Earth fault current flow

[tszpun]

Dear experts,
I have very basic question.
In fault current studies it often shows that whenever there is a fault, the current is flowing to the ground and it comes back flowing to the neutral. How is that even possible? Is it the same current that flowed to the ground? Can somebody explain me what is happenning there physically?

 

[davidbeach]

Define the system; define neutral, both physically and electrically; define ground, both physically and electrically. Not being pedantic, but the answer can depend on what you have and what you mean.

For instance, we run a multi-grounded neutral distribution system. So a fault phase to ground has many opportunities to hop back up onto the neutral to get back to the substation. Then back at the substation there's relays that operate on what's called "neutral current" which is actually residual current from the phase CTs and not a measurement of the current in the neutral. But then there is a CT on the transformer neutral and we can get true neutral current.

I’ll see your silver lining and raise you two black clouds. - Protection Operations

 

[tszpun]

Define the system; define neutral, both physically and electrically; define ground, both physically and electrically. Not being pedantic, but the answer can depend on what you have and what you mean.

For instance, we run a multi-grounded neutral distribution system. So a fault phase to ground has many opportunities to hop back up onto the neutral to get back to the substation. Then back at the substation there's relays that operate on what's called "neutral current" which is actually residual current from the phase CTs and not a measurement of the current in the neutral. But then there is a CT on the transformer neutral and we can get true neutral current.

Hello,
My question is general question (irrespective of the neutral system selected). I often see in text books that current hops back though the neutral. And this current is coming from the ground. How is that even possible?

 

[waross]

In a grounded system the neutral is connected or bonded to the a ground grid.

--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[dpc]

There has to be a complete path for the current to flow. During line-to-ground fault, current flow from one phase into the ground or ground conductor. To complete the circuit, the current must flow back into the transformer neutral. The earth is more or less conductive so current can flow through the ground.

 

[che12345]

Mr tszpun (Industrial)(OP)8 Apr 23 18:44
"...In fault current studies it often shows that whenever there is a fault, the current is flowing to the ground and it comes back flowing to the neutral. How is that even possible? Is it the same current that flowed to the ground? Can somebody explain me what is happenning there physically? "
I look at it this way:
1. A single-phase stand alone transformer with say L and N terminals, with the N is not grounded. When the L is dead short to the ground, there is NO current flow through the ground. The circuit is not complete because the N is isolated from ground.
1.1 Same as above, but with N connected to the ground. When the L is shorted to the ground, there is current flow through the ground. The current flow from the shorting to ground point > ..... ground (acting as a conductor) ......> the transformer N, which is grounded. The circuit is complete.
2. Now consider the single-phase source is from the utility where the N is grounded at the utility transformer. When a short from L to ground, the current flows through ground as above 1.1 .
3. If the utility source is 3-ph + N , where the N is grounded at the utility transformer, a phase short to ground will have current flow through the ground as explained in above 1.1 .
4. If the source is 3-ph without N or the N is NOT groaned (i.e. isolated from the ground):, any phase shorted to the ground will have NO current flow through the ground.
Note: capacitive current is not considered here.
Che Kuan Yau (Singapore)

 

[tszpun]

Mr tszpun (Industrial)(OP)8 Apr 23 18:44
"...In fault current studies it often shows that whenever there is a fault, the current is flowing to the ground and it comes back flowing to the neutral. How is that even possible? Is it the same current that flowed to the ground? Can somebody explain me what is happenning there physically? "
I look at it this way:
1. A single-phase stand alone transformer with say L and N terminals, with the N is not grounded. When the L is dead short to the ground, there is NO current flow through the ground. The circuit is not complete because the N is isolated from ground.
1.1 Same as above, but with N connected to the ground. When the L is shorted to the ground, there is current flow through the ground. The current flow from the shorting to ground point > ..... ground (acting as a conductor) ......> the transformer N, which is grounded. The circuit is complete.
2. Now consider the single-phase source is from the utility where the N is grounded at the utility transformer. When a short from L to ground, the current flows through ground as above 1.1 .
3. If the utility source is 3-ph + N , where the N is grounded at the utility transformer, a phase short to ground will have current flow through the ground as explained in above 1.1 .
4. If the source is 3-ph without N or the N is NOT groaned (i.e. isolated from the ground):, any phase shorted to the ground will have NO current flow through the ground.
Note: capacitive current is not considered here.
Che Kuan Yau (Singapore)

Hello Sir,

Thank you for taking time to respond to my question. But i think you've misunderstood my question. I understand the flow of currents during earth fault and the arrangement of NEUTRAL (grounded or isolated). However, what i wanted to know is that : during earth fault usually the current goes back from ground via neutral. Physically (in atomic or electron sense), how is that possible ? Imagine some of electrons at the fault location went to the ground, but how these electrons appear back at the neutral ? Usually there is no conductor connecting the neutral and fault location, except the ground. So does this mean that ground acts like a copper conductor ?

 

[wcaseyharman]

How it was explained to me was the earth acts like a large “sink.” It’s so large that any current is diffused to such a great degree there is essentially no resistance to current flow. The resistance is at interface, such as a ground grid, that has limited surface area.
I’m sure there are better ways to think of it, but that’s my understanding.

 

[che12345]

Mr tszpun (Industrial)(OP)
"......Usually there is no conductor connecting the neutral and fault location, except the ground. So does this mean that ground acts like a copper conductor ?..."
1. Consider that a phase is shorted to ground at point A. The source transformer Neutral is grounded at point B. Where Point A is some distance away from point B. There is NO physical electrical connection between Pt A and B. However, there is current flowing from point A to the Ground,>.... along the ground (which acts as a conductor) and >...to the Neutral (which is grounded); at the transformer.
Note: The exact same value of current that flows out from the phase conductor into the ground and back to the transformer Neutral. The current does NOT disappears/evaporated or flow to any other places except back to the particular transformer that is shorted to ground.
2. Yes, the ground acts as a conductor. You may treat it "equivalent" to as a [copper or aluminum] conductor.
Che Kuan Yau (Singapore)

 

[waross]

Even if the earth has a fairly high resistance, that shows up mostly as the resistance near the grounding electrode and the fault location.
As you get further away from the ground grid and the fault location, there are so many parallel paths through the earth that the increase in resistance is disproportionately slight compared to the distance between the grid and the fault.
Yes, the earth conducts the fault current.

Yes, the ground acts as a conductor. You may treat it "equivalent" to as a [copper or aluminum] conductor.

Where the resistance of a copper conductor increase directly as the length, the resistance of a ground path is not linear.
With a relatively long distance between the ground and the fault, the increase in resistance with increased length approaches zero.
Rather than a wire like conductor, consider the ground grid and the fault locations as connections at two points on an infinitely wide and thick copper mass.
Yes, the ground resistance is higher than copper, but the spread of the current and the drop in resistance as you get farther away fromm the contact points is analogous. (Perhaps not perfectly, but ground conditions may be good but are rarely perfect.)
--------------------
Ohm's law
Not just a good idea;
It's the LAW!