WHAT is the Difference between the expression Earthing and Grounding

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haitham said:
why RCD ill not trip
for TNC-S when N opened the current will path from Line to PE to the electrode backing to the source through mass of Earth there will be difference between the current in Line and N so RCD have to trip ?
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If N opens at the service, the power will flow through the RCD, into the load, back into the RCD, into the main panel and then get on the PE. Because of current flowing in and out of the RCD being equal, there is no trip. RCD only works on a current imbalance.
 
mbrooke said:
If N opens at the service, the power will flow through the RCD, into the load, back into the RCD, into the main panel and then get on the PE. Because of current flowing in and out of the RCD being equal, there is no trip. RCD only works on a current imbalance.
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you are right
I got that just two cases RCD will trip
1- unbalanced loads its common neutral opened at point between the panel and the loads
2- during ground fault whether N opened or not
 
haitham said:
you are right
I got that just two cases RCD will trip
1- unbalanced loads its common neutral opened at point between the panel and the loads
2- during ground fault whether N opened or not
Click to expand...

RCDs only trip if a current imbalance exists on their load side. Either the hot is leaking current to earth or the neutral is leaking current to earth.
 
mbrooke said:
RCDs only trip if a current imbalance exists on their load side. Either the hot is leaking current to earth or the neutral is leaking current to earth.
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yea if sum of lines current equal not N current ,with value high enough to operate the trip coil inside the RCD

i have another question that make me confused it is till relating to the voltage appeared on the metallic part of the equipment ...
I'm talking about normal operation cases and what u said is so useful and i got it and agree with it .. this voltage appeared on the chassis cus of connecting it to neutral whether directly TNC or likr in TNCS ,this voltage is called stray voltage ?

i read that connect not the N to the earth at any second point in the system will generate prevent existing of stray voltage ?
this mean for TNCS ,stray voltage will be existing and higher than it in TNC ? and i can't get it cus when there is electrodes connecting to n at services there will be two parallel impedance ,the neutral wire+electrode and earth mass ..so the resultant impedance will be less and stray voltage will be less ?

for tnc , is this consider connecting to N to earth , cus the equipment existing on the earth ?
 
haitham said:
[yea if sum of lines current equal not N current ,with value high enough to operate the trip coil inside the RCD
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Yup. The RCD is purely a L-N imbalance detection device.

i have another question that make me confused it is till relating to the voltage appeared on the metallic part of the equipment ...
I'm talking about normal operation cases and what u said is so useful and i got it and agree with it .. this voltage appeared on the chassis cus of connecting it to neutral whether directly TNC or likr in TNCS ,this voltage is called stray voltage ?
Click to expand...

Stray voltage is a broad term, but yes, it can be called that.




i read that connect not the N to the earth at any second point in the system will generate prevent existing of stray voltage ?
Click to expand...

Correct. One point is fine, but if you connect N at more than one point to earth current will flow between those one or more points in addition to introducing voltage gradients.



this mean for TNCS ,stray voltage will be existing and higher than it in TNC ?
Click to expand...


Stray voltage would be higher for TN-C than TN-C-S.


and i can't get it cus when there is electrodes connecting to n at services there will be two parallel impedance ,the neutral wire+electrode and earth mass ..so the resultant impedance will be less and stray voltage will be less ?
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Well, its a paradox. Decreasing the impedance back to the source will reduce the N voltage relative to remote earth, but it simultaneously creates voltage gradients across the soil.




for tnc , is this consider connecting to N to earth , cus the equipment existing on the earth ?
Click to expand...


Reducing the impedance back to the source reduces potential to remote earth in TN-C and TN-C-S, but if you really want a significant elimination thats where bonding comes in. When you bond everything together in a building, you reduce the voltage potential between all metal parts in that building. Remember, sometimes it just is not possible to get a good, low impedance ground back the the source. And even then, it can always vary for many reasons. And where 1 ohm ground resistance might be good for normal operation, during a fault that 1 ohm can cause some very high voltages to develop in relation to remote earth.
 
Yup. The RCD is purely a L-N imbalance detection device.



Stray voltage is a broad term, but yes, it can be called that.






Correct. One point is fine, but if you connect N at more than one point to earth current will flow between those one or more points in addition to introducing voltage gradients.






mbrooke said:
Stray voltage would be higher for TN-C than TN-C-S.
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this mean it is correct when consider in TNC-S the electrode and the reminder length of neutral wire is two parallel impedances


mbrooke said:
Well, its a paradox. Decreasing the impedance back to the source will reduce the N voltage relative to remote earth, but it simultaneously creates voltage gradients across the soil.
Click to expand...


you mean voltage difference between points along the earth due to the closer to the electrode the higher resistance of the soil
 
haitham said:
you mean voltage difference between points along the earth due to the closer to the electrode the higher resistance of the soil
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You could look at it that way. But the bulk resistivity of the soil is pretty uniform. The voltage drop can be approximated to take place across concentric shells of earth surrounding the electrode, with the shells closest to the rod having the least area and therefore the highest resistance.
But even without a higher resistance per foot of distance closer to the electrode there would still be a voltage drop which shows up as a voltage gradient at the surface of the earth.
The higher resistance closer to the rod just means that the largest part of the voltage drop takes place close to the rod and that the voltage difference between, for example, two feet next to each other on the ground will be greatest close to the rod.
The touch potential (hand touching rod and rest of body touching earth or ground) will be greatest the further the rest of your body is from the rod, while the step potential (from one foot to another) will be greatest closest to the rod.

The touch potential from metal surfaces connected metallically to the Ground Electrode System will depend on what else you are touching at the time, but will usually be the full voltage at the rod.
 
haitham said:
this mean it is correct when consider in TNC-S the electrode and the reminder length of neutral wire is two parallel impedances
Click to expand...



Yes. Unless of course the neutral is earthed many times between the source and service, but overall it is 2 paths: the neutral and the earth itself.


you mean voltage difference between points along the earth due to the closer to the electrode the higher resistance of the soil
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Yes, correct. When ever electricity passes through the earth, there will be a measurable voltage gradient between any 2 points. Google stray voltage, stray current, and stray voltage and dairy farms if curious.



This is a long video, but its a great stray voltage for beginners video:


https://www.youtube.com/watch?v=pAs_FmdxXhQ
 
mbrooke said:
I know this is not dealing with LV but rather MV, however page 132 (127 actual page number) has some interesting info in regard to ground resistance/bonding vs reducing voltage potential:
http://www.dbc.wroc.pl/Content/3458/...ngineering.pdf
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GoldDigger said:
You could look at it that way. But the bulk resistivity of the soil is pretty uniform. The voltage drop can be approximated to take place across concentric shells of earth surrounding the electrode, with the shells closest to the rod having the least area and therefore the highest resistance.
But even without a higher resistance per foot of distance closer to the electrode there would still be a voltage drop which shows up as a voltage gradient at the surface of the earth.
The higher resistance closer to the rod just means that the largest part of the voltage drop takes place close to the rod and that the voltage difference between, for example, two feet next to each other on the ground will be greatest close to the rod.
The touch potential (hand touching rod and rest of body touching earth or ground) will be greatest the further the rest of your body is from the rod, while the step potential (from one foot to another) will be greatest closest to the rod.

The touch potential from metal surfaces connected metallically to the Ground Electrode System will depend on what else you are touching at the time, but will usually be the full voltage at the rod
Click to expand...


mbrooke said:
This is a long video, but its a great stray voltage for beginners video:
https://www.youtube.com/watch?v=pAs_FmdxXhQ
Click to expand...


thank you for this so helpful video,file ,replies
I'll study all and back to ask :)

but cant wait till study wanna ask a question related what mentioned up about the paradox function of electrodes ,
can i say the benefit of the electrode to decrease the touch-fault- voltage is just effective some inches around the electrode if the whole body located inside it ,after these inches the problem of voltage gradient will appear specially during high g fault current ..
 
haitham said:
thank you for this so helpful video,file ,replies
I'll study all and back to ask :)

but cant wait till study wanna ask a question related what mentioned up about the paradox function of electrodes ,
can i say the benefit of the electrode to decrease the touch-fault- voltage is just effective some inches around the electrode if the whole body located inside it ,after these inches the problem of voltage gradient will appear specially during high g fault current ..
Click to expand...

Ask what you need :)


It is a paradox, ground rods can create voltage gradients as much as they might alleviate some.
 
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