Electrical grounding systems 2

Electrical grounding systems are a very wide field that includes the grounding of electric current generators, methods of grounding electrical transformers, grounding electronics to ensure that they are not subject to interference from the main grounding, as well as the relationship of grounding systems with various electrical protection devices. Despite the breadth of this field, the literature usually presents it by explaining the methods of grounding the different transformers and connecting the grounding point of them to the loads. The focus is on grounding systems, starting from the second end of the distribution transformer, because before this point from the primary end of the transformer to the electric generators, it is electrically isolated (but magnetically connected) from the secondary end of the distribution transformer, which makes it possible to overlook it to simplify the matter. Also, the secondary end of the low-voltage distribution transformers is what eventually feeds the loads, which makes the relationship between the grounding of the loads and this transformer a close relationship.

The field of electrical grounding systems is usually introduced by  IEC 60364  and  IEEE 80/81 standards  and the resulting literature in the field. Therefore, this article will start from the same base and give a brief introduction in this field through these two standards (without going too deep into  IEEE standards) with multiple additions, the most important of which is the  famous Schneider  Electrical Installation Guide .

The purpose of electrical grounding systems can be summarized in two main points:

A- Ground Protection:-

Connecting current-conducting surfaces, grounding points of sockets, etc. to earth protects us from exposure to electric  shock  by making sure that the voltage of the conductive surfaces is kept equal to the earth’s voltage. After the failure of the electrical insulation of any of the wires and their contact with any metal surface, the current will pass directly to the ground instead of passing through the body of any person touching that surface, or passing through any flammable material and causing a fire, or passing through devices that are not intended for such a high current. causing damage to it.

B- Functional grounding:-

What is meant by functional grounding is the current-carrying grounding line returning to the current source, as is practiced in some of the grounding systems presented in this article. One of the power transmission systems based on the return of current through the ground is the  Single-Wire Earth Return system

A –  Basic requirements for grounding electrical systems:

The parts of electrical grounding systems are defined as follows:

1- (No. 1 in the picture above) – the  Earth Electrode  , which is a single conductor (copper cable without insulation), or a group of conductors connected together and located underground (under the soil) with the possibility of  Rode rods  planted in the ground and connected with the conductors To form a grounding network under the soil. And the rods are planted in the ground at sufficient distances so that the maximum value of the current passing through one of them does not affect or raise the voltage of the rest. The following picture provides a more detailed illustration of the shape of the grounding grid placed under the soil.

2- The Earthing  Conductor  is a conductor (cable), or group of conductors, that connects the main grounding line (No. 6 in the image above) to the  Earth Electrodes  (No. 1). Its practical design looks as shown in the two pictures here:

3 –  Protective Conductor  or  Protective Earthing (PE)  responsible for connecting all metal surfaces of electrical devices together in the facility and connecting them to the same main earthing line (No. 6). An example of this protective conductor is the third line plug/plug (often the longest and middle to the top) found in homes. As well as ground lines for air conditioners and other loads connected to the body of the distribution board. They are usually yellow and green.

4 –  Extraneous Conductive Part Defined  by the British Standard  BS 7671  as devices that meet the three conditions together:

• 1- It must be made of a conductive material
• 2- It should be liable to bear the earth’s effort (such as if its surface is in contact with the ground).
• 3- It shall not be part of any electrical system

As you can see from the definition, you do not need to ground every metal in your facility or home because the second requirement is that this metal is liable to carry an electric potential. The reason for this condition is that if any of the live wires come into contact with the surface of any electrical device (such as the surface of a microwave oven, for example), then when a person touches the surface of the microwave on the one hand and the surface of any other metal object that carries the earth’s voltage on the other hand, the current will pass through his body, which is what should be banned. From this rule, you can exclude all objects that cannot support the effort of the earth, such as door handles (for wooden doors), window frames, and others. There is also another obvious condition that is not mentioned in the list above, which is accessibility and touchability of the device. Therefore, you can exclude pipes buried underground or behind a wall from the list. Metal surfaces covered with an electrical insulator such as plastic and others are also excluded.

From the same point of view, and based on the method of installing and connecting the various devices, it is possible to exclude pieces such as stair railings, metal tables, and others. As a general rule, pieces with electrical resistance must be grounded to earth equal to or greater than the following:

R < Vo/Ib

Where the voltage  Vo  is the voltage of the electrical system in the facility ( 230V  for example) and  Ib  is the limit of current allowed without causing harm to humans ( 30mA  according to  IEC standards ). As an example of  230V  , any piece with a resistance higher than  230/0.030 = 7.66OHM  between it and the earth must be grounded with a separate cable.

5 –  Bonding Conductor  , which aims to connect all metal parts together to ensure that no voltage difference forms between any of them and the ground, and creates what is known as  Equipotential Bonding . It is used to connect the different pieces that were explained in the previous point.

The linking of all metal pieces together is known as the  Equipotential Bonding  system according to  IEC  standards , and it was established with the aim that if any of the various metal surfaces in the facility (such as water or gas pipes) come into contact with any of the live electrical voltage lines, the voltage on all These surfaces will be equal (because it will become like a single metallic body without any electrical resistance between its parts), so no current discharge will occur anywhere else to save the lives of people and devices until the protection devices catch the error and separate the circuit.

6 – The main earthing  terminal  , which is  the bar  or the metal tape to which all the grounding points in the facility are connected from the connection connections (No. 5) and protection conductors (No. 3) as explained in the previous points. It is usually located inside the  Distribution Boards  (as the following picture)  , or  the Switchboard  , etc., but some local designs and standards require its presence outside the facility or building, as shown in some of the previous pictures. Also, there may be several bars  in  several different distribution boards, so they are also all connected to each other.

7-  Removable Link earthing system separation piece  for testing the resistance and validity of the ground rods.

B – Types of electrical grounding systems:

After presenting the methods of connecting the various loads and pieces to the grounding points in the facility, you must know the relationship between those grounding points and the electrical distribution transformer, which will be explained in this part of the article.

There are different types of grounding systems and the choice of any of them determines the type or protection requirements for the electrical system and how it is connected to the loads. These different types fall under three main types that are different from each other, and the network designer chooses any of them during his design of the electrical distribution system according to his needs and in accordance with the standards and requirements followed in his country. Choosing the type of grounding system by the network designer will result in three important and independent decisions:

• The method of connecting the neutral line in the network and its relationship to the apparent parts and pieces of the loads.
• Use a  separate Protective Earthing (PE) line/cable or combine  it with the neutral line and use only one cable.
• How to use ground fault protection devices (from a short circuit with earth or current leakage to earth) to separate the circuit or give an alarm of a ground fault.

The different earthing systems are also known as “Earthing  System Arrangements ” and describe the method of earthing the lower part of the network on the secondary end of the  MV/LV Transformers  (specifically the neutral connection  of  the transformer) and how to earth the visible parts of the electrical connections connected to the network. Grounding methods also describe whether the transformer’s neutral  (N)  and protective conductor  (PE)  are separate or connected together. Finally, choose whether the use of a protective device for the increase in current is sufficient, or is there a need for a special protection to read and disconnect the circuit when the insulation of the conductors fails and there is a leakage current to the ground. All these points have been combined together and covered by the standard as described throughout this article.

The  IEC 60364 standard  classifies the different grounding systems with two letters, the first letter of which identifies the method of connection between the ground and the current-providing equipment (whether it is an electrical transformer, generator, or other), while the second letter identifies the method of connection between the ground or the electrical network and the various electrical equipment (including loads) that are being fed. The characters available according to the standard are as follows:

• The letter  T : symbolizes a direct connection with the ground, which is an abbreviation of the French word  Terre  , which means “land.”
• The letter  I : means the absence of any connection with the ground, or the presence of a connection with high resistance, which is an abbreviation of the word  isole  in French and means “isolation”.
• The letter  N : It symbolizes the connection of the neutral coming from the transformer, and it is an abbreviation for the French word  Neutre  , which means “neutral.”

The first letter of the earthing system can only be either  T  or  I  , and the second letter can only be   either T  or  N. This just gives us the following combinations:

• TT
• IT
• TN
• IN  (it is a non-existent form because it is included in  IT  as you will see later)

The standard also allows the presence of sub-letters following the grounding system  , TN  , as follows:

• TN-C
• TN-S
• TN-CS

Where the letter  C is  an abbreviation for the word  Combined  , i.e. combined, and the letter  S is  an abbreviation for the word  Separate  , i.e. separated. Hence, we present all the installations of grounding systems in one picture, as you can see here:

B1 –  TT grounding system

We first start by talking about the  TT grounding system . As the type of this system indicates, the neutral end of the current-providing side (such as a transformer) is directly connected to the ground,  T  , through an  electrode . Likewise, the grounding point on the side of the load is connected to a different grounding that is directly related to it through another ground rod, and this grounding includes the grounding of the  Protective Conductor  and the  Extraneous Conductive Part . Also, the neutral point coming out of the transformer goes and is connected to the loads by means of a separate cable. The only connection between the grounding and neutral of a transformer is that the two are connected together at the transformer end only. Check out the picture of the  TT  system to learn more about it.

The method used to protect humans from the ground leakage current in this system is by using a  Residual Current Device  ( RCD ), which is usually set (according to the standards for each country) to disconnect the circuit breaker when the amount of ground leakage current exceeds  30mA . TT  earthing systems  separate the grounding line from the neutral allowing the use of four-phase circuit breakers, but it is necessary to ensure good soil between the loads and the transformer because the only path available for the earth-leaked current to return to the transformer is through the soil of the earth. If the loads are earthed at more than one point (as in the picture), RCD  protection must be provided  at each of these points.

The most important characteristics of this system can be summarized as follows:

• It is the easiest in terms of design and installation.
• It does not require permanent monitoring of the grounding state during the network operation, but a periodic examination of the current leakage protection devices must be carried out.
• Protection against leakage current is ensured by the use of  RCDs  , which also reduces the risk of fire.
• Any current leakage to the ground (such as the failure of the cable insulation, which causes it to connect to the ground or to the surrounding metal objects) will result in circuit separation and loss of the current load. However, this separation will be limited to the load in which the problem occurred, according to the settings and timing of the separation of the  RCD  circuit breakers connected in series.
• Some loads that by their nature produce a leakage current to the ground (such as welding machines) will disconnect the breaker frequently and undesirably during their work, so these loads will need a separate isolation transformer of their own, or that  specialized RCD  devices be used for them.

B2 – TN grounding system 

The TN  system  is similar  to the TT  system in the grounding point of the current source (transformer or generator) in terms of connecting the neutral point directly to the ground, but it differs in grounding the end of the loads. In this system, ground poles planted or connected to the soil are not constructed from the load side (except in some exceptions as you will see below), but it is sufficient to use the neutral and/or ground cable coming from the transformer for various grounding purposes. This system is divided into three types according to the arrival of the neutral cable from the current source to the loads as follows:

TN-S system 

In this system, five cables come out of the transformer, of which one cable is used as a neutral point for loads, while the second is used as a protective earthing  ( PE ) cable . It is one of the most popular and frequently used systems. This system is mandatory by  IEC  standard for circuits with a cable cross-sectional width of less than  10  millimeters ( mm2 ) or when portable devices (eg those connected to sockets) are used. Underground wire sheathing or metal reinforcement can be used to connect  PE  protection line and is generally applicable.

An example of using this system in electrical sockets is that one of the two cables is connected to the ground (  PE  ) and the other to the point of the cold line,  N ,  as in the picture:

This system saves the costs of establishing an earthing network for each load or facility separately, but it requires an additional grounding cable from the transformer (through the distribution substation) to the loads.

TN-C system 

It is completely similar to the previous system, except that instead of using two separate cables for neutral and ground, only one cable is used for the two purposes together, as in the picture below, and this cable is called the  Protective Earth and Neutral (PEN) cable . The disadvantage of this system is that if there is a need to open the current circuit breakers for the loads, the circuit breaker will separate the neutral and ground lines together (in order to ensure positive isolation of the  load  ), and this is not safe because the grounding point will lose the purpose of its existence. Also, separating the hot lines only without separating the neutral line is not recommended because it is not a positive separation, in the sense that it will not separate all the lines or cables through which the short current may pass.

The standard does not prevent a separate grounding point/bars from being provided at the end of the load and connected to the grounding line coming from the current source, and this also applies to the  TN-S system  as well. Also, it is forbidden to use the  TN-C system  with a neutral and earth conductor with a cross section of less than  10  square millimeters ( mm2 ) and it is forbidden to use it with portable devices (such as those connected to sockets).

When using this system, care must be taken to have an environment with equal voltage  in  the network, and that is done by planting earthing rods distributed at equal distances as much as possible, because the  PEN  line will carry the neutral current and the  3rd Harmonic currents . Therefore it is necessary to connect the  PEN  line with grounding.

TN-CS system 

Here we combine the two previous systems together, as only one cable comes out of the transformer, collecting the ground and neutral, and then it is divided in one of the distribution panels into two separate cables, as in the picture.

In this system, it is forbidden to use the  PE  ground line as a neutral point  N  to feed the load after the point of separation of the neutral cable from the ground as shown in the picture below. It is also forbidden to connect the ground and neutral lines together after their separation, because any unintended separation of the neutral cable at the top of the circuit, then it will result in the separation of the  PE ground protection line  at the bottom of the circuit, which is dangerous and unacceptable.

Below you can see how the breaker disconnects current in both  TN-C  and  TN-S systems . In the  TN-C system  , the current will not be separated from the neutral point of the load, because this point itself connects the ground protection line of  PE  loads and should not be cut off under any circumstances, while this can be done in the  TN-S system  because the neutral line is separated from the ground line.

Although Schneider’s  Electrical Installation Guide specifies  a way to protect the circuit only by using circuit breakers to increase the current, it is possible to use circuit breakers to protect against leakage current, but after studying each load separately and carefully, and this point will not be discussed in this article.

The characteristics of the TN  earthing system are  summarized as follows:

• The extension of this system in the future after its installation requires caution and study by specialized electricians so as not to confuse the different grounding methods.
• The TN-C earthing system   is relatively cheaper than the other two types. It is also forbidden to use it in facilities that have a high risk of fire (such as gas stations) and it is forbidden to use it in systems that have many computers because it will cause harmonics  carried  on the neutral line.
• The  TN-S system is characterized by its  ability to be used with small size conductors. It is also preferable to use it in networks where there are many computers because it will provide a  clean  PE  ground  protection line (as opposed to  Dirty ), which will prevent any interference or malfunctions in sensitive computers and servers.

B3 – IT grounding system 

The first letter  I  indicates that this earthing system is in which the neutral point of the transformer or generator is not connected to earth, i.e. it is isolated from earth.

However, this system allows the neutral point to be connected to the ground indirectly through a resistance to the ground, so the neutral grounding system is also called the  Impedance-Earthed Neutral resistance .

As for the load side, the grounding point is connected to the ground, which is indicated by the  letter T.

You may wonder why the load is connected to the ground (soil) directly without doing so at the generating end, unless it provides a path for the current to return to its source when any ground fault occurs in the circuit.

The answer is that the grounding point on the end of the load for systems in which the source is not grounded at all works as a point to ensure equal voltage on the material surfaces of the various electrical devices when any  Equipotential Bonding error occurs  .

A resistance can also be connected between the neutral point of the transformer and the ground, as in the following picture.

The value of this resistance is between  1000  and  2000  ohms.

IEEE  standards  classify this resistance into two types, the first is  High Impedance Grounding  and the second is  Low Impedance Grounding  , but this article will not delve further into this distinction between the two systems.

Here, the grounding point of the loads (which includes all the  Extraneous Conductive Part ) is connected to grounding rods.

The reason for using such a resistance is to adjust the amount of voltage relative to the ground in the current lines, especially when an unacceptable rise occurs in the medium voltage lines coming from the network and others.

 Ungrounded  IT systems use  a device to monitor the condition of the isolation of the current source’s neutral (such as a transformer) from ground, known as an Insulation Monitoring Device (IMD) .

In the event of a fault occurring between one of the power lines and the ground (let’s call it the first fault), there is no reason to disconnect the circuit, and an alarm can suffice for the maintenance team to investigate and find out the location of the fault.

However, in the event of another short circuit with earth (the second fault), the circuit must be disconnected to touch the two faulty lines together.

The characteristics of this system are summarized as follows:

• This system allows the circuit to continue working and provide current for as long as possible even when the first failure occurs
• Provides an alarm when the first earth fault (or current leakage) occurs in the circuit
• A maintenance team is required to locate the fault while the circuit is working

The following image from  IEEE  shows the type of grounding system used in different countries of the world.

It should be noted that all systems have their uses.

It is not black and white and there is no right or wrong answer. Therefore, the standards classified different grounding systems and left the choice of any of them to engineers around the world.

The choice of any of these systems does not depend on which one is safer because they are all equal in terms of human protection if the network is installed properly and in accordance with the conditions of each system.

Therefore, the choice of any of these systems depends first on the legal requirements in the facility or the country in which the system is installed.

Secondly, on the priority of continuing the network’s work and the danger of power outages, thirdly on the way and how to operate the network and the presence of individuals for that, and finally on the type of loads connected to the network.

The network designer can also use the following table from Schneider’s guide to see the difference between the different systems and determine what is suitable for him.

Note that system  IT  is mentioned twice in the table, the first time is  IT(1)  special in case of the first ground fault in the network and the second time is  IT(2)

Especially in the event of a second error (in the presence of the first error) in the network.

Also, you may need to use more than one grounding system in the same organization or network, according to the type of loads that will be connected to that system.

In this sense, you can see the most appropriate use in terms of type of load for each of the different grounding systems shown in the table: