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🎵

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Good morning to all, to the Unidad didáctica 2, basic concepts about electrical installations.

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In this unit we will take into account important aspects when selecting the different materials that will form our electrical installation.

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These aspects are the following.

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The first thing we are going to see is the voltage or potential difference, which is the work that we will have to do to move or boost electrons from one point to another.

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The greater the potential difference, the more electrons we will be able to move between those two points per unit of time.

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The unit of measurement is the voltage.

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The voltage is a very important element that must be considered when designing, since the materials that we select must be suitable for the nominal work voltage.

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The next concept that we are going to work on is the intensity.

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The intensity is the number of electrons that will pass through the conductor per unit of time, which we will measure in amperes.

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The elements that we select must withstand the nominal work voltage and the intensities greater than the nominal voltage, the time allowed by the protections that we select or configure.

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The next concept that we are going to work on is the frequency.

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What is the frequency?

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Well, it is the number of times that a wave, in this case sinusoidal, is repeated per unit of time.

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And here we have two examples.

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Imagine that both in this case and in this case, this segment is one second.

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Well, here we see that the wave occupies the entire segment, so we can say that this wave is repeated once per unit of time.

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It will have a frequency of one hertz.

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Here we see that the wave per unit of time is repeated five times, so we can say that this signal, this wave, has a frequency of 5 hertz.

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In the case of the Spanish power grid, the work frequency is 50 hertz, that is, it is repeated 50 times in one second.

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Another concept that we are going to see is that of power.

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The power is the amount of electrical energy delivered or absorbed by an element, or in this case we could also talk about our installation.

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It also has to do with what they are going to charge us, and well, later we will see how we calculate it.

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And the last concept that we are going to handle is also that of compatibility.

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Here we simply say that we are going to make sure that all the materials that we select, that we configure, are compatible with each other within our installation.

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We are going to put a practical example to illustrate the importance of the selection of materials and some of the variables that we have taken into account in the previous slide.

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The Joule effect is a phenomenon by which, if a certain electric current circulates in a conductor, part of the kinetic energy of the electrons is transformed into heat due to the constant shocks that occur between electrons and atoms.

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Well, as you can see, the trajectories of the electrons are chaotic, they are not straight and we also see that they collide with the atoms.

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The amount of collisions that occur will depend on the material in which the conductor is built. The better conductivity it has, the fewer collisions it will have.

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For example, copper is a good conductor, it has a high conductivity and a low resistivity, which is its inverse, and the electrons will circulate better.

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The fact is that when they collide with the atoms, they make the atoms vibrate and that kinetic energy is transformed into heat energy.

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This is an undesirable effect because we are already losing energy with an effect that we are not interested in, which is a heat effect in the wiring, and on the other hand it causes the heating of the conductor, which if it is not very well designed can end up damaging it.

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Well, we are going to be able to calculate all these losses. Here we have the Ohm's law, which establishes that the current intensity of a circuit varies directly proportional to the variation of the voltage and inversely proportional to the variation of the resistance.

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We also see that the resistance of the conductor is directly proportional to the resistivity and directly proportional to the length of the conductor. This makes sense because the longer the conductor is, the more collisions it will have.

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It will cost more to get from point A to point B simply because the path is longer. And it will be inversely proportional to the resistance of the conductor. The larger the pipe, the less problems it will have to move the electrons.

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They will be able to circulate more electrons since we have a larger pipe. Here I leave you a table with the different resistivities depending on the material. As an observation, you see that here copper has one of the lowest resistivities. It is the best conductor of this table.

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I also leave you here the formulas of the power depending on the voltage and the intensity. If we clear the voltage from the Ohm's law, we have that the power is equal to the resistance times the intensity squared.

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Notice the importance of selecting the right material. When selecting a cable, we have to take into account the length of it and the section of it to lower this resistance.

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Since lowering this resistance, we also reduce the power that will dissipate in the form of heat in the conductor. I also leave you the resistance of a material based on its temperature.

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We have here the resistance at a given temperature, which is equal to the resistance at 20ºC times 1 plus α, which is a coefficient that depends on the material, times ΔE. What does this mean? Basically, that the resistance of the material increases with the temperature.

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If we have not designed our cable well, for example, we have not given it an adequate section, that means that it will heat up more, because here the resistance will be greater.

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This is going to be translated into an increase in temperature, so we are going to increase the resistance of the cable, and if you look at this formula again, if we increase the resistance of the cable, it also increases the energy that is released in the form of heat.

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As a result, the temperature of the cable increases again, thus entering a circle of increase in temperature that can cause irreparable damage to our installation.

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Now we are going to see a small practical example of the Joule effect applied to a conductor in the classroom.

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For our practice, we will use a power supply composed of 4 batteries of 1.5V arranged in series, so we will have a 6V power supply to which we will connect a copper wire in terminals, which we are measuring and has a resistance of 0.3Ω.

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To measure the Joule effect, we are going to use a thermographic camera, which detects temperature variations in the environment.

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And with the dot that you see here, we can specify exactly at what temperature a material is located. This tool is used by many installers to detect hot spots in electrical circuits or in wiring that can cause an imminent failure.

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So, let's see what happens.

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We see that the cable is cold, it is at room temperature. Here we pass our hand and detect heat.

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Now we are going to get ready to cause the short circuit in the wiring.

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We see that the conductor is in good condition.

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We disconnect from the multimeter and connect to the power supply terminals.

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We already see a change in color in the conductor. Let's take a look with the thermographic camera.

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We see that there is an increase in temperature. Let's look at it with the cross and see that it is at approximately 62ºC. 64ºC is increasing its temperature.

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Perfect. Now we are going to disconnect it and measure the resistance in the wiring.

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Now we can see that the resistance is infinite. This is an open circuit, which means that the cable has suffered damage and in the end it has opened somewhere.

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We can clearly see that the cable is damaged. It is already blackish in color.