PROTECTION DEVICES AND GROUNDING SYSTEM (THEORY)
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In this theoretical lesson, we will address the main aspects concerning the protection devices and the grounding system.
The earthing system or grounding system connects specific parts of an electric power system with the ground, typically the Earth’s conductive surface, for safety and functional purposes. The choice of earthing system can affect not only the safety but also the electromagnetic compatibility of the installation.
In addition to the neutral and hot conductors, in our electrical system there is also the protective earthing conductor connected to the main earthing terminal of the home electrical system.
The protective earthing conductor, in general, is not important for the normal operation of electronic or electrical equipment.
In fact, we have seen in previous lessons that the electrical system can operate correctly in total absence of the earthing conductor,
but it is used as protection for things or people in the presence of fault currents, for example due to insulation failure between a hot conductor and an exposed conductive part.
Let’s consider for example the case of a chandelier. When the system works regularly, the neutral and hot wires are correctly connected to the lamp.
In these cases, if a person touches the chandelier, even if made of conductive material, there is no risk of electric shock and there is no risk for the person.
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Let’s suppose now that for some reason the hot conductor detaches from its contact with the bulb and comes into contact with the structure of the chandelier. In this situation, the lamp will no longer light, and it is likely that someone will go and check the chandelier, thus coming into contact with it.
This person will create a leakage current path that starts from the hot conductor in contact with the chandelier and reaches the floor. Unfortunately, this current will flow through the body of this person and can cause serious tissue damage or death.
There are also cases in which the presence of a fault is not an immediate danger to people, but can favor the development of a fire.
For example, suppose that the neutral and hot wires that power the bulb disconnect and come into contact with the metal structure of the chandelier.
The metal structure of the chandelier creates electrical continuity between the phase conductor and the neutral conductor and therefore a high electric current arises.
The presence of high currents in the conductors in turn creates abnormal overheating which can permanently damage the electrical system, or even trigger fires.
With these two simple examples we have identified two different danger scenarios associated with malfunctioning of the electrical system.
In the first example, we have an electrical leakage current which is typically low but can be fatal.
In the second example, we are in the presence of a short circuit that generates high currents and can also trigger fires.
There are solutions commonly adopted in electrical systems to deal with these danger scenarios. These solutions, which we will discuss in a moment, actually represent real obligations to be taken into consideration during the design and installation of the electrical system.
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Specifically, to cope with the hazard scenario number 1, it is necessary to use a residual-current circuit breaker, while to cope with the hazard scenario number 2, it is necessary to use an over-current circuit breaker.
The circuit breakers are placed in a dedicated electrical panel.
For simplicity’s sake, we will consider for the moment only one circuit breaker in the electrical panel.
We can see how this circuit breaker is closed in the normal operating conditions of the electrical system.
However, in the presence of abnormal conditions,
the circuit breaker will open with a reaction time that depends on the type used.
In this way the electric circuit is interrupted thus avoiding damage to things or people caused by the malfunction of the electric system.
In the specific case we have considered a double-pole circuit breaker, but there are also single-pole types. In this case it is important that the circuit breaker is connected to the hot wire as indicated in the following figure.
An important feature of the circuit breaker is that it can be reset to resume normal operation as soon as the fault in the system has been removed.
This represents the main difference between the circuit breaker and the fuse.
As we can see, in the normal operation of the electric system, the fuse permits the current to pass unobstructed across the filament.
If overloads occur,
the filament melts and stops the flow of electricity.
When a fuse is blown, it is to be discarded and replaced with a new fuse.
This is not the case of the circuit breaker that can be manually reset to resume normal operation as soon as the fault in the system has been removed.
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Well, it would seem that with the use of circuit breakers, we have designed a safe electrical system.
However, there are still some dangerous situations that can be avoided by introducing the grounding system.
Let’s consider the typical fault where the phase conductor detaches from its terminal and comes into contact with the structure of the chandelier.
In that case we have no fault current capable of activating the circuit breaker.
Only when a user of the electrical system touches the chandelier, he will be crossed by a leakage current
that will activate the circuit breaker.
Although the activation of the switch is very quick, we observe that its activation requires that the user be crossed by an electric current at least for a short period of time, and obviously we want to avoid this condition.
To prevent such episodes that can turn into real tragedies, a third conductor called protective earthing conductor is used.
The protective conductor is connected on one side to the metal structure of the chandelier,
and on the other side to a metal stake or a metal mesh or other conductive structure. The metal stake is also known as earth electrode and must be placed in intimate contact with the ground to increase the contact surface with the ground itself. In this way an electrical connection is made with the earth, thus favoring the dispersion of the fault current towards earth.
In correspondence with this electrode (which represents the collection point of the protective earth conductors), it is possible to find the indication of the earth or ground symbol shown in the following figure.
We remind you that the rules for the installation of electrical systems establish precise colors for the grounding wire. For example, in Europe the yellow-green color is used.
Well, made these clarifications, let’s go back to our electrical fault. We have seen how this fault is not detectable since it does not generate any leakage current.
We can instead note that with the use of the earth conductor, when the fault occurs, an electric current is created which from the hot conductor reaches the grounding conductor.
which is immediately detected by the residual-current circuit breaker, which is activated quickly thus interrupting the electrical circuit.
With the introduction of the grounding conductor we have introduced two advantages: first of all, the circuit breaker is activated as soon as the fault occurs and also the leakage current flows along the ground conductor and therefore does not require the presence of contact with things or people as seen for the circuit without the grounding conductor.
This is the main reason why the earth conductor is important in an electrical system and can save our lives when used in conjunction with the residual-current circuit breaker.
This also explains why all equipment or devices powered with dangerous voltages must be connected to the grounding system.
This approach is used not only for lamps, but also for electrical or electronic equipment, as in the case of a washing machine which has a metal structure and, in case of contact of the hot conductor with the metal structure, it could cause serious damage to the washing machine user.
By connecting the metal structure to the grounding system via the protective earth conductor, a leakage current is immediately generated as soon as the fault occurs.
If the electric system is equipped with a residual-current circuit breaker, this will detect the fault current and consequently will disconnect the electric system.
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Many will be wondering how the circuit breaker can recognize the presence of fault currents.
The principle of operation is conceptually very simple.
Residual-current circuit breakers work by comparing the current values through the hot and neutral wires.
In normal conditions, we have that the current flowing through the hot conductor is equal to the current flowing through the neutral conductor.
In the event of a fault, some of the electric current will flow through the earth conductor, or in the absence of the earthing system, through the body of the user.
This results in an imbalance between the current entering the appliance through the hot wire and the current exiting through the neutral wire.
This difference in electrical current is called the residual current.
Well, in summary we can observe that while the residual-current circuit breaker acts by detecting leakage currents or in other words small differences between the electric current of the hot and neutral wires.
We have seen that even the residual-current circuit breaker can operate without the grounding system, but in this case there is a worse degree of protection.
We have seen in fact that the use of the earth conductor is very useful for generating fault currents in order to trigger the activation of the residual-current circuit breaker as quickly as possible.
For this reason it is important to use the residual-current circuit breaker in combination with a grounding system well connected to all appliances and electrical devices.
For what concerns the over-current circuit breaker, it has the task of detecting high currents regardless of the presence of leakage currents.
In particular, the over-current circuit breaker doesn’t check if the current flowing through the neutral and hot wires is the same, but it checks whether the current flowing through the hot conductor exceeds a maximum value.
If this condition occurs, for example due to a short circuit,
the switch opens automatically.
We therefore observe that this mechanism can function correctly even without the grounding conductor.
It is interesting to note that there are particular types of circuit breaker that include both electromagnetic and thermal mechanisms. In this case, the electromagnet mechanism responds instantaneously to large surges in current (short circuits) whereas the thermal mechanism of the circuit breaker provides a delayed response depending on the current value, thus allowing smaller overloads to persist for a longer time. This allows short current spikes such as are produced when a motor or other non-resistive load is switched on. Of course, with very large over-currents during a short circuit, the magnetic element trips the circuit breaker with no intentional additional delay.
Well, we have therefore understood that both switches are important for the safety of the electrical system and also they perform different functions.
We therefore need to install both of them in the electrical system.
The simplest connection scheme is based on the series connection of the two circuit breakers.
In this way, any kind of fault current will be able to interrupt the power line.
A common mistake is to assume that an electrical system equipped with circuit breakers presents no risk of electric shock.
Let’s consider some typical cases. In the case illustrated in the following figure,
we have a person sitting on a wooden chair. The person is therefore isolated from the earth. If this person touches the hot wire, no current can be generated and therefore he will not receive any electric shock.
If the person is sitting on a conductive metal chair, he will no longer be isolated from the earth. If this person touches the hot conductor, a leakage current will be generated which will pass through the person, but which fortunately will activate the residual-current breaker to protect the person.
Fortunately, the presence of the circuit breaker will allow a rapid interruption of the electrical circuit.
Although the interruption of the circuit occurs very quickly, it should be noted that the person was still crossed by electric current, which is an event that, although brief, can cause problems.
From the example just made, it would seem that the wooden chair is able to offer more safety than a metal chair. In reality, this is not always true.
In fact, if the person touches the hot conductor and the neutral conductor at the same time, a current, not necessarily high, will be generated, which will pass through his body.
As stated, the generated current will not necessarily be high, given the resistance offered by the human body, and therefore the automatic switch will not detect any anomaly.
On the other hand, the wooden chair will prevent the generation of leakage currents and therefore the residual-current circuit breaker will not detect anomalies.
In other words, the current will continue to flow through the person’s body.
It is clear that if the person feels the electric shock, he can always move away from the point of contact that is generating it, but there are cases in which this is not possible: think for example of the case in which the person has an illness or the wire is wrapped around an arm or leg.
These cases can therefore quickly lead to the death of the person concerned.
With this last example, we can therefore observe that although the protection devices are a fundamental aspect for the safety of an electrical system, it is always important to avoid contact with the conductors of the electrical system.
Well, we have also come to the end of this lesson dedicated to the grounding system and protection devices.
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