Generator

[brian john]

Re: Generator

Hurk:

Grounding is required between the service, ATS and generator; second in all but a few circumstances (wood frame house for one) there would be some form of metallic connection. structural steel, rebar, sprinkler systems, and on and on, isolating the systems would be all but impossible or very expensive.

As for lack of maintenance that is up to the user we work in hospitals, some due maintenance others let the stuff fall apart. Should the facilities be designed with less than optimum electrical systems, just incase there may not be regular maintenance performed?

 

[hurk27]

Re: Generator

Brian
I still say that a NON-SDS is a more safer hook-up for a generator.
1. Not many check the condition of the transfer-switch
2. Having the neutral switched provides no more protection for ground faults than if they were not switched.
3. The switching of the neutral provides one more place for the failure of the neutral (with no back up of it) and looseing the neutral has caused alot of damage and fires.

as for the protection of electronic equipment if you loose the neutral there is no TVSS system that will withstand this amount of overvoltage or current and it would likely fail.
light bulbs can get very hot and the glass will melt and fall onto some flameables and ingnite them.
so the neutral to me would be the last wire that I would want to put on any kind of switch or contacts. And as with a generator you dont have the water ground to take up the loss of a neutral. unless you bring to water ground to the generator. but then you will have the parallel path again.

 

[russ]

Re: Generator

I have to agree with BJ.
When you run a circuit or feeder you must ground the metal involved with that circuit. That means grounding for metal conduit, disconnects, or metal wiring boxes on the generator.

Maybe if they come out with an all plastic generator we will be able to do away with the ground for the feed going to the generator. Until then it has to be grounded just like any other piece of equipment.

Russ

PS I go by Chicago code, (With very few exceptions, circuits are all run in metal conduit or metalic flex.)

 

[bennie]

Re: Generator

Please review Ed's drawing with the neutral bonded to the core.

The generator is grounded through the ground/neutral conductor with the green wire removed.

The star point should be bonded at the generator. An internal fault in the generator winding, will be contained at the generator and not the transfer switch.

 

[bennie]

Re: Generator

Installing a separate ground conductor, to the generator, is the same as installing an equipment ground conductor from the service to the utility transformer. It would be redundant and parallel with the ground/neutral and the earth. There is not really anything wrong, it is just not necessary.

 

[roger]

Re: Generator

Bennie,

. It would be redundant and parallel with the ground/neutral and the earth. There is not really anything wrong, it is just not necessary.

just a footnote.

Being that this seperate ground conductor is indeed in parallel with the neutral conductor, it would have to be sized as a paralleled conductor, not an EGC. And this would have to comply with 310.4

Roger

 

[Ed MacLaren]

Re: Generator

Installing a separate ground conductor, to the generator, is the same as installing an equipment ground conductor from the service to the utility transformer. It would be redundant and parallel with the ground/neutral and the earth.

Being that this seperate ground conductor is indeed in parallel with the neutral conductor, it would have to be sized as a paralleled conductor, not an EGC

Guys, In a correctly wired system that includes a generator configured as an SDS, at no time is the grounded (neutral) conductor in parallel with the equipment grounding conductor, or the earth.

There are two completely separate systems, only one of which can be active at any time, and connected only by the equipment enclosures, which cannot be isolated by any practical means.

When the load is on utility power, the neutral is X to Z, grounded to the earth, and bonded to the equipment, at the service only. (Diagram 4)

When the load is on generator power, the neutral is X to Y, grounded to the earth, and bonded to the equipment, at the generator only. (Diagram 5)

I have sketched a single phase installation, but the same principles apply to three phase systems.

Ed

[ May 24, 2003, 02:26 PM: Message edited by: Ed MacLaren ]

 

[roger]

Re: Generator

Ed, I'm stuck on diagram #6.

Bennie said,

Please review Ed's drawing with the neutral bonded to the core.

and I thought that was where we were.

Roger

[ May 24, 2003, 03:38 PM: Message edited by: roger ]

 

[bennie]

Re: Generator

Ed: Draw the green wire complete from the service neutral to the generator neutral. Don't leave gaps from the enclosures.

This is my point...The utility neutral is connected to the system, regardless of which one is on line.

The green wire between the generator and transfer switch has got to go.

The blue wire between the generator and ground rod has to go.

A separate ground electrode is required for the generator, when it is connected on the load side of the main.

[ May 24, 2003, 04:41 PM: Message edited by: bennie ]

 

[Ed MacLaren]

Re: Generator

Roger, Diagram 6 is an example of the Code violation that exists when a generator with it's neutral bonded to the frame is connected to a building wiring system with a two-pole transfer switch.

Of course, it is wrong. It is drawn that way on purpose to illustrate the violation.

Here is the situation I encounter often. A homeowner buys a portable generator at the home-and-garden store. These generators were designed for stand-alone use with cord-connected loads - construction sites, camping, etc. They are required to have the neutral bonded to the frame for such use.

The homeowner decides he would like to have this generator connected to his home wiring system. To maintain warranty and listing integrity, you can't lift the neutral/frame bonding jumper, even if you could get at it. So you use a three-pole transfer switch that breaks the neutral to avoid the problem illustrated in Diagram 6.

Diagrams 4 and 5 are two views of the same connection, the correct way to connect that generator.

Ed

P.S. BTW, I did not design these diagrams. They are redrawn from Square D transfer switch installation instructions, with a little color added for illustrative purposes.

During the Y2K period, many home-owners had generators installed, most of them incorrectly. To reduce his liability, Our Square D rep held a seminar re proper installation, at which he expressed great concern that electricians learn the proper installation methods.

[ May 24, 2003, 05:29 PM: Message edited by: Ed MacLaren ]

 

[Ed MacLaren]

Re: Generator

The utility neutral is connected to the system, regardless of which one is on line.

Could you kindly explain what problem that causes?

A separate ground electrode is required for the generator, when it is connected on the load side of the main.

It is my understanding that separately derived systems on the same premises are -
1. either grounded to a common grounding electrode system, or
2. if separate grounding electrode systems are used they must be bonded together.

Ed

 

[bennie]

Re: Generator

Ed: One of the most frequent hazards of high voltage being impressed on the secondary system is...During storms, when power lines are broken, the first object hit is the MGN. The high voltage appears on the neutral of the premises. The current will flow through the system taking everything out in its path.

When power is interrupted the generator goes on line. The neutral connection to the utility should be dis-connected for safety reasons.

When the generator is on line then there is a separately derived system. This is the only reason for switching the neutral, but it is very important in critical systems.

[ May 24, 2003, 05:30 PM: Message edited by: bennie ]

 

[Ed MacLaren]

Re: Generator

The neutral connection to the utility should be dis-connected for safety reasons.

If that were a common occurance, what about the millions of electrical utility consumers, including you and me, who have systems in which the MG neutral extends unswitched and unfused to the end of most branch circuits?

Does that mean you have installed a three-pole disconnect switch on your home service, and you open it during every power interruption?

Hey, this could happen without warning, right in the middle of Larry King Live.

Seriously, I have been thinking of getting a whole house TVSS unit. Hmmmm.

Ed

 

[roger]

Re: Generator

Ed,

Roger, Diagram 6 is an example of the Code violation that exists when a generator with it's neutral bonded to the frame is connected to a building wiring system with a two-pole transfer switch.

Not to be disrespectful, but, I know that. That is still what was being refered to in my post.

The homeowner decides he would like to have this generator connected to his home wiring system. To maintain warranty and listing integrity, you can't lift the neutral/frame bonding jumper, even if you could get at it. So you use a three-pole transfer switch that breaks the neutral to avoid the problem illustrated in Diagram 6.

I know the average homeowner wouldn't (and shouldn't) do this, but I use a jumper to backfeed a two pole breaker and completely lift the utility neutral when I need mine. All bonding and GE system stay in place. I'm to cheap to buy a xfer switch.

Roger

[ May 24, 2003, 06:16 PM: Message edited by: roger ]

 

[bennie]

Re: Generator

Ed: Would you let me know how to contact that Square D rep. I want an explanation of the transfer switch connection.

Do you have his Email address?

 

[hurk27]

Re: Generator

Ed

The homeowner decides he would like to have this generator connected to his home wiring system. To maintain warranty and listing integrity, you can't lift the neutral/frame bonding jumper, even if you could get at it

We have sevral portable generators in our shop in which we use on job sites.
they are 5000 watts and each have a 4-pole twist-lock receptacle on them plus a gfci. and one has a 3-wire 120 volt 30 amp recept.
they all have a rocker switch to switch between 120 to 120/240 when it is set for 120/240 the neutral is no longer bonded to the frame as it seems that this switch lifts the neutral/frame bond but only the twist-lock will have power and acording to the instructions that came with it, it says to use a 2-pole ts to use it as a backup source.

 

[don_resqcapt19]

Re: Generator

Wayne,
That generator switch seems to create a violation of 250.34(C). A 120/240 volt system is required to have a grounded conductor by 250.26.
Don

 

[hurk27]

Re: Generator

Don it does have a grounding conductor.
The ground termnal of the 4-pole twist lock receptacle is still bonded to the frame of the generator. I know if this twist lock is used for portable use then there would be no bond for the neutral but I will look closer at it tuesday when I go back to the shop.

 

[don_resqcapt19]

Re: Generator

Wayne,
It has to have a "grounded" conductor if it is a 120/240 volt 3 wire circuit. If it is a cord connection, the only way to have a grounded conductor is to ues the generator frame as the grounding electrode. This would require a "neutral" to frame bond.
Don