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Old 06-13-2015, 04:35 AM   #1
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Default Grounded Power Generator

Dear All

Sorry if I ask this very basic (yet fundamental) question "why ground (to earth) the Neutral of power source?". From many reading references I got the point that the necessity of grounding (to earth) the Load side is to provide protection against fault/stray current that would happen if there is any fault between "live" conductor and the body/chassis of equipment. The fault current will flow from live conductor to the chassis, then to the ground electrode (inside the earth), and then back to the power source (which could be very far away from the Load). But of course this could only happen if the Neutral point at the power source is connected to earth, since this condition creates a closed electrical loop so the current could flow. If the source's Neutral is not connected to earth, then the fault current can not exist. It implies that no electrical shock will happen if we touch the Load chassis that is in fault condition.

So, why most power plant would rather to earth their Neutral?
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Old 06-13-2015, 05:01 AM   #2
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Lightning spikes is why. Ground faults are mislabeled, fault current doesn't like going through the earth, too much resistance. Check out this thread. http://www.electriciantalk.com/f2/ar...ording-109857/

Last edited by backstay; 06-13-2015 at 05:03 AM.
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Old 06-13-2015, 05:03 AM   #3
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If your electrical system is TT it is necessary to earth neutral because the actual earth (soil) is used to clear a fault via RCD.

Now, for electric shock. If the system is small enough an unearthed neutral (IT earthed) will not shock. I could grab the line or neutral (but not both) and receive no shock. However, most system are large enough to cause shock, or even kill, despite the neutral being unearthed. Why so? Capacitive coupling. All electrical wires have potential capacitive and reactive coupling with other conductive or pseudo conductive objects like conduit, metal chassis, earth soil ect. This coupling allows current to flow even when no deliberate earthing of conductors is present.

Now, why earth? Earthing the neutral gives a reference. It guarantees the neutral will always be the earthed conductor, and thus close to earth potential under normal conditions.

In TN-C-S there is no choice as the neutral has to be connected to the building's bonding system which is in contact with earth at many points or can become so. In TN-S the PE which connects to ever building's bond system then runs back to the neutral bushing in the generator or transformer, connecting it to earth. In both these systems even if not intentional the neutral will always be connected to earth at many points. With the neutral being close to zero potential in buildings it allows single pole breakers in panels (being cheaper) and keeps the shell of light sockets safe for countries where its possible to come in contact with a lamp screw shell when changing a light bulb like here in the US. This is actually why are lamp cords are polarized.


In TT earthling the neutral at the transformer is necessary in order for the system to properly clear a fault as soil is used to complete a fault path.


There is IT earthing, however while it has advantages it also has disadvantages, which make it less suitable to residential, where a transformer serves more than on building, or where less a ground fault is not likely to be taken care of.
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Old 06-13-2015, 05:03 AM   #4
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Originally Posted by backstay View Post
Lightning spikes is why. Ground faults are mislabeled, fault current doesn't like going through the earth, too much resistance. Check out this thread. http://www.electriciantalk.com/f2/ar...ording-109857/

Doesn't mean the earth isn't used to clear a fault.
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Old 06-13-2015, 05:07 AM   #5
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Doesn't mean the earth isn't used to clear a fault.
No, but how many times do we read about people getting killed by touching a fence that's grounded and hot. Ground is not a reliable fault tripper.
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Old 06-13-2015, 05:12 AM   #6
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No, but how many times do we read about people getting killed by touching a fence that's grounded and hot. Ground is not a reliable fault tripper.
Trips RCDs fine from what I hear.
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Old 06-13-2015, 05:23 AM   #7
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Trips RCDs fine from what I hear.
That's great if you have one.
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Old 06-13-2015, 05:25 AM   #8
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That's great if you have one.
Which I am sure he does in Indonesia as the rest of the world.
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Old 06-13-2015, 07:46 AM   #9
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So the op's theory is that if the supply was not grounded ?
Then we wouldn't get shocks becsuse there is no return path ?
I think there is more than enough capacitve and inductive coupling
from the enormous distribution system to still kill you dead.
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Old 06-14-2015, 12:50 AM   #10
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Originally Posted by Hendra Tampang Allo View Post
Dear All

Sorry if I ask this very basic (yet fundamental) question "why ground (to earth) the Neutral of power source?". From many reading references I got the point that the necessity of grounding (to earth) the Load side is to provide protection against fault/stray current that would happen if there is any fault between "live" conductor and the body/chassis of equipment. The fault current will flow from live conductor to the chassis, then to the ground electrode (inside the earth), and then back to the power source (which could be very far away from the Load). But of course this could only happen if the Neutral point at the power source is connected to earth, since this condition creates a closed electrical loop so the current could flow. If the source's Neutral is not connected to earth, then the fault current can not exist. It implies that no electrical shock will happen if we touch the Load chassis that is in fault condition.

So, why most power plant would rather to earth their Neutral?
Mostly because of the remote possibility of a primary to secondary short at the service transformer. In that event the fault current can find a way back to its source via all of the available paths...water pipe...ground rods... and the like in order to trip the "cutout" in the line.
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Old 06-14-2015, 03:24 PM   #11
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Lightbulb The OP is an engineering student in Indonesia...

So his query comes from textbook /theory...

1) All major generation - across the planet - takes the form of synchronous alternators with salient poles energized independently from an extremely stable DC power source -- that is under tight control by the master operator.

2) The structure of the field windings around the rotating armature is always three-phase and is invariably wired for four or more poles. No major alternator revolves faster than 1800 rpm. Various hydro-power and atomic plants are wired with even more poles so that the prime mover need not spin all that fast - with hydro-electric alternators being the slowest, high torque, alternators on the planet.

3) By tuning the salient poles with DC current, their magnetic power -- within limits -- can be tuned. In a stable system the rotor is pulled into EXACT synchronous rotation (electrically -- the Hertz) with the rest of the infinite grid.

4) The internal wiring of these machines is invariably DELTA and they have NO NEUTRAL conductor. So at the point of generation the OP's query simply does not follow.

5) ALL main power synchronous alternators run at medium voltages: 4160VAC on up. There is the least uniformity in this value because they are ALWAYS mated to a co-designed step-up transformer that injects the current into the power grid at high voltage.

20-35 kV would be a low end value only seen in small, island sized grids. (Oahu, Hawaii)

The longer the distance to market, the higher the voltage. Even at this point, the 'system' is ungrounded / has no neutral. The tripled hot conductors can be seen strung from tower to tower across the planet. You never see a fourth conductor.

6) When the high voltage power is stepped down it can be sent on -- ungrounded, still -- or via a delta to wye transform.

It's at this point that a neutral connection to the planet Earth is established.

This, wye distribution is extremely favored for UNDERGROUND cables... which have taken over all modern urban primary distribution systems.

These primary cables wrap a concentric neutral conductor -- coaxial style -- around each hot leg.

When they are landed at local points of distribution, the labor to make them up is no small task. A single transformer can take a j-man hours to properly terminate. At these voltages, no flaws can be permitted.

These transformers are Poco property - typically - and are biased towards wye distribution -- as in 600Y347 or 480Y277 or 208Y120 in North America.(pad mount, serious loads -- pole mount transforms are typically single phase) But this is not always the case. Ungrounded Services are possible - - if requested.

&&&&&&

The OP query is most apt for micro-generators -- auxiliary power sets used by small customers -- particularly single family homes.

It has gotten to the point that there are so many variations on how these are built, controlled and wired -- I can't make any categorical statements.

Honda, and others are producing gen-sets that generate asynchronous power -- which is then converted to DC -- and then, via solid state power electronics, converted into synthetic single-phase AC current. With a couple of adjustments, these can crank out 50Hz or 60Hz and swing their voltage output, too.

Any neutral run to them is run to their electronics package.



Old style gen-sets use asynchronous generators -- with the motor 'over-sped' -- so that even with the induction 'slip' one still had near to perfect cycles per second AC power.

With this design scheme, every variant was tried at one point or another. The bias had been towards delta generator windings -- well insulated -- well isolated (double insulated, even)-- with the gen-set chassis and equipment ground from the alternator chassis brought together.

(Invariably the actual rotating machinery is on vibration mounts that are dielectrics.)

THIS is the point that is typically EARTHED.

Some of these are truck mounted rental units -- on rubber tires -- and have to be explicitly, independently earthed.

It (the made neutral) is not actually part of the internal wiring at all.



Even SMALLER units are in mass production. They are wired internally as 240/120 single phase and DO have a true neutral or earthed conductor. (Not as efficient, though)

You'll see MANY threads by new electricians trying to figure out exactly how to handle this conductor -- so as to not create multi-point bonding of the grounding electrode system and the neutral conductor -- with consequent circulating currents in the neutral.

The only generators that are wired for a true neutral return are DINKY, auxiliary, and are not representative of the electric power industry.

QED

Last edited by telsa; 06-14-2015 at 03:27 PM. Reason: polish
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Old 06-14-2015, 04:19 PM   #12
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[quote=telsa;2035986]

Quote:
4) The internal wiring of these machines is invariably DELTA and they have NO NEUTRAL conductor. So at the point of generation the OP's query simply does not follow.
Can you show my documentation? The vast majority of generators are connected in wye, and the center earthed via an impedance due to a single phase to ground fault having the ability to damage the gnerator.



Quote:
The longer the distance to market, the higher the voltage. Even at this point, the 'system' is ungrounded / has no neutral.

That is untrue 99% of the time. Almost every GSU I have seen is delta wye, with the XO earthed solidly. No one in there right mind would operate something above 35kv ungrounded, in fact capacitive coupling alone would produce fault current rivaling a solid ground.


Quote:
The tripled hot conductors can be seen strung from tower to tower across the planet. You never see a fourth conductor.
The lightning guard is what?

And no, its two circuits, not independent hots forming a 3 phase 6 wire system.




Quote:
6) When the high voltage power is stepped down it can be sent on -- ungrounded, still -- or via a delta to wye transform.
In some cases, although 345 to 115kv is via auto transformer.



Quote:
It's at this point that a neutral connection to the planet Earth is established.
No way, I have yet to see a 345kv system run ungrounded.





Quote:
Honda, and others are producing gen-sets that generate asynchronous power -- which is then converted to DC -- and then, via solid state power electronics, converted into synthetic single-phase AC current. With a couple of adjustments, these can crank out 50Hz or 60Hz and swing their voltage output, too.

Any neutral run to them is run to their electronics package.
Huh?



Quote:
Some of these are truck mounted rental units -- on rubber tires -- and have to be explicitly, independently earthed.

It (the made neutral) is not actually part of the internal wiring at all.
Show me the code requiring ground rods at portable generators.


Quote:
Even SMALLER units are in mass production. They are wired internally as 240/120 single phase and DO have a true neutral or earthed conductor. (Not as efficient, though)

You'll see MANY threads by new electricians trying to figure out exactly how to handle this conductor -- so as to not create multi-point bonding of the grounding electrode system and the neutral conductor -- with consequent circulating currents in the neutral.

The only generators that are wired for a true neutral return are DINKY, auxiliary, and are not representative of the electric power industry.
I don't know if you really believe all this or are just trolling...
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Old 06-14-2015, 10:28 PM   #13
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Lightning spikes is why. Ground faults are mislabeled, fault current doesn't like going through the earth, too much resistance. Check out this thread. http://www.electriciantalk.com/f2/ar...ording-109857/
If fault current doesn't flow through earth and back to the power source, how can we feel electric shock? Where does the current flow to? If I touch the live conductor and do not touch the neutral, then there should be no current flow through my body if the ground can not deliver the current.

I think lightning strike is only considered for outdoor system (equipment & wire). For equipment & wire that are inside a building, very low possibility that lightning can reach them. In out project we use separate grounding for building (lightning strike protection) and for the equipment.
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Old 06-16-2015, 10:27 PM   #14
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Originally Posted by Hendra Tampang Allo View Post
If fault current doesn't flow through earth and back to the power source, how can we feel electric shock? Where does the current flow to? If I touch the live conductor and do not touch the neutral, then there should be no current flow through my body if the ground can not deliver the current.

I think lightning strike is only considered for outdoor system (equipment & wire). For equipment & wire that are inside a building, very low possibility that lightning can reach them. In out project we use separate grounding for building (lightning strike protection) and for the equipment.

SOME of the fault current will go thru the earth,
But if it is a poor conductor (mostly is).
Then it is only a small portion.
But it only takes a small amount to shock someone.
Most of the fault current will go back via neutral line,
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Old 06-16-2015, 10:31 PM   #15
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Originally Posted by telsa View Post
So his query comes from textbook /theory...

1) All major generation - across the planet - takes the form of synchronous alternators with salient poles energized independently from an extremely stable DC power source -- that is under tight control by the master operator.

2) The structure of the field windings around the rotating armature is always three-phase and is invariably wired for four or more poles. No major alternator revolves faster than 1800 rpm. Various hydro-power and atomic plants are wired with even more poles so that the prime mover need not spin all that fast - with hydro-electric alternators being the slowest, high torque, alternators on the planet.

3) By tuning the salient poles with DC current, their magnetic power -- within limits -- can be tuned. In a stable system the rotor is pulled into EXACT synchronous rotation (electrically -- the Hertz) with the rest of the infinite grid.

4) The internal wiring of these machines is invariably DELTA and they have NO NEUTRAL conductor. So at the point of generation the OP's query simply does not follow.

5) ALL main power synchronous alternators run at medium voltages: 4160VAC on up. There is the least uniformity in this value because they are ALWAYS mated to a co-designed step-up transformer that injects the current into the power grid at high voltage.

20-35 kV would be a low end value only seen in small, island sized grids. (Oahu, Hawaii)

The longer the distance to market, the higher the voltage. Even at this point, the 'system' is ungrounded / has no neutral. The tripled hot conductors can be seen strung from tower to tower across the planet. You never see a fourth conductor.

6) When the high voltage power is stepped down it can be sent on -- ungrounded, still -- or via a delta to wye transform.

It's at this point that a neutral connection to the planet Earth is established.

This, wye distribution is extremely favored for UNDERGROUND cables... which have taken over all modern urban primary distribution systems.

These primary cables wrap a concentric neutral conductor -- coaxial style -- around each hot leg.

When they are landed at local points of distribution, the labor to make them up is no small task. A single transformer can take a j-man hours to properly terminate. At these voltages, no flaws can be permitted.

These transformers are Poco property - typically - and are biased towards wye distribution -- as in 600Y347 or 480Y277 or 208Y120 in North America.(pad mount, serious loads -- pole mount transforms are typically single phase) But this is not always the case. Ungrounded Services are possible - - if requested.

&&&&&&

The OP query is most apt for micro-generators -- auxiliary power sets used by small customers -- particularly single family homes.

It has gotten to the point that there are so many variations on how these are built, controlled and wired -- I can't make any categorical statements.

Honda, and others are producing gen-sets that generate asynchronous power -- which is then converted to DC -- and then, via solid state power electronics, converted into synthetic single-phase AC current. With a couple of adjustments, these can crank out 50Hz or 60Hz and swing their voltage output, too.

Any neutral run to them is run to their electronics package.



Old style gen-sets use asynchronous generators -- with the motor 'over-sped' -- so that even with the induction 'slip' one still had near to perfect cycles per second AC power.

With this design scheme, every variant was tried at one point or another. The bias had been towards delta generator windings -- well insulated -- well isolated (double insulated, even)-- with the gen-set chassis and equipment ground from the alternator chassis brought together.

(Invariably the actual rotating machinery is on vibration mounts that are dielectrics.)

THIS is the point that is typically EARTHED.

Some of these are truck mounted rental units -- on rubber tires -- and have to be explicitly, independently earthed.

It (the made neutral) is not actually part of the internal wiring at all.



Even SMALLER units are in mass production. They are wired internally as 240/120 single phase and DO have a true neutral or earthed conductor. (Not as efficient, though)

You'll see MANY threads by new electricians trying to figure out exactly how to handle this conductor -- so as to not create multi-point bonding of the grounding electrode system and the neutral conductor -- with consequent circulating currents in the neutral.

The only generators that are wired for a true neutral return are DINKY, auxiliary, and are not representative of the electric power industry.

QED
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