I am a little surprise to know that some people do not know how a switch works or how to read and interpret the symbol of a switch. There are, of course, many different designs and different ways to use the switch and different switches as well.
I would like to start with the very basic and may be a simple old switch:
The diagram shows how the switch is made. The switch indicated is “open “, that is the lever is in UP position and there is no connection and to close it, just lower the lever to make the connection. This is why when we say “open" the switch, we means the connection is open or simply say, not connected. To “close" the switch, we means the connection is made, or a circuit is completed with the connection.
The symbol shown in the diagram is just an illustration of how this actual switch is created so I usually think that it is self explanatory. I guessed I was wrong. I hope those who do not know how a switch works is now able to see how it works.
This switch is then called the normally open switch. Lowering the lever will close the switch and make a connection.
Some switch is made with the connection normally closed, so when you see a label NC and NO, you should know that NC means “Normally Closed" and NO means “Normally Open".
This is also why some switches have 3 terminals, which is different from the diagram shown above with 2 terminals or connections. The 3 terminals switch is normally a latch which means that the switch can be latched and connected with either one terminal or another from a specific terminal. It means that you can throw the lever to NO or NC from the C (common or center) terminal and leave it in the position until you want it otherwise. So the label for the 3 terminals is usually labelled with C, NO, and NC. This kind of switch is called SPDT – single pole double throw to distinguished from the original one which is called SPST – single pole single throw. See the diagram below to see how it is made:
The SPST switch can be a latch or just momentary make the connection and is implemented like a key. The connection is broken as you release the lever or the pole. To keep the connection, you need to hold on or keep pressing the key or switch. This kind of switch is usually designed as a push button.
There are other switches called DPST or DPDT. Can you tell now what they are and how they work?