LAMPS CONTROLLED FROM THREE LOCATIONS: THE INTERMEDIATE SWITCH (THEORY)
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Welcome to the home’s electrical systems course by NPR Online Technologies!
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In this lesson, we will see how to control a group of lamps from three different locations. To this end, we will introduce a new electrical component: the intermediate switch.
In the previous lessons, we have seen that two single-pole double-throw switches can be connected together to control a lamp or a group of lamps from two distinct points in a room or corridor.
But, how do we solve the problem if our room has three entrances?
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It is evident that with two switches, we are not able to turn the lamp on and off from the third entrance.
This means that we are forced to turn the lamp on and off by moving to one of the other two entrances, and it is clearly an uncomfortable and undesired action.
For this reason, there are other devices commonly available in electrical stores that solve this problem.
These are the intermediate switches, also known as crossover switches, which are a particular type of double-pole double-throw switches.
By combining two single-pole double-throw switches, and one or more intermediate switches, it is possible to control a group of lamps from a number of points equal to the sum of the two SPDT switches and the intermediate switches used.
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We can see that, the intermediate switch, is equipped with four terminals.
We then have that the SPST switch has two terminals, the SPDT switch has three terminals, and, finally, the intermediate switch has four terminals.
This is why in USA they are respectively called two-way, three-way and four-way switches.
Unfortunately, this is a terminology that is not widely accepted.
For example, in Europe, the SPST switch is named one-way switch and the SPDT switch is named two-way switch, because in this case the name refers to the number of throws of the switches. Of course this terminology generates confusion when compared with the one adopted in USA.
This is the main reason that leads us to use SPST, SPDT and intermediate names for these switches.
Well, after these considerations, we are now ready to analyze the connection diagram in the case of two SPDT switches and one intermediate switch.
In this case, as already stated, we will be able to control a group of lamps from three different locations.
The circuit is very similar to the case that uses only two SPDT switches, with the substantial difference that in this new configuration an intermediate switch has been inserted between them.
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Let’s analyze what happens if we flip the first SPDT switch.
In this case, the hot wire reaches the lamp without interruptions, and consequently the lamp will light up.
Starting from this situation, let us now see what happens when the intermediate switch is flipped.
We recall that an alternative name for this electric component is crossover switch. As can be understood from this alternative name, the two levers made of conductive material which establish the connections with the four terminals, invert the electric connections every time the intermediate switch is flipped.
In other words, we can see that the intermediate switch has two positions, also known as states or configurations.
In the first configuration, the terminal number 1 is connected to the terminal number 3, while the terminal number 2 is connected to the terminal number 4. On the contrary, in the second configuration, these electrical connections are reversed. We have that the terminal number 1 is connected to the terminal number 4, while the terminal number 2 is connected to the terminal number 3.
Well, we are now ready to continue the analysis of our circuit.
More specifically, when we flip the intermediate switch, this creates an interruption of the hot wire continuity, and consequently the lamp will be turned off.
Of course, if we flip the intermediate switch again, the two levers will change position and reverse the connections, thus allowing the lamp to turn on again.
Finally, we can operate the second single-pole double-throw switch.
We can see that by flipping this switch, we are able to change the state of the lamp.
There are many other combinations to test. However, this is enough to state that with these examples, we have shown that by operating any of the three devices, it is always possible to change the state of the lamp: if it is on, it turns off, while if it is off, it turns on.
This is precisely the mechanism used to control a group of lamps from three distinct points, and it is precisely the problem we had to solve.
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Let’s apply it now to the practical case of a room with three entrances.
As can be seen, compared to the case of the previous lesson, there are two SPDT switches and an intermediate switch.
We have chosen to place an SPDT switch at the left entrance:
and one at the bottom entrance:
while the intermediate switch has been placed at the right entrance:
It is obvious that these positions can be modified according to your needs. For example, the intermediate switch could be positioned at the bottom entrance or at the left entrance.
We observe that the connections here implemented follow exactly the connection diagrams analyzed before.
More precisely, we have the hot wire connected to the first SPDT switch,
then we have two traveler wires connected to the intermediate switch,
then we have other two traveler terminals connected to the second SPDT switch,
and, finally, a wire connected to the light bulb.
This is exactly the circuit installed in our room, where we have the hot wire connected to the first SPDT switch, then we have two traveler wires connected to the intermediate switch, then we have other two traveler terminals connected to the second SPDT switch, and, finally, a wire connected to the light bulb:
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Well, after this check, we can carry out simulations to verify the correct functioning of the system.
We can see that in the current situation, the light bulb is off.
If we now enter the room from the right door, we can flip the intermediate switch,
which swaps the connections of the hot wire and allows the lamp to turn on.
If we leave the door placed at the bottom of the figure, we can flip the second SPDT switch, which interrupts the electric continuity of the hot wire by turning off the lamp.
Assuming now to re-enter from the door placed on the left of the figure, in this case we will operate the first SPDT switch, which will change the connections of the hot wire, thus creating electric continuity with the lamp. Consequently the lamp will be turned on:
Well, from this check we have verified the correct functioning of the system.
We can continue the analysis of the circuit considering the laying of the wires and the positioning of the various elements of the system.
It is observed that the intermediate switch and the SPDT switches are placed on the walls at the room entrances.
Note the use of the letter “X” to indicate the position of the intermediate or crossover switch, and the letter “D” to indicate the position of the SPDT switches. It should be noted in this regard that there are specific symbols to indicate all the elements of an electrical system. However, this symbolism is not useful for understanding the operating principle of the circuits we are analyzing, and for simplicity we have decided not to adopt it at this stage of the course.
Let’s go back to the system analysis, observing the presence of the junction box from which the hot and neutral wires are taken.
As regards the installation of the electric conduits, a possible configuration, uses 3 conduits that start from the junction box and reach the intermediate switch, the SPDT switch placed on the left side of the room, and the lamp.
We have also a fourth conduit that allows the connection of the intermediate switch to the SPDT switch placed at the bottom side of the room.
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This is not the only possible configuration.
Another solution, for example, is based in replacing the fourth conduit with one connecting directly the SPDT switch placed at the bottom side of the room with the junction box. In these cases, generally, it is adopted a solution that allows to reduce the length of conduits and wires.
Well, with this last consideration of this lesson, we have come to the end of this lesson. We have analyzed how to control a group of lamps from three different locations by using the intermediate switch.
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Thanks for your attention and … see you next time!
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