- Education and Science
The Electricity Guy: Completing the Circuit
Previously, we looked at how electricity gets to you. In this entry we will look at what happens after electricity gets to you.
Transforming To A Whole New Voltage
I was going to wait to discuss how transformer work for a later entry, but knowing how they work now will help to better understand this entry.
A transformer is basically used for one of two things. To change an AC voltage either up or down, or to isolate circuits,
There are three types of transformers.
The most common is called a step-down transformer. It will take a higher voltage and make it lower.
Less common is a step-up transformer. These are usually only used at the generating plant to make the generated voltage higher for transmission.
There is also an isolation transformer which does not change the voltage at all, but rather just isolates a circuit from others, since a transformer has no physical connection inside (I will explain later). There are many different reasons to isolate a circuit, but the most common is used in public transit. Electric trains, light rail, and trolleys get their power from a network of overhead wires. These wires are all connected together so that vehicles can move from on line to another easily. Each line has its own power supply, but what if for some reason you need to disconnect the power to only one line? You isolate each line with an isolation transformer. Since there is no physical connection, cutting power to one area of the system will not affect others.
A transformer works using a magnetic field. There are two large copper windings inside. The input winding is called the primary and the output winding is called the secondary. Which ever winding is the largest has the higher voltage. Therefore, the common step-down transformer will have the larger winding on the primary. If both windings are exactly the same, there is no voltage change and you only have isolation.
Larger transformers, those used at transformer stations and on poles have an iron core that the copper wraps around. Smaller ones for consumer use generally don't have a core, but the two copper windings are just wrapped around each other. This makes the transformer smaller, lighter, and less expensive. Also, the larger transformers are filled with oil used as a coolant.
The life span of a transformer is 20 - 25 years and is dependent a lot on it's load and the ambient temperature. A transformer that is frequently under a heavy load and is in an area with a lot of hot weather will not live as long as one with less of a load in a colder climate.
As a transformer gets older, the core becomes loose and begins to vibrate. This is what causes the hum you hear coming from large transformers. If a transformer is newer (less than five years) it will likely not make any sound.
Continuing the Circuit
Now we look at what happens once electricity gets to your home or business.
First we will look at single dwelling units, since high-rises are a bit more complicated.
In older installations buildings are fed from pole transformers to the side of the building and into a stack. The stack brings the wires into the building's main panel.
In newer installations, buildings are fed from underground, but still go directly into the panel.
Circuits then branch from that panel. Generally in residential buildings, each circuit is designed to handle 15 amps. Some newer building have 20 amp circuits, but typically it is 15.
There can be more than one outlet on a single 15 amp circuit. However, some electrical safety codes may specify different.
For example, in the area where I reside, kitchens can only have one outlet on each circuit. That means that a modern kitchen could have as many as four circuits feeding it, while the rest of the house is serviced by only one or two. This is because most kitchen appliances require a lot of electricity to operate. Coffee makers and toasters for example, can draw as much as 10 amps each.
While on the subject of electrical code, some areas do not allow a circuit to be split inside an outlet box. Such as the area I reside, a junction box must be used. The junction box is connected to the panel as one circuit, then each outlet is connected to the box (diagram below).
There are typically four color-coded wires used within a circuit. These color codes are different depending on where you are in the world. For this purpose will will use the North American colors.
Since there is only one phase entering the building, there is only one supply wire. That is the black wire. It is called 'live' or 'hot'. It will have an alternating voltage and polarity up to 60 times per second.
However, you need to complete the circuit, so another wire is used. It is the white wire, and that is called the neutral. That wire returns the electricity back to the panel.
Uncommon, but sometimes used is a red wire. This is called the switch return and is only used when a light is controlled by two different switches on the same circuit branch.
Finally is the ground. This wire is labeled as green, but in most cases it is an un-insulated copper wire. The ground is used to connect metal chassis and boxes to the panel which is also grounded to a grounding rod, a copper rod that goes directly into the ground or connected to a water pipe.
Why do we need that? The purpose of the ground is for safety. If you remember from "Back To Basics", electricity is lazy and always wants the easiest path and to the ground. Here is a scenario: Somehow, the live (black) wire in an outlet box somehow manages to make contact with the metal chassis it is mounted on. That will make the metal box and everything around it 'live'. Then you, your toddler, or your pet comes along and happens to touch a screw that is holding the outlet plate on. That does not end well. That is where the ground comes in. If the box is grounded, the electricity will take the easiest path to the ground. That is through the lower resistance copper wire rather than the higher resistance of you, your child, or your pet. That will of course cause an overload that will trip the breaker or blow the fuse. That is a lot better than what would happen otherwise.
Below is a photo of a junction box at one of the buildings where I work. It clearly shows all of the connections, but this box would not pass code. Can you see why?
The cover is missing. To pass code, all junction boxes must have a cover. There is a light switch right next to this junction box. Do you have any idea how many times I have reached for the light switch, and ended up sticking my fingers in here? A lot of times, but nothing has happened...yet.
What's With the Orange Outlets?
In most cases, the color choice for electrical outlets is preference. However, sometimes not. You may have noticed in hospitals some outlets are orange. Some houses even have them. These are orange for a reason. They have isolated grounds. Normally, the ground wire of all boxes are connected together, but an isolated ground goes directly to the panel. The reason for this is to reduce electrical 'noise' created by other devices on the same circuit. Although, in practicality this should not happen anyway since the ground is only connected to the chassis and should never be energized unless there is a fault. I am not exactly sure how having an isolated ground really makes any difference, but hospitals use them anyway.
Electrical Distribution In High-rises
High-rises generally have their transformer inside the building. In this case all three phases are output, but are distributed between the different floors. For example, floors 1,4,7 get one phase, floors 2,5,8 get another, and floors 3,6,9 get the third. However, since all three phases are available within the building, if a medical center or radio station was to move in, it would be very easy to supply the required three-phase cycle.
Below are photos of the inside of the main electrical room inside a downtown office building. An explanation will follow.
You Mean I Can Turn Off the Whole Building ?
Yes, you can.
A master switch is provided to cut power to the whole building.
...so I can just pull down that switch to turn off the building? Great!
Well, yes you can, but you won't have to worry about it because you won't be here any more.
This is a switch that can handle thousands of amps. Although the voltage is only 120, for one tiny split second a large quantity of electricity will jump across the switch contacts causing something called an arc flash. An arc flash is not good for you, and in many cases is a life-limiting situation.
What I am trying to say is, never touch the master switch. There is a procedure on how to shut down the power in a large building and pulling the master is not it.
The master is turned off after all the sub switches and panels are turned off.
The opposite procedure is done when power is restored. Turn on the master first, and then everything else.
That's all for this time.
Look for more electricity and electronics articles coming soon from 'The Electricity Guy'.