Understanding Earthing and Bonding Safety

Hello everyone, I welcome you all to my YouTube channel. We have started with a new playlist for earthing and bonding. In this we will be dealing regarding what is earthing, why we require it, how to design earth mat for a substation or for a structure and then at later we will be dealing on how to design an earthing bonding scheme for AC and DC traction. In this lecture, the first one we will be dealing regarding the objectives of earthing bonding, how the impedance of the body varies and the range of a tolerable current. So, let's start the lecture first. In this, I have mentioned the three objectives. The first one being the safety of a person. The second is the protection of the installation. That means in case of fault, the earth provides the return path. so that the breaker trips and avoids overheating of the installation. And the last is EMC i.e. electromagnetic compatibility. Like math is the language of physics, earthing bonding forms the basis or the language for electromagnetic compatibility. Now for the safety of person, I have taken an example of an equipment which can be your washing machine which is getting supply from the transformer secondary which is connected, which is next to your home or your apartment. and there is an MCB, there is no switch and all, I have just directly provided an MCB and 10A is not the rating but it is the tripping current at which the MCB will open instantaneously. So, this is your 230V supply, this is your phase, this is your return neutral and here it is connected to earth, the secondary of the transformer. For initial, the equipment frame is not connected to earth. Now, once you turn on this MCB, the path gets closed and the current starts flowing. One thing to remember is that the current will always come back to its source. No matter what happens, the current will have, if it is flowing, it has to come back to its source. Now, suppose the phase wire has broken and it is touching the equipment frame. and somebody comes and touches the equipment frame at the same point of time. This voltage which is in the phase wire will now get to the frame and from there this body or this human will provide the return path or the completion of the circuit and eventually the current will start flowing. Initially, if you see here, there was a dedicated conductor. Now, there is no dedicated conductor as the frame is not connected through a conductor to this point. The frame is being connected from the body and then through the earth. So, the current will start flowing. Let us suppose this is 230 volts. The impedance offered by the body is around 1000 ohms and the earth is suppose 0. So, the circuit will have now of 1000 ohms. So, the current flowing will be around 230 milliamps. So, will the breaker trip? No, as the breaker is set for 10 amps. So, the current will then become a continuous flow for indefinite time. Now, we will check whether this current is harmful or not in the last slide. Let's go ahead further. Suppose you provide a thing to this equipment frame through an electrode suppose and it's having a resistance of 1 ohms. Now, as you turn on this MCD and there is a fault, then what will happen? The current will flow. Some current will flow through human body also because it's not infinite. It's some 1000 ohms and this is 1 ohm. your model will become like this this is 230 volts 1 ohm and 1000 ohm in parallel most of the current will flow through this low resistance path and the current flowing will be having a very high value of around 230 amps which will cause the tripping of this MCB. So, that means you have protected your installation also within tripping within time and also your human which is not getting exposed to current for a longer duration. Now, As this is an earth electrode, what is it? It is nothing but a conductive part which is embedded inside the earth to have an electrical contact with this earth. What is Local Earth? Local Earth is the region or the part of the Earth which is near to this electrode and which is having a potential not necessarily equal to zero. It can be but mostly it is not. This region is known as Zone of Influence. To have a more feel of it, you can refer to IEEE 487. for the definition of zy and the region outside this zone of influence which is conventionally taken as voltage zero at this region is known as remote earth or reference earth this blue region these definitions are provided for local earth electrode in 50522 i have taken the reference from them Next, we move on to the next slide. In this, we will be talking about the impedance of the body. Now, we know that there is a current that was flowing through the body. It can be dangerous in case the magnitude is high and also if the duration of the fault current flowing through the body is higher. So that means it is dependent on both the current magnitude as well as the shock duration. But, the necessary condition which we deal with while designing is the admissible touch potential. This admissible touch potential is the product of current flowing through body and the impedance offered by the body. by looking at this it is looking like a simple linear equation like a straight line but it is not as the impedance of body is a function of touch voltage path of the current path means whether it's going from hand to hand like this or from hand to leg then the shock duration it is dependent upon frequency that is whether it's ac dc Moisture in the skin, area of contact like suppose this is the frame and how much surface area I am using to hold it. The pressure, how much pressure I am exerting and the temperature. Now this body impedance is divided into two. One is internal and the other one is skin. It is the sum of both. Internal is mainly resistive part and it is mainly due to the flow of current. It is not dependent on other factors. it is only dependent upon the path like it is going from hand to hand or from hand to leg. This is mainly the internal impedance. The skin impedance is comprised of both resistance and capacitance. and it is dependent upon the touch voltage, duration, frequency, moisture, area, pressure and temperature. Now, as you increase the touch voltage, there is a breakdown in the skin and the value of skin impedance falls. It goes considerably low, making this body impedance only dependent upon the internal impedance. This is a simplified model. this is the skin impedance suppose this is hand and other hand and you are touching it so this is the skin impedance which is resistance and capacitance in parallel in between you have internal impedance of the body and then again the skin so this is your hand and this is your other hand so in case of dc This capacitor will offer infinite impedance as it is 1 by omega c, its frequency is 0, so it becomes an open circuit. So, it will be completely resistive in case of DC. as in case of AC, it will have both the components and as these two being parallel, the net impedance offered will be comparatively lower. But we understood that skin impedance is dependent upon voltage only till a certain point in a certain limit as it becomes higher the skin breaks down that means after some time if the voltage is higher like around 200 volts and above in case of both ac and dc the dependence becomes equal as it is only dependent upon internal not on the skin Now, I have given an example of 100V here. Above 200V both are similar because no effect of skin. At 100V, impedance offered by AC is 990 whereas in case of DC it is 1100 Ohms. this is in case of dry condition it depends upon moisture and all so I have not taken that example to refer to other values you can go back to this icn check which is 6479-1 now we understood the impedance the nature of impedance how it works now we will understand what is the amount of current that can impact the impact There are four types. One is the threshold of perception which is suppose this is the body from which the current is flowing this marker and perception means if I touch it and there is current flowing and I feel some tingling effect some current is flowing through me also. So, this amount is equal to 1 milliamps as per IEEE 80. For threshold of let go, let go means I am holding this, I am touching the equipment, the frame of the washing machine, the current is flowing through me but still I can remove my hand, I can remove my hand easily, not a problem. That is wonderful. 1 to 6 mA, star marked as per IEEE 18, 6 mA is for women, 9 mA for men. Threshold for immobilization, that means if I am touching the frame of the washing machine or this pen and the current is flowing through me also due to a fault, then I am not able to remove my hand. I am trying but I am not able to remove. I am not able to remove my right hand. So, what will happen? My muscles will contract in this case and I am unable to remove it till this current stops flowing. That means the circuit breaker gets open. So, this value is 9 to 25 milliamps as per 80 and threshold for ventricular fibrillation. This is the most important. This leads ventricular fibrillation then eventually leads to stoppage of heart or inhibition of respiration. and causing injury or even death. This is the value which is 60-100 mA as per 80 whereas as per IEC 6479-1 this is the curve it is not to scale and I have not drawn it properly also but okay just to have a feel I have made it. As per 6479-1 this value for perception lies in this region as soon as I come and reach this point of 0.5 million. for any time it is independent of time above this I will feel the perception once I move into this region I will go for let go current for let go it is time dependent as well as magnitude so this is the curve that we follow generally from 0.5 to 5 milliamps you can assume and after at 5 it is independent of time straight asymptotic then for immobilization it it is in this region. The main is ventricular fibrillation which is this area. In case of blue shaded, AC 4.1 curve is there which tells that the probability of having is around 5% that means out of 100 around 5 people will go into this range. In case of red shaded region that is AC 4.2, if this amount of current is flowing for this amount of time. Then, the threshold of ventricular fibrillation will be achieved by upto 50% of the population and above that is AC4. Normally we use AC4.2 which is 80mA if you draw it. In case of DC, this is 0.5 is 2 and this is 5 is 25 mA and 80 is around 300 mA. That is ventricular fibrillation threshold. So, I hope you have got an idea of how it is dependent on current and time. We will be dealing regarding touch and step potential in the next lecture in which I will be using these values to further go on. Thank you for listening and God bless you all.

In this lecture, the first one we will be dealing regarding the objectives of earthing bonding, how the impedance of the body varies and the range of a tolerable current. So, let's start the lecture first. In this, I have mentioned the three objectives.

The first one being the safety of a person. The second is the protection of the installation. That means in case of fault, the earth provides the return path.

so that the breaker trips and avoids overheating of the installation. And the last is EMC i.e. electromagnetic compatibility. Like math is the language of physics, earthing bonding forms the basis or the language for electromagnetic compatibility. Now for the safety of person, I have taken an example of an equipment which can be your washing machine which is getting supply from the transformer secondary which is connected, which is next to your home or your apartment. and there is an MCB, there is no switch and all, I have just directly provided an MCB and 10A is not the rating but it is the tripping current at which the MCB will open instantaneously.

So, this is your 230V supply, this is your phase, this is your return neutral and here it is connected to earth, the secondary of the transformer. For initial, the equipment frame is not connected to earth. Now, once you turn on this MCB, the path gets closed and the current starts flowing.

One thing to remember is that the current will always come back to its source. No matter what happens, the current will have, if it is flowing, it has to come back to its source. Now, suppose the phase wire has broken and it is touching the equipment frame.

and somebody comes and touches the equipment frame at the same point of time. This voltage which is in the phase wire will now get to the frame and from there this body or this human will provide the return path or the completion of the circuit and eventually the current will start flowing. Initially, if you see here, there was a dedicated conductor.

Now, there is no dedicated conductor as the frame is not connected through a conductor to this point. The frame is being connected from the body and then through the earth. So, the current will start flowing. Let us suppose this is 230 volts. The impedance offered by the body is around 1000 ohms and the earth is suppose 0. So, the circuit will have now of 1000 ohms.

So, the current flowing will be around 230 milliamps. So, will the breaker trip? No, as the breaker is set for 10 amps.

So, the current will then become a continuous flow for indefinite time. Now, we will check whether this current is harmful or not in the last slide. Let's go ahead further. Suppose you provide a thing to this equipment frame through an electrode suppose and it's having a resistance of 1 ohms. Now, as you turn on this MCD and there is a fault, then what will happen?

The current will flow. Some current will flow through human body also because it's not infinite. It's some 1000 ohms and this is 1 ohm.

your model will become like this this is 230 volts 1 ohm and 1000 ohm in parallel most of the current will flow through this low resistance path and the current flowing will be having a very high value of around 230 amps which will cause the tripping of this MCB. So, that means you have protected your installation also within tripping within time and also your human which is not getting exposed to current for a longer duration. Now, As this is an earth electrode, what is it?

It is nothing but a conductive part which is embedded inside the earth to have an electrical contact with this earth. What is Local Earth? Local Earth is the region or the part of the Earth which is near to this electrode and which is having a potential not necessarily equal to zero. It can be but mostly it is not.

This region is known as Zone of Influence. To have a more feel of it, you can refer to IEEE 487. for the definition of zy and the region outside this zone of influence which is conventionally taken as voltage zero at this region is known as remote earth or reference earth this blue region these definitions are provided for local earth electrode in 50522 i have taken the reference from them Next, we move on to the next slide. In this, we will be talking about the impedance of the body.

Now, we know that there is a current that was flowing through the body. It can be dangerous in case the magnitude is high and also if the duration of the fault current flowing through the body is higher. So that means it is dependent on both the current magnitude as well as the shock duration.

But, the necessary condition which we deal with while designing is the admissible touch potential. This admissible touch potential is the product of current flowing through body and the impedance offered by the body. by looking at this it is looking like a simple linear equation like a straight line but it is not as the impedance of body is a function of touch voltage path of the current path means whether it's going from hand to hand like this or from hand to leg then the shock duration it is dependent upon frequency that is whether it's ac dc Moisture in the skin, area of contact like suppose this is the frame and how much surface area I am using to hold it. The pressure, how much pressure I am exerting and the temperature.

Now this body impedance is divided into two. One is internal and the other one is skin. It is the sum of both. Internal is mainly resistive part and it is mainly due to the flow of current. It is not dependent on other factors.

it is only dependent upon the path like it is going from hand to hand or from hand to leg. This is mainly the internal impedance. The skin impedance is comprised of both resistance and capacitance. and it is dependent upon the touch voltage, duration, frequency, moisture, area, pressure and temperature. Now, as you increase the touch voltage, there is a breakdown in the skin and the value of skin impedance falls.

It goes considerably low, making this body impedance only dependent upon the internal impedance. This is a simplified model. this is the skin impedance suppose this is hand and other hand and you are touching it so this is the skin impedance which is resistance and capacitance in parallel in between you have internal impedance of the body and then again the skin so this is your hand and this is your other hand so in case of dc This capacitor will offer infinite impedance as it is 1 by omega c, its frequency is 0, so it becomes an open circuit. So, it will be completely resistive in case of DC.

as in case of AC, it will have both the components and as these two being parallel, the net impedance offered will be comparatively lower. But we understood that skin impedance is dependent upon voltage only till a certain point in a certain limit as it becomes higher the skin breaks down that means after some time if the voltage is higher like around 200 volts and above in case of both ac and dc the dependence becomes equal as it is only dependent upon internal not on the skin Now, I have given an example of 100V here. Above 200V both are similar because no effect of skin. At 100V, impedance offered by AC is 990 whereas in case of DC it is 1100 Ohms. this is in case of dry condition it depends upon moisture and all so I have not taken that example to refer to other values you can go back to this icn check which is 6479-1 now we understood the impedance the nature of impedance how it works now we will understand what is the amount of current that can impact the impact There are four types.

One is the threshold of perception which is suppose this is the body from which the current is flowing this marker and perception means if I touch it and there is current flowing and I feel some tingling effect some current is flowing through me also. So, this amount is equal to 1 milliamps as per IEEE 80. For threshold of let go, let go means I am holding this, I am touching the equipment, the frame of the washing machine, the current is flowing through me but still I can remove my hand, I can remove my hand easily, not a problem. That is wonderful. 1 to 6 mA, star marked as per IEEE 18, 6 mA is for women, 9 mA for men.

Threshold for immobilization, that means if I am touching the frame of the washing machine or this pen and the current is flowing through me also due to a fault, then I am not able to remove my hand. I am trying but I am not able to remove. I am not able to remove my right hand. So, what will happen? My muscles will contract in this case and I am unable to remove it till this current stops flowing.

That means the circuit breaker gets open. So, this value is 9 to 25 milliamps as per 80 and threshold for ventricular fibrillation. This is the most important.

This leads ventricular fibrillation then eventually leads to stoppage of heart or inhibition of respiration. and causing injury or even death. This is the value which is 60-100 mA as per 80 whereas as per IEC 6479-1 this is the curve it is not to scale and I have not drawn it properly also but okay just to have a feel I have made it.

As per 6479-1 this value for perception lies in this region as soon as I come and reach this point of 0.5 million. for any time it is independent of time above this I will feel the perception once I move into this region I will go for let go current for let go it is time dependent as well as magnitude so this is the curve that we follow generally from 0.5 to 5 milliamps you can assume and after at 5 it is independent of time straight asymptotic then for immobilization it it is in this region. The main is ventricular fibrillation which is this area. In case of blue shaded, AC 4.1 curve is there which tells that the probability of having is around 5% that means out of 100 around 5 people will go into this range.

In case of red shaded region that is AC 4.2, if this amount of current is flowing for this amount of time. Then, the threshold of ventricular fibrillation will be achieved by upto 50% of the population and above that is AC4. Normally we use AC4.2 which is 80mA if you draw it. In case of DC, this is 0.5 is 2 and this is 5 is 25 mA and 80 is around 300 mA. That is ventricular fibrillation threshold.

So, I hope you have got an idea of how it is dependent on current and time. We will be dealing regarding touch and step potential in the next lecture in which I will be using these values to further go on. Thank you for listening and God bless you all.

Transcript for:Understanding Earthing and Bonding Safety

Transcript for:
Understanding Earthing and Bonding Safety