chris kennedy
Senior Member
- Location
- Miami Fla.
- Occupation
- 60 yr old tool twisting electrician
Question via PM;
Where does the neutral come from?
- Thread starter chris kennedy
- Start date
- Status
- Location
- New Jersey
- Occupation
- Journeyman Electrician
For a residence with a 1?, 120/240 volt system the neutral would be derived from the center point on the utility transformer. Here's a rudimentary graphic:
- Location
- Wisconsin
- Occupation
- PE (Retired) - Power Systems
Ground rods have nothing to do with the source of the neutral. The neutral is a 'creation' of the transformer winding connections.
Shared Neutrals?
Shared Neutrals?
Isn't the neutral shared with all the residences served by the transformer, and for that matter isn't it also tied to the wye transformer neutral?
Shared Neutrals?
For a residence with a 1?, 120/240 volt system the neutral would be derived from the center point on the utility transformer. Here's a rudimentary graphic:
Isn't the neutral shared with all the residences served by the transformer, and for that matter isn't it also tied to the wye transformer neutral?
kwired
Electron manager
- Location
- NE Nebraska
The fact that we (usually) ground the neutral conductor, is what ties it to grounded conductors of other systems.
You can have a system that has no neutral but still has a grounded conductor. That grounded conductor is both intentionally "earthed" somwhere and is tied to grounded conductors of other systems.
As far as going back to the generating plant, quite often once you get higher than the local distribution level and into the transmission and generation levels of the grid, there is no neutral as there is not really a need for line to neutral loads.
In my system it appears that the neutral is carried in metal all the way back to the substation. It is grounded at every pole and every house.
kwired
Electron manager
- Location
- NE Nebraska
That is the local distribution I mentioned. They usually have a grounded neutral there as there are often many single phase services supplied by the local distribution and they supply them via transformers connected line to neutral.
Check out the primary side of the substation, the transmission side of things. Most likely there is no neutral, and any grounded lines are just shield wires and not normal current carrying conductors. All loads at this level are generally three phase and no need for a neutral conductor.
gar
Senior Member
- Location
- Ann Arbor, Michigan
- Occupation
- EE
130525-1152 EDT
In my area there are a great number of 3 wire 3 phase primaries with 2 post primary transformers to supply the load. Here the residential neutral, secondary side, is only associated with the pole transformer and earth in at least two places. There will be very little current in this neutral that flows back to the sub-station. There may be neutral current from different homes on the same transformer that result from ground rod paths, water pipes, and other conductive paths between homes. Transformer neutral current inside the transformer is only related to the transformer. Outside of the transformer the neutral from the transformer to the home may carry a small current from some other source, or local path.
There are also a lot of residential areas with single post single phase transformers. Here the neutral of the primary and secondary are connected together, and some primary neutral current may, and almost certainly will, flow thru the neutral from the transformer to the home. The power company primary neutral has many ground points, and the home has at least one grounding point. This is why primary neutral current shows up on the secondary neutral. Once you pass the home main breaker panel there should not be any primary neutral current flow.
.
In my area there are a great number of 3 wire 3 phase primaries with 2 post primary transformers to supply the load. Here the residential neutral, secondary side, is only associated with the pole transformer and earth in at least two places. There will be very little current in this neutral that flows back to the sub-station. There may be neutral current from different homes on the same transformer that result from ground rod paths, water pipes, and other conductive paths between homes. Transformer neutral current inside the transformer is only related to the transformer. Outside of the transformer the neutral from the transformer to the home may carry a small current from some other source, or local path.
There are also a lot of residential areas with single post single phase transformers. Here the neutral of the primary and secondary are connected together, and some primary neutral current may, and almost certainly will, flow thru the neutral from the transformer to the home. The power company primary neutral has many ground points, and the home has at least one grounding point. This is why primary neutral current shows up on the secondary neutral. Once you pass the home main breaker panel there should not be any primary neutral current flow.
.
kwired
Electron manager
- Location
- NE Nebraska
Once you pass the main breaker panel that risk is typically greatly reduced, there likely is little or no current, but any voltage above earth is still there. All your equipment grounding conductors are still at same potential above earth as the MGN, because they are all tied together via the bonding jumpers between systems. This has been the cause for many electric shock and electrocution incidents especially around things like marinas, docks, boat hoists. Nothing normally considered wrong with anything, just simple rise in voltage on the MGN because of normal operation voltage drop will cause this problem.
gar
Senior Member
- Location
- Ann Arbor, Michigan
- Occupation
- EE
130525-1449 EDT:
kwired:
For a 3 wire 3 phase primary there is no neutral, nor any sort of shield ground. Unless a primary wire falls on and connects to a secondary wire, hot or neutral, there is no current in the secondary side from the primary.
The maximum voltage from my neutral to anywhere in my yard is less than 0.1 to 0.2 V. If there is a downed primary wire to earth nearby my neutral to earth voltage won't be changed much. You would need to get somewhat close to the arcing point to see much change.
I will need to do a 120 V test to provide an idea of what somewhat means. By recollection it will be in the 10 to 20 ft range when the current is supplied from my transformer.
.
kwired:
For a 3 wire 3 phase primary there is no neutral, nor any sort of shield ground. Unless a primary wire falls on and connects to a secondary wire, hot or neutral, there is no current in the secondary side from the primary.
The maximum voltage from my neutral to anywhere in my yard is less than 0.1 to 0.2 V. If there is a downed primary wire to earth nearby my neutral to earth voltage won't be changed much. You would need to get somewhat close to the arcing point to see much change.
I will need to do a 120 V test to provide an idea of what somewhat means. By recollection it will be in the 10 to 20 ft range when the current is supplied from my transformer.
.
kwired
Electron manager
- Location
- NE Nebraska
Understood, your first paragraph in that post was talking about this kind of system. The second paragraph was talking about a three phase 4 wire wye distribution, the neutral of that primary is bonded to any grounded conductor of any secondary system connected to it and is subject to carrying primary neutral currents.
You can't have both without having two systems present.
- Location
- Chapel Hill, NC
- Occupation
- Retired Electrical Contractor
In our area there are 3 conductors on the 7200 V primary feeding the residential single phase areas. I assume that the middle leg is the neutral- not sure why it is needed but it appears it comes from the substations.
ceb58
Senior Member
- Location
- Raeford, NC
Chris they are all wrong. You see you have a boy transformer and a girl transformer and they in fall in love and get married. Then one day they have a little neutral..............:happyno:
- Location
- Placerville, CA, USA
- Occupation
- Retired PV System Designer
Chris they are all wrong. You see you have a boy transformer and a girl transformer and they in fall in love and get married. Then one day they have a little neutral..............:happyno:
But what happens when it grows up?
PS: Those who are interested in grounding/bonding and neutrals should take a quick look at this ongoing thread and chime in.
Last edited:
ceb58
Senior Member
- Location
- Raeford, NC
It gets loose and causes all kinds of problems because mommy and daddy transformer didn't tighten up on it when it was young.
gar
Senior Member
- Location
- Ann Arbor, Michigan
- Occupation
- EE
130525-2123 EDT
Dennis Alwon:
If there are two insulating posts on the transformer primary, then you can probably consider the primary to look like a delta source even though it may be a wye source at the sub-station with the wye center point grounded, but no neutral provided outside of the substation.
This you could determine by tracing the three wires from one transformer to another and see how the transformers are connected.
In our neighborhood and many others in the DTE area we have essentially a 3 wire delta primary on the poles. When a downed wire occurred beyond my house on one of my primary phase wires there was only a small variation in my voltage. A couple hours later to stop the arcing the one phase wire that was down was opened up between the substation and me, probably at the sub-station, but possibly at an intermediate point.
After the problem phase was opened ahead of me, then my voltage started jumping from possibly 20 to 90 V. This was not real rapid. My reasoning of the cause was that many customers with my problem had refrigerators or other loads that would trip automatic thermal overloads randomly and thus change the impedance loading from the bad phase that is now floating. Equivalent to what you would see in an open neutral situation with changing loads.
I pulled my main fuses and went to generator otherwise I could have had damaged equipment. But also I really needed to keep the freezers running so the generator was connected.
.
Dennis Alwon:
If there are two insulating posts on the transformer primary, then you can probably consider the primary to look like a delta source even though it may be a wye source at the sub-station with the wye center point grounded, but no neutral provided outside of the substation.
This you could determine by tracing the three wires from one transformer to another and see how the transformers are connected.
In our neighborhood and many others in the DTE area we have essentially a 3 wire delta primary on the poles. When a downed wire occurred beyond my house on one of my primary phase wires there was only a small variation in my voltage. A couple hours later to stop the arcing the one phase wire that was down was opened up between the substation and me, probably at the sub-station, but possibly at an intermediate point.
After the problem phase was opened ahead of me, then my voltage started jumping from possibly 20 to 90 V. This was not real rapid. My reasoning of the cause was that many customers with my problem had refrigerators or other loads that would trip automatic thermal overloads randomly and thus change the impedance loading from the bad phase that is now floating. Equivalent to what you would see in an open neutral situation with changing loads.
I pulled my main fuses and went to generator otherwise I could have had damaged equipment. But also I really needed to keep the freezers running so the generator was connected.
.
ceb58
Senior Member
- Location
- Raeford, NC
Slow Saturday night :blink::thumbsup:
Roger
kwired
Electron manager
- Location
- NE Nebraska
Most likely the system is 12.5/7.2 kV three phase Y. They brought two phases and the neutral through the area for two reasons, first to distribute loading across two phases - and they can alternate which two phases they run down different segments for load balancing. Second they can easily derive an open delta should the need arise that a customer would need three phase.
Most if not all the transformers are 7200 volts and operate from phase to neutral. If they operated phase to phase they would have two high voltage insulated bushings to connect the primary to. If they only have one bushing then the second primary conductor is intended to connect to a grounded conductor.
jumper
Senior Member
- Location
- 3 Hr 2 Min from Winged Horses
Ok, I am grounding the neutral for life after I give it a good talking to about its wanton and loose behavior.
- Status