Why this thing happens?

[REEngineer]

Assume two electrical motors. One of them is directly connected to earth and the other is connected to the body of the first motor via a cable. The distance between motors is about 20 meters.

Now a ground fault occurs in the first motor (the one which was directly connected to earth). If a person touches this motor, nothing will happen to him/her and the earthing does the desired protection, but if someone touches the second motor (the one which was 20m apart) he/she will get electrocuted!

Is such thing possible ? and why?

 

[don_resqcapt19]

Why this thing happens?

It doesn't!!!!

If a person touches this motor, nothing will happen to him/her and the earthing does the desired protection, but if someone touches the second motor (the one which was 20m apart) he/she will get electrocuted!

Earthing or grounding just doesn't work that way. This is a very common myth and it is the source of an number of installation problems. The earth does not provide much safety in a case like this. The only way to make this type of installation safe is to use the code required EGC.
There will be little difference in the potential between the motors and the earth. The grounding electrode at the first motor does not remove the potential to earth. It does not lower the voltage of the motor to that of the earth and it does not raise the voltage of the earth to match that of the motor. This only works in close proximity to the grounding electrode. If you are touching motor one and standing on the grounding electrode there is no voltage across you, but as soon as you move away from the grounding electrode, you will have voltage across you. At 12" away from the grounding electrode you will have ~58% of the full voltage, at 3' ~75%.
Don

 

[George Stolz]

Don is 100% correct. To add to his post, read 250.4(A)(3):

Bonding of Electrical Equipment. Non?current-carrying conductive materials enclosing electrical conductors or equipment, or forming part of such equipment, shall be connected together and to the electrical supply source in a manner that establishes an effective ground-fault current path.

(The housing of the motor is required to be connected to an effective ground fault current path.)

Then read 250.4(A)(5):

(5) Effective Ground-Fault Current Path. Electrical equipment and wiring and other electrically conductive material likely to become energized shall be installed in a manner that creates a permanent, low-impedance circuit facilitating the operation of the overcurrent device or ground detector for high-impedance grounded systems...

(This permanent path is designed to 'kick breakers.')

...It shall be capable of safely carrying the maximum ground-fault current likely to be imposed on it from any point on the wiring system where a ground fault may occur to the electrical supply source. The earth shall not be considered as an effective ground-fault current path.

(You can't use the earth as this path. It has to be an Equipment Grounding Conductor (EGC).)

 

[bphgravity]

Again proof that 100 years of inappropriate code terminology and poor explanation and understanding of grounding and bonding principles is alive and well. Not even electrical engineers are educated properly. I really cannot believe the task group rejected the terminology change in Article 250. This would have least been a step in the right direction... :evil:

 

[jwelectric]

What you all have to realize is that most electrical engineers do not have an understanding of the grounding concept as we know it.

Most of what they learn about grounding through school is chassis grounding. Most of the classes that they take have more to do with electronics than electrical constructions as we know it.

 

[charlie b]

I gotta jump on you here, Mike. I've agreed with you twice in recent days, and I need to make amends for that transgression.

What you all have to realize is that most electrical engineers do not have an understanding of the grounding concept as we know it.

Neither do most electricians.

Most of what they learn about grounding through school is chassis grounding. Most of the classes that they take have more to do with electronics than electrical constructions as we know it.

No true at all. I hated electronics, and preferred courses related to power distribution and analysis.

 

[jwelectric]

I gotta jump on you here, Mike. I've agreed with you twice in recent days, and I need to make amends for that transgression.

What you all have to realize is that most electrical engineers do not have an understanding of the grounding concept as we know it.

Neither do most electricians.

Most of what they learn about grounding through school is chassis grounding. Most of the classes that they take have more to do with electronics than electrical constructions as we know it.

No true at all. I hated electronics, and preferred courses related to power distribution and analysis.

Please accept my apologies Charlie I didn?t mean this to be offensive.

I dropped out of my engineering classes because after two years I hadn?t covered one thing that had to do with electrical construction. In the time and money that I spent I felt that I would have been better off taking a TV repair course. In the courses that I did take and was able to pass a ground was nothing more than a reference point. Not once did I cover the grounding (bonding) of a circuit in order to facilitate the opening of an over current device. The ground was always referenced as the chassis which is indirectly connected to earth. I do believe that it is my knowledge of the electrical construction trade that hindered me in those courses that I took.

Now don?t take me the wrong way there is a need for engineers and I have nothing but respect for anyone that can endure the four years of school. It is not an easy thing to accomplish. I quit so that tells me something. :wink:

I just don?t see where a degree in electrical engineering would help the installer of the electrical system. Looking at my course material for the next semester I didn?t see anything that pointed toward any code or installation criteria. All I saw was more math and electronics.

When working in the field and I run across something in a PC board that is not working properly I just replace the board. In 37 years in the field I have not ever used an oscilloscope nor have I had to figure the X sub l of a circuit. The only time that I worked on a power factor in a plant I just installed what someone else had designed (an engineer). When I ran into a problem with the electrical serviced I just called the power company and they took care of the problem.

Again please accept my apologies if I offended you or anyone else with my statement. The engineering field is just not my bag of bones. I fail to see where it would benefit anyone who is going into the electrical trade as an installer of the electrical system.

I can and do see where it would be of GREAT use for someone that is designing the system or a component of the system as long as they continue their education into the code aspects of the trade.

 

[charlie b]

No offence was taken; no apology was needed. My real comment was to echo the regrettable fact that far too many of us (and I include myself in this group) do not understanding grounding, bonding, earthing, and lightning.

 

[al hildenbrand]

REEngineer,

I went looking for a post that spent a lot of time on this very subject, but it is, for some reason, just beyond reach in the Archived Post section of this forum.

What are the particulars of the verbal picture you gave us in your opening post? Especially, the properties of the conductors involved in what you are thinking of.

I, for one, can think of a way to read Don's image

This only works in close proximity to the grounding electrode.

That would lead me to counter Don's It doesn't!!!! with an exclamation of Wait a minute, It DOES. :wink:

Give us some circuit characteristics, and just maybe we can find a bone of contention to pass around for a bit. :twisted: :twisted: :twisted: