Portable Generator GEC & EGC

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Location
Virginia
277/480-125KW generator on wheels
Are most portable generators wired as SDS or not?
If SDS,
Questions:
1. Do you drive a ground rod solely for the portable generator to be connected to?
2. If so, do you connect this ground rod to the ground rod for the pump house OR can you just provide a place on the Pump House utility service ground rod to connect the generator?
 

bennie

Esteemed Member
Re: Portable Generator GEC & EGC

A generator is a generator, a transformer is a transformer, there is nothing else.

The premises wiring may, or may not, be a separately derived system.

A transfer switch, ahead of the main service switch, and disconnecting the utility MGN, will create a separately derived system.
 

dereckbc

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

If you switch the neutral at the transfer switch, or if the generator neutral is bonded to the frame, then it is considered SDS by code.

If it is configured SDS then you must provide a GEC to the service ground electrode. By code you cannot simply drive a ground rod for the generator without bonding it to the service electrode.
 

Ed MacLaren

Senior Member
Re: Portable Generator GEC & EGC

As Dereck stated (he types faster than I do :) ), the determining factor is whether the generator is connected to a fixed wiring system by means of a transfer switch, and how that switch is configured.

If the generator is the only power source, and is not interconnected with another (utility) source, the term Separately Derived System does not even apply.

Assuming it is interconnected, the type of transfer switch (3-pole or 4-pole) selected, and the grounding/bonding required, depends on whether or not the generator winding wye point (neutral) is bonded to the generator frame.

The sketches below may help.
Note- They do not represent actual installations, and are offered to illustrate basic principles only.

Some of the other people here can help with the applicable NEC rules.





Ed

[ March 05, 2003, 11:02 PM: Message edited by: Ed MacLaren ]
 

bennie

Esteemed Member
Re: Portable Generator GEC & EGC

Read carefully the definition of a separately derived system, in Section 100.

A premises wiring system can be a separately derived system. A generator, transformer or the other power sources are not separately derived systems.
 

Ed MacLaren

Senior Member
Re: Portable Generator GEC & EGC

Hi Bennie, glad to hear you're health has improved.

"A rose by any other name is still a rose"

No matter what we call them, there are power sources that are "separate" from the main service, and a stand-by generator is one of them. In Canada we call them "isolated systems"

I am using the term Separately Derived System here because that is what the NEC calls them, and this is an NEC forum.

Ed
 

bennie

Esteemed Member
Re: Portable Generator GEC & EGC

Hello Ed, thanks for the thoughts. I don't mean to appear rude, my comments may be taken that way. I don't have a flowered writing style :) .

My only concern is that the erroneous application of the phrase, Separately Derived System, is causing a lot of objectionable common mode current and EMF.

Grounding a system in one earth location, is correct, grounding a transformer in two locations is an error.
 

charlie b

Moderator
Staff member
Location
Seattle, WA
Occupation
Electrical Engineer
Re: Portable Generator GEC & EGC

A rose by any other name would smell as sweet. A carnation can also smell sweet, but that doesn’t make it a rose.

My contribution here will relate to language only, not to code. A generator is not, and cannot ever be, a “Separately Derived System.” Nor can a transformer, nor any other power source. The reason is simply that none of these is a “system,” in the context of the NEC definition of SDS. The “system” is the premise wiring system. It can be powered by a transformer or by standby generator, but it is the wiring system that constitutes the SDS, not the power source.

Let’s be careful with words. The NEC is full of them, and therein lies the hazard. If you look up the word “separately” in a common dictionary, you will find such concepts as isolated, divided, disconnected, and detached. But can’t go from these meanings directly to an understanding of a “Separately Derived System,” even though that phrase does contain the word “separately.” You need to do as Bennie suggests, and look up the phrase in Section 100.
 

dereckbc

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

Bennie, I agree with your statement: "Grounding a system in one earth location, is correct, grounding a transformer in two locations is an error". This can be practiced if the electrician or engineer plans carefully. It is rarely practiced however. I agree this is the major cause of common mode noise on ground systems.

The problem is by code the transformer or any SDS can be grounded from multiple points. (The points have to be bonded, but not single point) One is from the EGC on the primary of a transformer, and another from the GEC on the secondary Xo. By code they are not required to come from the same central location (SPG). In addition the equipment being served by the transformer probable has incidental contact to ground via concrete anchors, and contact with other metallic objects forming multiple paths to ground through the EGC.

Glad to see you back and share your knowledge.

Dereck
 

bennie

Esteemed Member
Re: Portable Generator GEC & EGC

Gentleman: You have made my day. This is an area that really needs interest from learned professionals on this forum.

I have researched this topic back to when it went sideways. My information indicates that after 1984, when the handbook was written by J.F. Mac Partland, the equipment ground wire was added to a transformer primary.

A lot of problems in grounding will go away when the true definition of a separtely derived system is considered.

I have puzzeled over this subject, for a long time. The answer I needed was in plain sight, Article 100.

My Email is: palmerbenray@aol.com if anyone wants more data.
 

Ed MacLaren

Senior Member
Re: Portable Generator GEC & EGC

How often have any of you ever installed a transformer or generator and not connected some circuit conductors and a load to it?
That makes it a "system" and the grounding of that system is what Doug asked about in his post.

Doug, this is what OSHA has to say about the grounding of portable generators in one of their interpretation publications.

QUOTE "OSHA Standard 29 CFR 1926.404(f)(3)(i) and NEC 250-6(a)(1) and (2) states the conditions where portable generators do not have to be grounded.
Specifically, 29 CFR 1926.404(f)(3)(i)states "Under the following conditions, the frame of a portable generator need not be grounded and may serve as the grounding electrode for a system supplied by the generator: (A) The generator
supplies only equipment mounted on the generator and/or cord- and plug-connected equipment through receptacles mounted on the generator, and (B) The
noncurrent-carrying metal parts of equipment and the equipment grounding conductor terminals of the receptacles are bonded to the generator frame." END QUOTE

However when a so-called "portable" generator is connected to a fixed wiring system it would then not meet the definition of a portable generator, and then the system it supplies should be grounded and the equipment "bonded to ground" by the same methods as any other system.

The sketches illustrate the way I believe it should be done, from an engineering point of view. Perhaps someone will comment on NEC compliance.

Ed

[ March 06, 2003, 12:08 PM: Message edited by: Ed MacLaren ]
 

bennie

Esteemed Member
Re: Portable Generator GEC & EGC

Ed: I know we have walked this dog before, it still won't hunt. :)
From an engineering aspect the two switches are electrically the same.

This drawing appears in the handbook, and in Chapter 12 of the Soares Book of grounding.

This drawing is as ludicrous as the statement " All transformers, except an auto-transformer, are separately derived systems".
 

Ed MacLaren

Senior Member
Re: Portable Generator GEC & EGC

Bennie,
But she's a hard dog to keep under the porch. :)

Could you tell me, and the other good people, in technical laymans's terms, just what is wrong with the installation as sketched.

Ed
 

bennie

Esteemed Member
Re: Portable Generator GEC & EGC

Trace the path of the green wire, in the switch, indicated as a separately derived system.. The switch is shunted.
 

david luchini

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

Bennie,
As I read the definition of separately derived systems in the NEC, it is a premise wiring system with no direct electrical connections, including a solidly connected grounded circuit conductor, to SUPPLY conductors originating in another system. I do not see how the green wire in the sketch could be considered a SUPPLY conductor of either system. What does it supply, and would the system stop supplying power if that conductor is disconnected? The definition of an SDS appears not to address grounding conductors as you indicate.

Dave
 

don_resqcapt19

Moderator
Staff member
Location
Illinois
Occupation
retired electrician
Re: Portable Generator GEC & EGC

We will have an official panel statement on this in July. I have submitted a proposal to add the following words to the Article 100 definition of SDS:
"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". This is proposal 1-134. A similar proposal, 1-135, has been submitted by Mr. Stromberg of Dow Chemical. These proposals will put this question to rest, as they will result in an offical position statement by the code making panel.
The 2003 proposals are on line. These are just the proposals, without any panel action or comment.
Don
 

Ed MacLaren

Senior Member
Re: Portable Generator GEC & EGC

Trace the path of the green wire, in the switch, indicated as a separately derived system. The switch is shunted.
Yes, I agree, it is. And what is wrong with that? What problem or hazard does that cause?

Ed
 

bennie

Esteemed Member
Re: Portable Generator GEC & EGC

The reason for disconnecting the MGN during an emergency situation, such as a storm, is due to the unknown state the neutral may achieve.

A broken MGN from the distribution system, back to the source, can cause the medium voltage to appear on all grounded conductive surfaces of the premises.

A transfer switch must be before the main service switch to insure disconnecting the utility MGN system. With the MGN disconnected, the premises wiring is then a separately derived system. Grounding must be done, if the system is required to be grounded.`
 

bennie

Esteemed Member
Re: Portable Generator GEC & EGC

The Soares book implies that current does not flow in green wires.

The article also states that auto-transformers are not separately derived systems. Who ever wrote that does not know anything about transformers. A grounded wye/grounded wye, has a neutral connected.

No power source is a separately derived system. The power source may supply a separately derived system.

A transformer supplying a separately derived system must have an ungrounded primary supply.

An isolation transformer has a grounded primary with an ungrounded secondary.
 
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