Portable Generator GEC & EGC

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doug

Member
Location
Virginia
Re: Portable Generator GEC & EGC

ED,
On your diagram for SDS, what does the dotted blue line represent?

Doug Glasscock
 

hurk27

Senior Member
Re: Portable Generator GEC & EGC

I think what the intent of the NEC is that there is no solid connection between the generator/transformer and the primess wiring as it says if the second part 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.
and in a FPN that also discribes it:

250.20
D) Separately Derived Systems.
FPN No. 1: An alternate ac power source such as an on-site generator is not a separately derived system if the neutral is solidly interconnected to a service-supplied system neutral.
So if a transfer switch switches the neutral then it would be a separately derived system, and would be required to be bonded as per 250.30

FPN No. 2: For systems that are not separately derived and are not required to be grounded as specified in 250.30, see 445.13 for minimum size of conductors that must carry fault current.

[ March 06, 2003, 11:48 PM: Message edited by: hurk27 ]
 

bennie

Esteemed Member
Re: Portable Generator GEC & EGC

Correct, provided the switching is done before the equipment ground wires are in the equation.

Install a conductor of any color, or name, across a switch and the switch will not function. Why switch the neutral when the equipment ground conductor is in parallel.

The switching, as shown, must have been designed by the auto-transformer expert. :)
 

Ed MacLaren

Senior Member
Re: Portable Generator GEC & EGC

Doug, the dotted blue line represents the alternative location to connect the generator grounding electrode conductor.

Install a conductor of any color, or name, across a switch and the switch will not function
Bennie and I have been through this before, but I think it should be addressed again in case the new folks are interested.

A double throw switch (transfer switch) is actually two switches in one, and in this SDS installation it’s purpose is to completely isolate the circuit conductors of the two systems.

There are actually two separate systems and it helps to visualize them one at a time.
Note - The sketches below are for a single-phase installation, but the principle applies to three phase also.

When the load is on utility power, it is as if the generator does not exist. Diagram 4 The unbalanced load (neutral) current path is from X to Z, and there is no parallel path.

Likewise, when the load is on generator power, it is as if the service (utility) does not exist. Diagram 5 The unbalanced load (neutral) current path is from X to Y, and again there is no parallel path.

Each system is grounded and each system’s EGC lands at the point where it’s GEC is connected, (Single Point Grounding) and the function of the transfer switch is not impaired by the EGCs.

Ed



 

bennie

Esteemed Member
Re: Portable Generator GEC & EGC

Visualize the utility neutral (MGN) becoming 13 KV to ground, during a storm, when the MGN is broken upstream.

Basic electrical theory, a shorted switch is no switch.

Bennie...palmerbenray@aol.com

[ March 07, 2003, 08:10 AM: Message edited by: bennie ]
 

bennie

Esteemed Member
Re: Portable Generator GEC & EGC

Another reason the MGN must be disconnected is to prevent the ground electrode system from conducting fault current from non-related, near by power lines, during a LG fault.
 

bennie

Esteemed Member
Re: Portable Generator GEC & EGC

Regardless of the Soares Book on Grounding, a green wire will carry current, it's presence excludes the premises wiring from the definition of being a separately derived system.

Place the transfer switch ahead of the service switch. Switch the neutral. Install equipment ground conductor from the transfer switch to the service switch. You now have a separately derived system when operating in the generator mode.
 

david luchini

Moderator
Staff member
Location
Connecticut
Occupation
Engineer
Re: Portable Generator GEC & EGC

Bennie,
Out of curiousity, where would the service Main Bonding Jumper (NEC 250-28) be installed on the system you just described, and where would the SDS bonding jumper (NEC 250-30(a)(1)) be installed?

Dave
 

bennie

Esteemed Member
Re: Portable Generator GEC & EGC

Hello David. The intertie to the earth, equipment, and MGN, will be at the service switch.

The ground electrode system for the utility service will serve the generator. There is not as much concern with a lightning strike transient.
A lightning strike can affect the neutral.

Another reason for disconnecting the neutral (MGN) is...During storm conditions and ice buildup on power lines the lines break and make first contact with the MGN below the lines. The premises service elevates to the high voltage. This is also part of the reason for the speed requirement of a generator starting and coming on line. Voltage on the conductive surfaces is quickly removed.
Compares to the GFCI principal.
 

david luchini

Moderator
Staff member
Location
Connecticut
Occupation
Engineer
Re: Portable Generator GEC & EGC

Bennie,
I'm not sure I understand. Are you saying that there is no SDS bonding jumper as defined in NEC 250-30(a)(1)? If so, what is the purpose of that section?

Dave
 

bennie

Esteemed Member
Re: Portable Generator GEC & EGC

David: The premises wiring is the separately derived system. The same service intertie serves the same purpose for both the utility and generator.

Alternating-current Systems Without External Connections Section 2514, now Separately Derived Systems, originated during the REA era of electrical distribution.
As power lines started appearing in the rural areas. Many farm houses had generators for electrical power.
This premises wiring system is a separately derived system, it is separately derived from the utility system, and not grounded to the utility grid MGN.

The code statement is to point out this feature, and require the separately derived system to be grounded according to proper procedure.

The definition in Article 100 is quite clear and substantiates this statement.
 

bennie

Esteemed Member
Re: Portable Generator GEC & EGC

Another fact: Dairies were powered by a transformer supplied by an ungrounded secondary power line. The derived neutral was not electrically connected to the supply MGN.

This premises wiring is a separately derived system, the transformer is a transformer, nothing else.

This system is not used due to through fault containment problems.
 

david luchini

Moderator
Staff member
Location
Connecticut
Occupation
Engineer
Re: Portable Generator GEC & EGC

Bennie,
I think the definition in article 100 is quite clear that the SDS involves the interconnection of SUPPLY conductors (including the grounded circuit conductor) and not of the grounding system.

If the SDS is also the service entrance, then why give instructions on how to ground the SDS in section 250-30. It is already grounded by section 250-28. The SDS grounding section is completely redundant under this example. It seems to me that under your definition that every three phase service fed from a utility padmount transformer is also an SDS (whether or not there is a generator and transfer switch) because there is no connection from the primary system to the secondary system.

Dave
 

Ed MacLaren

Senior Member
Re: Portable Generator GEC & EGC

I am mystified by Bennie’s statement-
Another reason the MGN must be disconnected is to prevent the ground electrode system from conducting fault current from non-related, near by power lines, during a LG fault.
Why would the (Multiple Grounded) Neutral have to be "disconnected" in this particular system, and not in others.
The neutral conductor, in virtually every residential and three phase four-wire service in North America, is not switched.

In my house, my workplace, and in most building services that I have seen, the multiple grounded utility neutral extends un-switched, and un-fused, all the way to the load devices.

The purpose of switching the neutral conductor in the transfer switch, in the systems illustrated above, is to prevent a parallel path created by the internal bonding of the generator neutral to the frame.

If the generator neutral is bonded to the frame, and the neutral were not switched, a portion of the unbalanced (neutral) current would flow in the EGC. Diagram 6.

Generators that do not have their neutral bonded to the frame (floating neutral) can be used with a solid neutral transfer switch. Diagram A – Page 1



Ed
 

hurk27

Senior Member
Re: Portable Generator GEC & EGC

If we look closer at 250.30 (1) it says that we can bond the gec at any point eather at the supply or the first means of disconect and a transfer switch could be the first means of disconect. but it does say that The point of connection shall be the same as the grounding electrode conductor as required in 250.30(A)(2). (point is singler)
Now where does it say that we have to bond the EGC at both the generator and the disconect?

(4) Grounding Electrode. The grounding electrode shall be as near as practicable to and preferably in the same area as the grounding electrode conductor connection to the system.
Again the word connection is singler


Now here it even permits us to have a single connection
Except as permitted by 250.24(A)(3),

250.24(A)(3) Dual Fed Services. For services that are dual fed (double ended) in a common enclosure or grouped together in separate enclosures and employing a secondary tie, a single grounding electrode connection to the tie point of the grounded circuit conductors from each power source shall be permitted.

I might be wrong on the intent but after rereading it over and over it seems to not allow a second connection.
And the bonding would seem to serve as both the egc and gec after the connection and this would comply with 250.20 FPN No. 1: An alternate ac power source such as an on-site generator is not a separately derived system if the neutral is solidly interconnected to a service-supplied system neutral.

Grounded Conductor. A system or circuit conductor that is intentionally grounded.

Grounding Conductor. A conductor used to connect equipment or the grounded circuit of a wiring system to a grounding electrode or electrodes.

Grounding Conductor, Equipment. The conductor used to connect the non?current-carrying metal parts of equipment, raceways, and other enclosures to the system grounded conductor, the grounding electrode conductor, or both, at the service equipment or at the source of a separately derived system.
Grounding Electrode Conductor. The conductor used to connect the grounding electrode(s) to the equipment grounding conductor, to the grounded conductor, or to both, at the service, at each building or structure where supplied from a common service, or at the source of a separately derived system .

[ March 07, 2003, 10:08 PM: Message edited by: hurk27 ]
 

dereckbc

Moderator
Staff member
Location
Plano, TX
Re: Portable Generator GEC & EGC

Bennie, I am trying to follow along with you, but I am still not clear what you are trying convey.

Are you saying if the service has a MGN, it is not SDS?

For example: If the sevice was a 480 ungrounded delta.

It looks to me like you are trying to say if there is a grounded conductor involed it is not SDS.
 

doug

Member
Location
Virginia
Re: Portable Generator GEC & EGC

Remember me,
I started all this.
My generator is 125KW, 277/480 on wheels.

Kohler says that a customer can specify whether to bond the neutral to the frame or not when it is ordered.

So lets say I order it with the neutral not bonded to the frame.

If I were to install a service rated transfer switch with utility and generator 3-POle main breakers built in to the transfer switch (Cutler Hammer makes this type of switch). I would then carry five wires (L1,L2,L3,N,bonding G) from the generator to the transfer switch and only four wires from the utility (L1,L2,L3,N) to the transfer switch. The GEC would be bonded at the transfer switch which would work for both services with a single ground rod.

1. Does this sound right?

2. Since the generator can be moved from site to site and it will have a cable with a large plug to connect to the pump house. Is it also safer/redundant to provide a place to hook on to this same ground rod, a connection from the generator frame?
I am thinking about this in the unlikely event that the operator starts the generator prior to plugging it in for operation.

Doug Glasscock

[ March 07, 2003, 11:23 PM: Message edited by: doug ]
 

ronaldrc

Senior Member
Location
Tennessee
Re: Portable Generator GEC & EGC

I'm not siding with anyone here but I did find these in my old 1971 NFPA NEC Handbook.

These where a direct scan from the pages and unedited.

Article 700-15 Please read paragraph #2 Solid Neutral on A-C and D-C Systems

:)

[ March 09, 2003, 09:57 AM: Message edited by: ronaldrc ]
 

hurk27

Senior Member
Re: Portable Generator GEC & EGC

Doug glad to see your still around but if you order the generator with the neutral to ground bonded then you must provide a 4-pole transfer switch look at ED's last diagram in his last post as this explaines it best. but if you order it without the bond then you can use a 3-pole transfer switch but you must run a egc along with the feeders from the generator fram to the transfer switch now you can bond the neutral at the transfer switch or at the service but where you do this bond you must bond your gec too. and the service is treated like a subpanel and will not have a neutral to ground bond. a neutral to ground bond can only be done at one location in a non sds system.

Wayne
 

hurk27

Senior Member
Re: Portable Generator GEC & EGC

Ed In the first post of the two diagrams (A&B) the B (SDS) will have a parallel path of the neutral between the generator and the transfer switch. I think this is why we have to connect the gec at the same point that we bond the neutral?

But I have learned alot from these post as the way I was thinking with the gec connecting to both sides of the transfer switch neutral contact that there had to be a paralleling of the neutral but after tracing the current path's I see now that it wouldn't since when the transfer switch is in the generator position the current would not flow back through the bond at the service disconect. even though there is an electrical connection accross the contact it is because that the connection is broken between the two neutral connections (poco&generator) at the transfer switch.

this could be duplcated by taking a dpdt switch and aranging the circuit with the supplys and loads with the same grounding to mimic the same circuit and an amp probe could be used to show which way the current goes when the switch is changed.

And as per the definition goes the neutral of the premiss wiring is not solidly connected to the service neutral or to the generator's neutral there just connected together (the service to generator) by the gec's
Ed Thanks again
Wayne

[ March 08, 2003, 01:46 AM: Message edited by: hurk27 ]
 
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