4 pole ATS

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jeremy.zinkofsky

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nj
I know :)


I guess my question primarily is: what is the advantage of a separately derived system vs no having one.

A lot of my questions are around the design of building power systems from scratch, wondering what I should consider vs not doing so.

Besides the cases where GFP is required (1000A and above systems), another advantage would be to prevent fault current from traveling through the solidly grounded neutral and into the normal distribution system.
 

ActionDave

Chief Moderator
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Durango, CO, 10 h 20 min from the winged horses.
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Licensed Electrician
Besides the cases where GFP is required (1000A and above systems), another advantage would be to prevent fault current from traveling through the solidly grounded neutral and into the normal distribution system.
Fault current is moving through the system to the main source or the back up source in either case. I don't see how switching the neutral makes any difference.
 

dkidd

Senior Member
Location
here
Occupation
PE
Fault current is moving through the system to the main source or the back up source in either case. I don't see how switching the neutral makes any difference.

Multiple neutral to ground bonds result in paralleling the neutral and any grounded conductive items. Both normal neutral and fault current will be shared by the parallel paths.
 

mbrooke

Batteries Included
Location
United States
Occupation
Technician
What if your system is 3 wire? Obviously you ground the generator neutral, but are you required ground rod and all that good stuff?
 

kwired

Electron manager
Location
NE Nebraska
Multiple neutral to ground bonds result in paralleling the neutral and any grounded conductive items. Both normal neutral and fault current will be shared by the parallel paths.

:?
There's only one neutral to ground bond in a SDS or Non SDS.
Correct, or at least only one connected at a time through the switched neutral on the SDS. You don't connect current carrying neutrals to the EGC's beyond that bonding point, which leaves only one path for neutral current to return to the source.

Return current of a ground fault takes the many paths that are bonded together, but that is only intended for abnormal circumstances and should be for short time when it does happen.
 

mivey

Senior Member
:?
There's only one neutral to ground bond in a SDS or Non SDS.
Incorrect.

Fault current is moving through the system to the main source or the back up source in either case. I don't see how switching the neutral makes any difference.
It removes a parallel path problem

Perfect illustration why you don't need to switch the neutral.
Are you objecting to people objecting to objectionable current or is it something else?

Do you have some prior posts on how your particular position negates code requirements or negates the objectionable current concerns or do you feel like posting your argument here?
 

dkidd

Senior Member
Location
here
Occupation
PE
I finally tracked down a good graphic that illustrates the parallel path problem.
If you bond the neutral to ground at the generator you must switch the neutral at the transfer switch, and vice versa.
 

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mivey

Senior Member
I finally tracked down a good graphic that illustrates the parallel path problem.
If you bond the neutral to ground at the generator you must switch the neutral at the transfer switch, and vice versa.
Precisely. Nice pic.
 

RD35

Senior Member
My understanding is that separately derived vs non-separately derived systems is all about the location of the grounded and grounding conductor bond point. If the two systems are bonded separately on the line side of the transfer switch then a switched-neutral is required to avoid having two bond points. If only one of the two is bonded prior to the switch then the transfer switch cannot switch the neutral and it is a non-separately derived system. If the bond is on the load side of the transfer switch, then switching the neutral will have no effect one way or the other and would not be necessary. This is also a non-separately derived system. I hope I am understanding this correctly.
 

dkidd

Senior Member
Location
here
Occupation
PE
My understanding is that separately derived vs non-separately derived systems is all about the location of the grounded and grounding conductor bond point. If the two systems are bonded separately on the line side of the transfer switch then a switched-neutral is required to avoid having two bond points. If only one of the two is bonded prior to the switch then the transfer switch cannot switch the neutral and it is a non-separately derived system. If the bond is on the load side of the transfer switch, then switching the neutral will have no effect one way or the other and would not be necessary. This is also a non-separately derived system. I hope I am understanding this correctly.

You have it right.

From NEC 2014

100 I.

Separately Derived System. An electrical source, other
than a service, having no direct connection(s) to circuit
conductors of any other electrical source other than those
established by grounding and bonding connections.


250.30

Informational Note No. 1: An alternate ac power source,
such as an on-site generator, is not a separately derived
system if the grounded conductor is solidly interconnected
to a service-supplied system grounded conductor. An example
of such a situation is where alternate source transfer
equipment does not include a switching action in the grounded
conductor and allows it to remain solidly connected to the
service-supplied grounded conductor when the alternate
source is operational and supplying the load served.

250.30 (A)

(A) Grounded Systems. A separately derived ac system
that is grounded shall comply with 250.30(A)(1) through
(A)(8). Except as otherwise permitted in this article, a
grounded conductor shall not be connected to normally
non–current-carrying metal parts of equipment, be connected
to equipment grounding conductors, or be reconnected
to ground on the load side of the system bonding
jumper.



If the neutral of a generator is solidly connected to an electrical system,
it is not a separately derived system.
 
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