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Electrical circuits & short circutis, home electrical wiring basics & definitions:
This article answers basic questions about electricity and electrical wiring such as "What is an electrical circuit" and what is a "short circuit".
Sketches at page top and at left are provided courtesy of Carson Dunlop Associates
[Click to enlarge any image]
At the top of this page we show a very simple electrical circuit. Electrical energy (here very roughly defined as a flow of electrons) moves through a circuit to perform work such as lighting a light or heating a room.
At left we show a common analogy using water flowing in pipes to help understand electrical circuits. Some people explain volts as similar to water pressure in a pipe, and amps as water current or total quantity flow. We discuss volts and amps below and in detail at this website.
A closed path of electron movement is required to put electrical energy into use.
This path is called a circuit. Sketch courtesy of Carson Dunlop Associates.
Every electric circuit has four basic parts:
The real source electrical power at most buildings is an electric utility company which operates an electrical generator (a power station). The electric utility brings power from its power generators into a neighborhood where power is to be used by means of electrical transmission lines or power lines (big heavy wires).
For efficiency, electrical power is usually delivered into a neighborhood at very high voltage levels. In the neighborhood high voltage is converted by local power transformers (those big boxes or "cans" you see on some electric utility poles) to the lower voltage levels (240V or 120V) used in most buildings.
From the overhead (or in some communities buried) power transmission lines and transformer, a local power distribution wire (also overhead or buried) brings electrical power close to the building being served with power.
A service entry cable (SEC) (which is the responsibility of the building owner) connects the local power distribution wire to an electric meter mounted on or close to the building, and from the electrical meter, the service entry cable continues in to one or more main electrical panels on or inside the building. Sketch courtesy of Carson Dunlop Associates.
The electrical panel provides a place for mounting of fuses or circuit breakers which protect the building wiring from overheating and short circuits as a way to reduce fire risk. Inside of a building electrical power is distributed to various rooms through individual circuits, each of which is fed from the electrical panel(s).
However, for practical purposes, the source of electricity at an individual home or other building may be thought of as the circuit breaker panel or fuse box from which electrical power is distributed throughout a home or other building. Junction boxes and wall outlets may be thought of as secondary sources of electrical power since they convenient points at which a building occupant can connect electrical devices anywhere in a building.
See ELECTRICAL DEFINITIONS for definitions of "Volts", Watts, Amps, etc.
Electrical Conductors are the wires that carry the electric energy to the point at which it will be used. Conductors offer little (but not zero) resistance to current flow because atoms in the metals they are made of have lots of free electrons and therefore can transport it efficiently.
The electrical load is the amount of electrical energy that any device (a light bulb, your toaster or your washing machine, for example) uses to perform some work. The electrical load (unlike the electrical conductors) offers resistance to current flow.
This resistance makes it possible for the device to convert electric energy to another form (heat, light, mechanical movement, etc.).
The load resistance determines how much current will flow in a circuit. Increase the resistance in the load and you automatically decrease the current flowing in the circuit.
If the voltage applied to a circuit is multiplied by the current in amperes, the result is in units called watts. The watt is a unit of power and indicates the rate at which the load is consuming power.
The electrical load on a circuit is measured in Amps, a measure of electrical current.
The fourth element in an electrical circuit is a means of controlling the energy. This is a device that can interrupt and restore current flow as desired by the user. Switches used in home electric systems control current flow by inserting a very high resistance (air gap) in the circuit to stop the current flow and by) removing the resistance (closing the air gap) to start current flowing again.
A basic principle of electrical wiring is that switches should always be wired into the hot (black or red) line leading directly to a device or outlet. When switches in the hot line are turned off, no hot line power is present in the load device. This greatly reduces the possibility of injury or damage resulting from accidental grounding.
An electrical short circuit is a fault that occurs when a low resistance path exists between the hot lead and some grounded points.
When an electrical short circuit occurs, heavy current will flow in the circuit for as long as it takes for the circuit breaker to trip to OFF or for the fuse to blow.
Because a properly-working circuit breaker or fuse cuts off the current flow quickly, the circuit wiring will not have time to overheat to the point where fire may occur.
Sketch courtesy of Carson Dunlop Associates.
Safety Warning: an over-sized or improperly-installed fuse, and also some circuit breakers do not work reliably; a circuit breaker that is damaged by rust, corrosion, modification, or a circuit breaker which was poorly designed in the first place may not trip - resulting in a fire.
CIRCUIT BREAKER FAILURE RATES for some examples of problem circuit breakers.
An open circuit is a fault that cuts off current flow in a circuit. The break that causes the open circuit may be in the hot line, the neutral line, or the load. An open circuit appears to be off. The break that causes an open circuit may be quite small. Vibration or even changes in temperature may suddenly turn the circuit on again. When looking for the cause of an open circuit, be sure power is off at the source.
Watch out: Safety Warning: Do not attempt to work on your electrical wiring, switches, or outlets unless you are properly trained and equipped to do so. Electrical components in a building can easily cause an electrical shock, burn, or even death.
Even when a hot line switch is off, one terminal on the switch is still connected to the power source. Before doing any work on the switch, the power source must be turned off by setting a circuit breaker to OFF or removing a fuse.
The electrical energy that flows in circuits and operates loads can be generated in either of two different forms. One form is called alternating current (AC); the other form is called direct current (DC).
Alternating current is almost universally used for home electric power and is, therefore, the kind this article is primarily concerned with. In an AC circuit, the amount of voltage applied to the circuit is constantly changing from zero to a maximum and back to zero in one direction and then from zero to maximum and back to zero in the other direction. The maximum voltage is set by the generating plant.
Direct current is most commonly found in homes in the form of electrical energy stored in batteries. In a DC circuit, the amount of voltage and the direction of application are constant. The amount of voltage is determined by the type and size of battery. The direction of current flow is also constant and, as in AC circuits, the amount of current flow is determined by the load resistance.
DEFINITIONS of ELECTRICAL TERMS defines alternating current and direct current in more detail.
This website provides information about a variety of electrical hazards in buildings, with articles focused on the inspection, detection, and reporting of electrical hazards and on proper electrical repair methods for unsafe electrical conditions.
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Recently-posted questions & answers about short circuits and electrical shorts in building electrical wiring
On 2016-09-30 (mod) said: - Suggested basics of electrical wiring for kitchen designers
On 2016-09-30 17:58:39.824194 by TracyWhat are the basics in terms of residential applications should a kitchen designer know
On 2016-07-30 23:02:07.410793 by (mod) - what do you do with that red wire?
As there are risks of electrocution and death for you messing with wiring, I'm nervous about pretending anyone can know for sure what wire is doing what in your system.
But typically if there is an un-used electrical wire terminating in an electrical box, it is capped off with a twist-on connector.
On 2016-07-30 16:08:31.052109 by AnonymousI am getting ready to install a light with ground, black and white wires. the line into the box has a ground, black, white and red wire. what should I do with the red wire?
On 2016-03-17 22:02:25.632921 by (mod) - how to keep the electrical receptacle "hot" when the switch is "closed" ?
Gil Just to keep our terms consistent, a "closed" switch means it is conducting power and an "open" one means it is not.
That nit picking over and done-with, you'll have to run a separate hot wire to your receptacle, as the switch is basically interrupting the hot line.
Examine the circuit, follow the wires. There may be an always-on "hot" already there, depending on how the switch was wired.
On 2016-03-17 21:59:12.606457 by gilI have a switch going to a light and to a plug receptacle, how can I keep the receptacle hot when the switch is closed
On 2016-01-10 14:40:01.499070 by Kelvin
There was a junction box with three wires connected inside,two of the wires was connected black to white,/black to white and the third was connected to a wire that was coming out the junction box on one side and went back into it on the other.
Although the connection was working it seem to me this was incorrect. So I splice all three wires and connected the whites together,the blacks and the grounds,but when I engage the breaker it tripped. What could this problem be?
On 2015-06-30 15:55:03.097310 by (mod) - can I used a single 110 v curcuit that is switched to 3 overhead lights
Sure, Lee, but you will most likely need to add switch wires from individual fixtures to individual wall switches.
On 2015-06-30 14:59:06.511620 by leecan I used a single 110 v curcuit that is switched to 3 overhead lights. can I use that circuit to run 2 switches and split how the lights come on. currently all the light come one with the singles switch . what I want is for two to operate from one stitch and the 3rd from a separate switch.
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