Explanation of Electrical Short Circuit
Forward: In this article I explain the phenomenon of electrical short circuit and how it is related to electrical shock.
By: Chrysanthus Date Published: 1 Sep 2012
Introduction
Prerequisite
If you are not an electrician (engineer) or an electrical student, then you have to read the articles with the following titles first:
- Electricity Basics and Explanation of Shock
- Alternating Current Basics and Explanation of Shock
To reach the articles, just search the web.
Simple Electrical Circuit
A simple electrical circuit consists of a voltage source (say on the left of a board), one conductor from the top of the voltage source to the top of a load, one conductor from the voltage source to the bottom of the load. Say the voltage source is on the left and the load is on the right. I use this simple circuit to explain short circuit.
Current and Heat
When current flows through a conductor it heats it up. When current flows through a load, the heat is even more. If a circuit is not switched off, then it will continue to heat and finally burn, everything being equal. Circuits that are on through out must have a cooling system. A computer that may be on the whole day in the office has fans inside it; the fans cool the computer as currents in many circuits (loops) inside the computer are always flowing.
For other circuits with a low rate of emitting heat, the atmosphere is enough to cool them.
Any load offers resistance to current flow in a circuit. The higher the resistance, the less the current that flows through it! Assume that in a circuit, you have 2 loads (at the right end); the conductor from the top load goes from the top of the voltage source to the tops of the 2 loads; the conductor from the bottom goes from the bottom of the voltage source to the bottoms of the 2 loads. Assume that the 2 loads are close together physically but rather distant from the voltage source.
Now, the load with the lower resistance will have more current flowing through it than the one with the higher resistance. The total current from the voltage source will be more for the 2 loads than if there were only one load; that will always be the case when you compare the effects of 2 loads to a voltage source with one load.
Now, if one of the loads is very low in resistance, then it will pull a very large amount of current. The following possibilities can occur.
- The voltage source (e.g. battery) may not be able to supply the high current demanded by the very low resistance load and so may be damaged in the course of trying to supply the current. In this case the current flow will ultimately stop. You have to be lucky if this is your occurrence in life. In real life it is the next option that will likely happen.
- If the voltage source can supply the current, which in many cases it will, then the load with very low resistance and/or the conductors from the voltage source will be overheated and burn. The voltage source may also burn, if it cannot support the very high current (and resulting heat) pulled by the very low resistance. This rise in current and burning will take a very short time (say as soon as the circuit is completely connected by a switch). The burning results from the heating of the materials, which results from the high current (surge). The high resistance load is not affected in anyway. However, if the burning leads to fire, then the fire may extend and burn the high resistance load.
In practice a conductor is assume to have zero resistance (extremely low). If you replace the very low resistance load of the above situation, with a conductor (never try this) the current (surge) will even be higher, considered as infinite. The effects and the resulting damages would be worse. As said above, the lower the resistance the more the current that flows through it! So, if the resistance is zero, the current through it will be extremely large, considered as infinite.
The conductor that replaces the resistance above is the short circuit. Now, in an electrical appliance (e.g. electrical iron or TV Set) or an electrical wiring system of a house, there is at least one circuit (electrical loop). If there is more than one circuit, then the circuits should be related (I will not go into such relationships in this article).
Now, an electrical appliance has a number of component(s) and load(s). Components have life or can be damaged. A component can be damaged by becoming a short circuit and may lead to burning inside the appliance. In some cases the high current surge goes backward in the appliance to the mains cable of the appliance, to the socket and to the house electrical wiring system.
The resistance of a rat, animal or man is very low, like a short circuit. In a house roof, if a rat somehow bridges the 2 conductors that carry the mains current, that is a short circuit.
If you are lucky, in your electrical appliance, a component may be damaged by breaking up across. If you remove the component (never try this), you may not see the break because some components have envelope coatings. A gap across a load is an open circuit. Remember, the higher the resistance, the less the current that flows through it. An open circuit is the same as an extremely high resistance. The resistance of an open circuit is considered to be infinite and so allows no current through it.
Note: short circuit of a load leads to infinite current though it (and damage), while open circuit leads to zero current through it (and possibly no damage). Open circuit offers but infinite resistance.
Short Circuit and Shock
The human body is of very low resistance. In fact the only resistance offered by the human body is by the skin. Inside the skin, the human body is a very good conductor of electricity. Whatever is the case, the human body including the skin cover, is still of very low resistance.
If you are standing bare footed and you touch the inside of an electrical appliance or you touch the mains conductor that carries current, you would likely have a shock. Current will move from what you have touched, through your body to the ground. From the ground the current goes right to the voltage source (wherever it is outside the house). Under this condition, you are a short circuit. Remember, the flow of current is extremely fast.
If you are wearing protected shoes and you use one hand to touch one of the conductors (or one terminal of a socket) of the mains and the other hand to touch the other conductor (or the other terminal of the socket), you will definitely have a shock. This is because current will flow into your body by one hand and out of your body by the other hand as you are connected to the 2 conductors. This time current does not flow to the earth and you are not much different from the rat case mentioned above. The short circuit you present here is from one hand through your chess to the other hand. In the above case, the short circuit is from your hand, through your chess, stomach and then legs.
In this section, I explain how short circuit is prevented in the electrical wiring system of a house. The explanation here does not include electrical appliances like TV sets and electric cookers. The electrical system of a house refers to the wires in the walls of the house and above the ceiling.
If you go outside your house (or building), you can find where the cable from your electricity company reaches your house. Just next to that on the wall, there are 2 components. One is called a circuit breaker and the other is called a fuse. The fuse is normally inside some small container. There may be 2 fuses (do not touch any of the fuses).
A short circuit may occur in the house electrical wiring system, like in the rat case above. If a short circuit occurs inside an electrical appliance connected to the mains, the current surge may go backward in the appliance and enter the mains. The effect to the electrical wiring system of the house may still be like that of the rat.
The function of a fuse is to blow (open circuited) when the current through it is too high (crosses a limit). The function of the circuit breaker is similar, but it is a switch that opens.
When a short circuit occurs in a house, a fuse may blow. Blowing of a fuse means it becomes an open circuit preventing current from further entering the house. The fuse is in the current path from the electrical supplier company. Even the circuit breaker is in the path. When the fuse blows, the house is prevented from burning and the duration of the short circuit current is short.
The circuit breaker and the fuse are serving a similar purpose.
In some cases, the short circuit current is so high that by the time the fuse or the circuit breaker operates, damage (fire, human death from shock, dead of electrical appliance) has been done. Such cases are rare with standard electrical wiring. In the developing countries, such cases are not rare because some of their electrical wirings are not of standard and protective devices (fuse, circuit breaker) may not even be provided.
We have come to the end of the article. I hope you now know the meaning of short circuit.