LAMPS CONTROLLED FROM A SINGLE POINT: THE SPST SWITCH (THEORY)
Welcome to the home’s electrical systems course organized by NPR Online Technologies!
In this lesson we will see how to turn on and off lamps by using a single-pole single-throw switch.
Let’s go back to our two lamps connected in parallel.
If we want to turn them off, we have to disconnect at least one of the two conductors that supply them.
Instead of disconnecting them, we can decide to interrupt one of the two wires. If we do that, the two lamps turn off immediately.
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Of course, if we restore the connection, the two lamps turn on again.
It follows that with this interruption and restoration mechanism, we are able to turn the lamps on and off.
This is exactly the mechanism we frequently use when we operate the switches placed in our homes.
The switch is a small box that encloses two terminals with a movable electric contact capable of interrupting or reactivating the connection between the two contacts.
We will say that the switch is closed when it establishes the electric continuity between the two contacts.
On the contrary, we will say that the switch is open when it interrupts the electric continuity between the two contacts.
We also note that the switch has been connected to the hot wire. However, nobody can prevent us from connecting the switch to the neutral wire, as indicated in the following circuit diagram.
From a circuit point of view, there is no difference and the switch could be equivalently connected to the neutral wire.
In fact, by opening and closing the switch, we are able to turn off and on the lamps. However, for safety reasons, it is mandatory to use a switch connection that can break the hot wire.
We are talking generically about a switch, but to be precise we should be talking about a device called “single-pole, single-throw switch” or briefly “SPST switch”.
If you live in the USA, electricians are much more likely to talk about two-way switches, while if you live in Europe, you will hear electricians talking about one-way switches.
There is a long debate on the Internet about the origin of this terminology and about the best name to assign to these switches.
Our opinion is: if you want to be precise and don’t want to be misunderstood then use the term SPST switch, but keep in mind that the other names are frequently used in technical jargon.
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Let’s analyze a now a real situation where this switch is used.
Let’s imagine we have a room with a single entrance and with a lamp or a group of lamps to be controlled from a single point at the entrance to the room. Well, this is exactly the situation that can be easily managed by using a switch and in particular a single-pole single-throw switch.
The switch must be placed in the point where we want to control the lamp and by connecting the neutral and hot wires as previously explained, it is possible to turn on and off the lamp as desired:
In particular, it can be seen that the neutral wire is constantly connected to the lamp, while the hot wire passes through the switch and is connected to the lamp only when the switch is closed.
The switch is mounted by embedding it in the wall with a suitable plastic or metal electric box.
We indicate with the letter “S” the point where we are placing the electric box used to install the switch:
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In technical jargon, there are different ways to indicate an electrical box. For example: pattress, pattress box, fitting box, electrical wall switch box, electrical wall outlet box, electrical ceiling box, switch box, outlet box and finally “electrical box” which is the term we will use throughout our course.
Note that electric boxes may be designed not only for embedding in the wall but there are various types designed for surface mounting.
All the components of the electric system are connected with wires running through electric conduits.
An electrical conduit is a tube used to protect and route electrical wiring in a building or structure. Electrical conduit may be made of metal or plastic, and can be rigid or flexible.
The tube can be embedded in the wall (or ceiling or floor) but there are also various types designed for surface mounting (with cabling running along the wall surface).
To embed the electric conduits, we will cut chases in the walls, floors or ceilings.
It is extremely important to avoid errors at this phase of the work.
In fact, if during electric wiring we realize that we need an additional conduit then this will require extra work especially if the wall plastering has already been done.
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In the case of our system, it is simple to infer that we need two chases, one for the electric conduit that leads the cables to the switch, and a second one for the electric conduit that leads the cables from the switch to the lamp.
Well, we have come to the end of this lesson. We have seen how to use and install a single-pole single-throw switch in order to control a lamp or group of lamps from a single point of a room.
We suggest you immediately consolidate the concepts you have just learned by carefully reviewing this lesson and above all by going through all our videos on the subject, by accessing our playlists.
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Thanks for your attention and … see you next time!
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