is smaller than the incoming one, the circuit senses the difference and automatically cuts off the circuit. Such devices are sensitive enough to save a person from serious harm.
To remember:
Tidbit: Why do switches and circuit breakers click? ...Because they make and break contact very quickly, with a steel spring adding speed to the motion. A switch opening slowly will create a larger spark, a momentary plasma--gas (here, air) hot enough to conduct electricity. It is even possible (though very, very unlikely) that with a powerful current source an electric arc will form, as in an arc welder, bridging the opening gap. An arc (a hot atmospheric plasma) is extremely hot and could melt contacts, but even a spark, though brief, is hot enough to gradually corrode the electrical contacts.
Arcing is a more serious concern in high voltage switches of the electric grid, where switches in the open air open wide to break any possible arc, or else, they are immersed in insulating oil.
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