I was trying to explain to someone the importance of ground mats that are bonded to enclosures and the response I got was that these ground mats put the person at more of a risk for electrician when accidently coming in contact with a live circuit then they would be without the ground mat due to the fact the ground mat provides less resistance to earth.
I explain first off that the purpose of these ground mats are not to protect person when contacting live circuit but rather to protect person when they make contact with an enclosure that may have come in accidental contact with an energized circuit. Secondly I argue that even with the ground mat providing a more direct connection to remote earth I do not think this makes the conditions any worse if someone were to come directly in contact with an energized conductor.
Most of the time when installing a ground rod the achieved resistance to earth is aproximately 25ohms. However I have heard and even seen proved that this same resistance to earth can be achieved in some cases by simply resting a ground rod on the surface of the earth and in driving it in the ground. A ground rod resting on the earth would be a similar case of a human standing on the earth and therefore would go to show that there would be no difference between someone standing on the ground without a ground mat connected to earth with a ground rod and with a ground mat connected to earth with a ground rod.
Another way I look at this is that in most cases a ground rod at minimum will only provide a 25ohm resistance to ground. Most of the impedance when coming in contact with an energized conductor will be across the body lets say 500ohms. This high impedance value compared to the impedance value of the body to ground will be much greater and therefore the body to ground impedance will become irelevent and so would weather you were standing on bare ground or a mat connected with a rod to ground when contacting an energized conductor. With a ground rod connected to earth the current through the body would be 277V / 500ohms + 25ohms = .52A for contact on a 480V system. I dont know how much this would vary if someone was standing on earth without a ground rod driven but I dont expect it to vary much because I dont expect the resistance to remote earth to increase by much.
Do I have my explanation correct or am I way off. I wanted to hear others thoughts.
I explain first off that the purpose of these ground mats are not to protect person when contacting live circuit but rather to protect person when they make contact with an enclosure that may have come in accidental contact with an energized circuit. Secondly I argue that even with the ground mat providing a more direct connection to remote earth I do not think this makes the conditions any worse if someone were to come directly in contact with an energized conductor.
Most of the time when installing a ground rod the achieved resistance to earth is aproximately 25ohms. However I have heard and even seen proved that this same resistance to earth can be achieved in some cases by simply resting a ground rod on the surface of the earth and in driving it in the ground. A ground rod resting on the earth would be a similar case of a human standing on the earth and therefore would go to show that there would be no difference between someone standing on the ground without a ground mat connected to earth with a ground rod and with a ground mat connected to earth with a ground rod.
Another way I look at this is that in most cases a ground rod at minimum will only provide a 25ohm resistance to ground. Most of the impedance when coming in contact with an energized conductor will be across the body lets say 500ohms. This high impedance value compared to the impedance value of the body to ground will be much greater and therefore the body to ground impedance will become irelevent and so would weather you were standing on bare ground or a mat connected with a rod to ground when contacting an energized conductor. With a ground rod connected to earth the current through the body would be 277V / 500ohms + 25ohms = .52A for contact on a 480V system. I dont know how much this would vary if someone was standing on earth without a ground rod driven but I dont expect it to vary much because I dont expect the resistance to remote earth to increase by much.
Do I have my explanation correct or am I way off. I wanted to hear others thoughts.
