SDS/Generator grounding help

[browningcw]

My name is Charlie Browning. I'm an electrical engineer with Black & Veatch in Charlotte, NC (please don't hold the EE against me ).

I have a very interesting situation with a waste water treatment plant. We have three utility services, three generators, and five pieces of power distribution switchgear. All loads are 480-volt. We have no 277-volt loads and we have not installed any neutrals in any of the building feeders, only equipment grounds.

Per my design, the contractor has bonded the generator neutrals at the first overcurrent device. We then installed 3 phase conductors and an equipment ground from the generator switchgear to each of the utility service switchgear. Each utility service switchgear has a service entrance rated circuit breaker and the utility neutral is bonded at this location. Transfer between utility and generator is accomplished via circuit breakers for two of the utility services (Russelectric switchgear automatic transfer scheme) and via a 3-pole automatic transfer switch at one of the switchgear. I can e-mail you the jpeg or ACAD drawing file if it will help explain the situation.

The electrical inspector is telling us that we must install 5 conductors (3 phase, 1 grounded [neutral], and 1 equipment ground) from the generator switchgear to each of the utility service switchgears. He does not accept my explanation/reasoning that we are treating the generators as SDSs and bonding at the first overcurrent device. He is saying that the generator and utility neutrals are connected together via the equipment grounds (which is true, they are connected via the main bonding jumper at each of the utility service switchgears).

If you're willing to help, my e-mail address is browningcw@bv.com My phone number is 704-510-8416.

Again, I can send you a JPG or ACAD 2000 drawing file to help explain. Or I can fax a drawing to you and call you to discuss.

Thanks in advance for your assistance and time.

Charlie Browning
Black & Veatch

 

[tom baker]

Re: SDS/Generator grounding help

If your transfer switch does not transfer the neutral then you do not have a SDS.
See definition of SDS in Article 100.

 

[browningcw]

Re: SDS/Generator grounding help

Tom,
Thanks for the very prompt response.

To clarify, the only place the service neutral is installed is in the service entrance. The generator neutrals are not installed to the utility service gear (they are bonded at the first overcurrent device). The utility and generator neutrals are only connected via the generators' equipment grounding conductor and the main bonding jumpers at the generator and utility service switchgear.

If I could find a place to post it, I would post a JPG that shows the entire system...

 

[don_resqcapt19]

Re: SDS/Generator grounding help

Charlie,
No matter how you make this inatallation there will always be a connection between the utility grounded conductor and the generator grounded conductor via the equipment grounding conductors. Even when you install a 4 pole transfer switch this connection between the two grounded conductors will exist. I see nothing wrong with your design.
don

 

[charlie b]

Re: SDS/Generator grounding help

To Charlie the EE in NC: This is Charlie the EE in WA. I promise not to hold your EE against you.

To Tom: I?m not convinced that Charlie does not have an SDS. There is no neutral to switch.

Suppose we consider a different facility that has 277V loads, and that uses the neutral. Suppose we install a 4-pole transfer switch, and bond N-G at both the utility switchgear and the DG switchgear. At both locations, the G bus will have a GEC that connects the G to planet earth (and to building steel, and to other stuff). This is the SDS setup that you are describing. But take note that the two N buses have a solid connection to each other, since each is bonded to a G bus, and since the two G buses are each connected to building steel. I conclude that this path from N to N does not contravene the definition of SDS in Article 100.

I have drawn a circuit diagram that I think matches what Charlie has described. I looked at normal current flow, and at current flow with an imbalanced load, and at current flow with a fault at a load. I see a difference between a 480/277V system (that would need a 4-pole transfer switch to call it an SDS) and a 480V system. I conclude that the 480V system would not need a 4-pole transfer switch to call it an SDS. It appears to me that his setup is acceptable. By that I mean that it appears safe to me, in that a fault would be cleared by a trip of the breaker, and that the grounding electrode system is not at risk of becoming energized by a load imbalance.

To Don: It seems that you and I are in agreement. I was about to post my reply when I saw yours.

 

[Ed MacLaren]

Re: SDS/Generator grounding help

Charlie Browning,

I agree with others that the installation seems to fit the criteria for a SDS system.

If you can send me the .jpg diagram file at edmac@attcanada.ca I will try to post it here.

Ed

[ August 12, 2003, 12:20 PM: Message edited by: Ed MacLaren ]

 

[browningcw]

Re: SDS/Generator grounding help

charlie b (and others),
Great news. I have finally convinced the chief electrical inspector that we do, in fact, have an SDS and that the N-G-N connection will always exist, even with a 4-pole transfer switch.
I drew it up and showed him how the fault current would clear the overcurrent device and protect the system.
He said in his mind the NEC is requiring a neutral (or 4th pole) but in his heart he couldn't see any reason to add either one.
I have sent a jpg to Ed and hopefully he can post it here for everyone to review (and add comments).
I realize this is a strange situation, but the principles still apply and this is a great example to demonstrate a true SDS.
Thanks to everyone for their assistance and advice.
Charlie Browning

 

[Ed MacLaren]

Re: SDS/Generator grounding help

Charlie, I am posting your diagram as a separate thread.
http://www.mikeholt.com/cgi-bin/codeforum/ultimatebb.cgi?ubb=get_topic&f=7&t=000336

I couldn't reduce the size any smaller, and you will have to use your scroll bar to see all of it.

That will prevent this whole thread from being really wide.

Ed

[ August 12, 2003, 05:53 PM: Message edited by: Ed MacLaren ]

 

[don_resqcapt19]

Re: SDS/Generator grounding help

Charlie,
After reviewing the drawing, I see no code or safety reason to change your design.
Don

 

[websparky]

Re: SDS/Generator grounding help

Charlie,

After looking at the drawing, I do not see a grounding electrode at any of the generators?

 

[Ed MacLaren]

Re: SDS/Generator grounding help

I think he intends each of these symbols-

to represent a connection to an acceptable grounding electrode system.

The neutral bus of each switchgear appears to be grounded.
What purpose would a grounding electrode adjacent to the generator serve?

Ed

[ August 12, 2003, 09:12 PM: Message edited by: Ed MacLaren ]

 

[websparky]

Re: SDS/Generator grounding help

Hi Ed,

I was thinking that this would apply.

NEC 2002 250.30 (A)(2) Grounding Electrode Conductor. The grounding electrode conductor shall be installed in accordance with (a) or (b). Where taps are connected to a common grounding electrode conductor, the installation shall comply with 250.30(A)(3).
(a) Single Separately Derived System. A grounding electrode conductor for a single separately derived system shall be sized in accordance with 250.66 for the derived phase conductors and shall be used to connect the grounded conductor of the derived system to the grounding electrode as specified in 250.30(A)(4). Except as permitted by 250.24(A)(3) or (A)(4), this connection shall be made at the same point on the separately derived system where the bonding jumper is installed.

 

[browningcw]

Re: SDS/Generator grounding help

websparky,
"...this connection shall be made at the same point on the separately derived system where the bonding jumper is installed."

The grounded conductor of each of the generators is grounded at the first overcurrent device (via a MBJ).

Or am I missing the point?

 

[browningcw]

Re: SDS/Generator grounding help

Originally posted by don_resqcapt19:
After reviewing the drawing, I see no code or safety reason to change your design.

Don,
I came to the same conclusion a long time ago (I designed this system in 2001) and it made it even tougher to 'defend the design'. The interesting thing (at least to me) is that if one applies a VERY strict definition of "connection", it appears that it's nearly impossible to have an SDS (where the utility and back-up power sources have a grounded condcutor) due to the MBJ connection at the service.

Do you agree?

[ August 13, 2003, 06:07 AM: Message edited by: browningcw ]

 

[don_resqcapt19]

Re: SDS/Generator grounding help

Charlie,
Most code authorities say that the grounding conenctions are not "direct electrical connections". If EGCs are direct electrical connections, then transformers would not be SDS. I submitted a propsal on this subject but it was rejected.

1-134 Log #1868 NEC-P01
(100?Separately Derived System)
Final Action: Reject
Submitter: Donald A. Ganiere Ottawa, IL
Recommendation:
Add new text as follows:
For the purposes of this section, a connection between the grounding conductors on the primary and secondary side of a transformer shall not be considered a direct electrical connection.
Substantiation:
The existing wording causes much confusion in the application of the rules for the installation of transformers. Many feel that the connection of the grounding conductors on the primary and secondary are direct electrical connections and therefore transformers are not separately derived systems. There have been numerous discussions of this on the various Internet forums devoted to the NEC. It is always very difficult to convince a number of people that the grounding connection is not a direct electrical connections and that transformers, other than autotransformers are SDS.
Panel Meeting Action: Reject
Panel Statement:
The proposed definition contains a requirement. The panel refers the submitter to 2.2.2 of the NEC Style Manual.
Number Eligible to Vote: 12
Affirmative: 12 Ballot Results:

Don

 

[don_resqcapt19]

Re: SDS/Generator grounding help

There was another proposal on this same subject and the panel statement makes it clear that a grounding connection is not a direct connection.

1-135 Log #3160 NEC-P01
(100? Separately Derived System)
Final Action: Reject
Submitter: Eric Stromberg, The Dow Chemical Company
Recommendation:
Revise text as follows:
A premises wiring system whose power is derived from a battery, solar photovoltaic system, generator, transformer, or converter windings, and whose ungrounded conductors have no direct electrical connection to the ungrounded supply conductors originating in another system. The grounded conductor at the separately derived system can be, and most often is, connected to the grounded conductor of other systems through the bonding connection to an effective grounding system.
Substantiation:
The current definition of a Separately Derived System leads one to believe that it is an isolated system that has no connection to the upstream, or downstream, system. In fact, the neutral of the separately derived system is connected to the neutral of the other systems through the grounding system. The proposed definition makes it clear that the systems are, in fact, connected. Understanding the connectedness of the entire electrical system will help people understand the potential for multiple paths for neutral current and will result in the installation of better electrical systems.
Panel Meeting Action: Reject
Panel Statement:
The existing definition is clear and states that there not be a direct connection of a solidly grounded circuit conductor to a similar conductor in another system. Connections of these conductors to a common grounding system are not connections of the circuit conductors directly to each other.
Number Eligible to Vote: 12
Affirmative: 12 Ballot Results:

 

[roger]

Re: SDS/Generator grounding help

And this is proof that not all (unlike our Charlie E ) panel members are sane people, and in fact they are downright scarry.

In otherwords if they are going to spend such time and effort to perfect grounding, and then these same people do not realize all grounding and associated bonding conductors are "direct electrical connections", we are definately in trouble and are being led by the blind.

Roger

[ August 13, 2003, 12:56 PM: Message edited by: roger ]

 

[iwire]

Re: SDS/Generator grounding help

Originally posted by don_resqcapt19:

Panel Statement:
The existing definition is clear and states that there not be a direct connection of a solidly grounded circuit conductor to a similar conductor in another system. Connections of these conductors to a common grounding system are not connections of the circuit conductors directly to each other.

OK, I am aware many you have talked about this before but I have missed most of it.

Can anyone explain the logic of this?

I could understand if it said "connections solely through the earth are not connections of the circuit conductors"

But I can not understand it as worded, to my mind what they are saying is the equivalent of saying conductors are not conductive.

Bob

[ August 13, 2003, 12:58 PM: Message edited by: iwire ]

 

[Ed MacLaren]

Re: SDS/Generator grounding help

Guys, I wonder if any more of you are getting a bit tired of this SDS name argument?

Try thinking of it this way.

When step-down transformers and/or generator sets came into common use in facilities, there was a need for a term, or name, to describe a wiring system that was supplied from one of these sources of voltage that was separate from the supply service source.

For a period of time these were called isolated systems, and still are in Canada. The NEC has subsequently changed the name to Separately Derived System, because their supply is derived from a separate source, but, here's the main point, it's just a name.

There will always be wiring systems in buildings that are supplied from transformer secondaries, generators, photo-voltaic/batteries, and other sources other than the utility service.

These systems are often required to be grounded to the building's grounding electrode system. Does that mean that they shouldn't be allowed to exist, or if they do, must have their name changed? Of course not!

I hope you will pardon my sarcasm, but didn't that make sense.

Ed

 

[don_resqcapt19]

Re: SDS/Generator grounding help

The code rule is only intented to apply to the actual circuit conductors and not to the grounding conductor. That is what I was taught and this has been clarified by the panel statement. Look at the wording of the definition.

Separately Derived System. A premises wiring system whose power is derived from a battery, from a solar photovoltaic system, or from a generator, transformer, or converter windings, and that has no direct electrical connection, including a solidly connected grounded circuit conductor, to supply conductors originating in another system.

The grounding conductors are not supply conductors. I still think that the panel should accept my proposal to make their position very clear. Not too many users of the NEC read the ROPs and ROCs. Yes, I know that many of the members here do read these documents, but the normal code user does not.
Don