Utility to earth fault

[leerhinson]

Been thinking about when utility conductors come in contact with earth ,by either trees or downed lines and it conducts, is that because each transformer is bonded and earthed at each pole and if they wasnt earthed wouldnt that make it safer incase a single line came in contact with earth,maybe im confused I was just understanding how the earth is conducting a path for a fault to return to its source,or so it seems ,thanks guys.

 

[electrofelon]

Been thinking about when utility conductors come in contact with earth ,by either trees or downed lines and it conducts, is that because each transformer is bonded and earthed at each pole and if they wasnt earthed wouldnt that make it safer incase a single line came in contact with earth,maybe im confused I was just understanding how the earth is conducting a path for a fault to return to its source,or so it seems ,thanks guys.

It depends on the type of distribution system. An MGN, I would think, would allow enough fault current to flow thru the earth back to the MGN to trigger an OCPD or recloser. For ungrounded systems, I am not as clear on how faults are detected and cleared. Lots of ungrounded 4800 here in upstate NY. Maybe @Hv&Lv can comment.

 

[jaggedben]

Been thinking about when utility conductors come in contact with earth ,by either trees or downed lines and it conducts, is that because each transformer is bonded and earthed at each pole and if they wasnt earthed wouldnt that make it safer incase a single line came in contact with earth,maybe im confused I was just understanding how the earth is conducting a path for a fault to return to its source,or so it seems ,thanks guys.

Generally speaking, the advantage of grounded systems is that the first fault of an ungrounded conductor to ground or grounded parts can be detected via current from the normal source. i.e. current flows through ground back to the grounded conductor. This can be detected by monitoring current on the grounding electrode or by monitoring for imbalance between currents on the grounded and ungrounded conductors, even if the fault current isn't big enough to trip an overcurrent device. By contrast, the first fault on an ungrounded system may not cause current to flow by another path. So other detection methods are needed, or else the fault just sits there until a second fault develops.

 

[Hv&Lv]

Ground fault current magnitudes depend on the grounding method used by the POCO.
We generally use ground overcurrent (50/51G and/or 50/51N) and directional overcurrent (67) relays and a single or combination of these pickups are the way we handle ground fault protection.
Basically reading and tripping on the N element. Without an actual CT on the N, it's a calculated value. If the three phase system is unbalanced, an element setting too low can trip the circuit. Another reason to keep the three phase balanced.

Solid grounded systems see high levels of fault currents. We generally rely on line tripping to remove the fault. (Reclosers) Again, we use pickup values for 50/51 phase elements to measure overcurrent.



Ungrounded Delta systems have no intentional ground. Single-line-to-ground fault on these
systems have to flow through the line-ground capacitance of the system.
Remember the old Delta system where three lights are wired L-G? If one line goes to ground it puts phase at the ground potential. Think of grounded B phase banks where one line is at ground potential.

You could get into discussion of Peterson Coils, which go between the N and the G, but no one would be very interested in them.

 

[electrofelon]

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Ungrounded Delta systems have no intentional ground. Single-line-to-ground fault on these
systems have to flow through the line-ground capacitance of the system.
Remember the old Delta system where three lights are wired L-G? If one line goes to ground it puts phase at the ground potential. Think of grounded B phase banks where one line is at ground potential.

You could get into discussion of Peterson Coils, which go between the N and the G, but no one would be very interested in them.

Thanks for the explanation. In my head I was imagining ground fault monitoring of a distribution line spanning miles and miles to be more challenging than a 480 or 600 V ungrounded system in a building. But maybe it's easy peasy? How well do these "float" at center?

 

[Seven-Delta-FortyOne]

In theory, an ungrounded system is safest. The problem, like most theories, is reality comes into play.

In an ungrounded system, the first ground fault goes unnoticed. The second ground fault is deadly. So, we intentionally create the first ground fault, and make a low impedance ground fault return path to open the OCPD. That is how we know we have the second ground fault.

On ungrounded utility systems, because they do not have the first intentional ground fault, they require monitoring. How that is done is beyond my pay grade and understanding, even after reading @Hv&Lv post.

 

[Hv&Lv]

Thanks for the explanation. In my head I was imagining ground fault monitoring of a distribution line spanning miles and miles to be more challenging than a 480 or 600 V ungrounded system in a building. But maybe it's easy peasy? How well do these "float" at center?

You measure phase imbalance return on the neutral..
You have 50 amps on one phase, 20 amps on the other phase, and 15 amps on the third phase, that is 35 amps on the neutral. It’s a little more than that though..50G in an SEL relay is a mathematical phasor summation

A tree falls on a phase wire now the phase currents go to 2000A, the relay sees the imbalance and N return also.
Some of our solar installs are set at 200A phase, 100 amp 50G.

 

[ptonsparky]

What is G, as in 50G?

 

[electrofelon]

You measure phase imbalance return on the neutral..
You have 50 amps on one phase, 20 amps on the other phase, and 15 amps on the third phase, that is 35 amps on the neutral. It’s a little more than that though..50G in an SEL relay is a mathematical phasor summation

A tree falls on a phase wire now the phase currents go to 2000A, the relay sees the imbalance and N return also.
Some of our solar installs are set at 200A phase, 100 amp 50G.

Sounds straightforward.....but that is for grounded systems right? Can you give a quick explanation of ground fault monitoring for an ungrounded system? Seems like you would need to differentiate between high and low impedance faults

 

[Hv&Lv]

Quite honestly, Zero Sequence (ANSI 9) can detect ground faults in ungrounded systems and is usually the easiest to use. You set the relay to measure zero sequence and compare it with an under voltage element(27)
So I've been told.
Now with that being said I've never dealt with delta protection. I generally turn the zero sequence elements off and seldom even use a 27 or 59 element.
Most solidly grounded distribution systems simply use 50/51 elements. Making the relay do too much can result in unwanted trips and can create a protection nightmare.

 

[TX+ MASTER#4544]

Been thinking about when utility conductors come in contact with earth ,by either trees or downed lines and it conducts, is that because each transformer is bonded and earthed at each pole and if they wasnt earthed wouldnt that make it safer incase a single line came in contact with earth,maybe im confused I was just understanding how the earth is conducting a path for a fault to return to its source,or so it seems ,thanks guys.

2023 NEC
Sections 250.4 (A) (5)(and (B) (4) read the last sentence of each paragraph.

Two identical reasons why we need an electrical conductor such as a (grounded) neutral at the service.

To carry fault current back to the source.

TX+MASTER#4544

 

[TX+ MASTER#4544]

You measure phase imbalance return on the neutral..
You have 50 amps on one phase, 20 amps on the other phase, and 15 amps on the third phase, that is 35 amps on the neutral. It’s a little more than that though..50G in an SEL relay is a mathematical phasor summation

A tree falls on a phase wire now the phase currents go to 2000A, the relay sees the imbalance and N return also.
Some of our solar installs are set at 200A phase, 100 amp 50G.

 

[TX+ MASTER#4544]

poster ......Hv&Lv said.....

How did you arrive at 35 amps on the neutral?
......a little more than that though.....
You describe a 3 phase 'Y' neutral.

TX+MASTER#4544

 

[Hv&Lv]

poster ......Hv&Lv said.....

How did you arrive at 35 amps on the neutral?
......a little more than that though.....
You describe a 3 phase 'Y' neutral.

TX+MASTER#4544

I didn't do the math when I said 35 amps.
I was going for a simplified example.

It's actually 32.79A with an online calculator.
(I’m lazy today)