Separately derived source grounding

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shoffnerm

Member
Looking for a proper grounding method from source feeder conductors through metal conduit and a disconnect to a separately derived source with a secondary conductor service to new load.

I want to install a ground conductor with the feeder conductors to a disconnect for the high side of a separately derived source transformer. From the disconnect on the high side to the transformer container, I am using a flexible metallic tubing with metal flexible connectors. On the secondary side of this transformer the neutral is grounded which includes the container of transformer. This ground conductor is then distributed with the new voltage loads.

From the disconnect to the high side of the transformer I am questioning if I have to connect this source ground conductor??? When looking at it on paper, I still see a loop being created through the metal frame of the transformer container and the conduit back to the disconnect. I do not want to create any loops in the system when I connect the new grounded electrode with my system ground of the building. I have to protect the high side conductors to create the low impedance requirement for tripping the source breaker. I also have to create the same type path for the secondary source. This way the new distribution system breakers have their low impedance path.

It appears that a break in the conduit between either the disconnect or after the separately derived source should be incorporated to prevent loops ? since it is required to bond all ground electrodes or grounded electrode conductors.

I do not have a NEC Handbook ? but I do recall it had a diagram for this type installation. If anyone has this available and can forward a copy it might help.

Thank you ALL? This forum is an excellent resource.
 

Ed MacLaren

Senior Member
Re: Separately derived source grounding

This is not from the NEC Handbook, but I believe this sketch illustrates the basic principles of grounding and bonding for the type of installation you describe.

In any case, it can serve as a starting point for others to comment on.

I'm not sure what "loops" you are concerned about?




Ed

[ September 02, 2003, 07:40 AM: Message edited by: Ed MacLaren ]
 

shoffnerm

Member
Re: Separately derived source grounding

Hello Ed,

Thank you for the reply. The loop that I see starts at the ground symbol and intern loops around right back to the same ground sysmbol. I guess I am confused about the low impedance path for the phase conductors up to the line side of the delta-wye transformer. I would think that if there is a fault on the phase conductor of the line side of the transformer... then the lowest path would have to return back to the sou8rce N-G bond to ground. Would the low path in the sketch actually be the N-G bond on the secondary side of the transformer. OR Would there be a problem for the fault current ...because either way... it would find a path to ground and cause the fuse to blow.

Thanks, Martin
 

dereckbc

Moderator
Staff member
Location
Plano, TX
Re: Separately derived source grounding

Shoffnerm, you can run the GEC with the primary feeder conductors provided you bond the GEC at the point where it enters and leaves the raceway, providing building steel and water pipes are not closer than the service ground. However if the building steel or water pipe is closer you can still run an GEC with the feeders provided you also bond to either the building steel or water pipe if you wish.

The loops you are concerned with do not exist. You are mearly paralleling the EGC of the primary feeder circuit. Look at ED's drawing closely and you should see this. The point is at the secondary of the transformer is single point grounded to the grounded conductor. What happens before this point is moot.

[ September 02, 2003, 02:15 PM: Message edited by: dereckbc ]
 

bennie

Esteemed Member
Re: Separately derived source grounding

Most end user transformers are not connected back to the service ground. The X-O is connected to a water pipe or the building steel.

I maintain that the two green wires, as shown, back to the service are not necessary. Plus regrounding the X-O at a different location is multi-point grounding, creating ground loops.

The X-O of the transformer and the ground neutral bus, of the service, are at different potentials.

Can anyone explain why it is necessary to re-dirt the X-O at two locations?

[ September 02, 2003, 11:33 AM: Message edited by: bennie ]
 

bennie

Esteemed Member
Re: Separately derived source grounding

Most end user transformers are not connected to the same electrode.

Why does the transformer need two conductors making paths to earth?

Why is the ground electrode conductor, on a transformer, required to be connected to the closest ground electrode, yet the main service ground conductor can be any length?

As I have stated before, a bond is a low impedance short circuit. A ground electrode conductor is an inductor and will present a degree of impedance.

I still stick to my own understanding that 250-30 applies to the premises wiring system not to the power source. This misunderstanding is creating the ground loops.

[ September 02, 2003, 11:54 AM: Message edited by: bennie ]
 

Ed MacLaren

Senior Member
Re: Separately derived source grounding

Most end user transformers are not connected back to the service ground.
The sketch is a schematic, and as such, simply indicates that the SDS is to be grounded to an acceptable electrode system.

Plus regrounding the X-O at a different location is multi-point grounding, creating ground loops.
Notice that the SDS (the system supplied from the secondary of the transformer) has grounding electrode conductors connected to it at only one point, at X0. In my opinion, that is what is meant by "single point grounding", not the number of connections to the earth.

The X-O of the transformer and the ground neutral bus, of the service, are at different potentials.
The way I see it, the two points that you refer to belong to two different systems. There is no relevence to to the term "potential difference" between the two systems.

Under no-fault conditions, the grounded conductor of each is at 0 volts potential in relation to the earth. The earth thus becomes the common reference, and only in relation to the earth does it make any sense to compare the "voltage to ground" of the two systems.

Ed
 

don_resqcapt19

Moderator
Staff member
Location
Illinois
Occupation
retired electrician
Re: Separately derived source grounding

shoffnerm,
Would there be a problem for the fault current ...because either way... it would find a path to ground and cause the fuse to blow.
The fault must have a path back to XO, not to ground, to cause the OCPD to operate.
Don
 

brian john

Senior Member
Location
Leesburg, VA
Re: Separately derived source grounding

In posting a reply how do I include a quote from a previous poster. As Ed and Don have done in this Item.

Brian
 

dereckbc

Moderator
Staff member
Location
Plano, TX
Re: Separately derived source grounding

Ed, well said and I agree 100%.

Sorry Bennie but I share the oppinion that the SPG is in reference to the source not earth.
 

bennie

Esteemed Member
Re: Separately derived source grounding

I know we have kicked this dead horse around, a lot. It still has a bit of life left, so here goes.

According to 250.30(4), the only time a separately derived system can be grounded to the building ground is when it is part of a listed system. This section is assuming the transformer is a separately derived system. Which I believe is a serious error in interpretation.

I plan to run this by the McPartland group, along with a request for voice lessons. :p

Ed, draw the ground electrode conductor from the transformer to the main service as a connection to building steel. The earth is not a bond, so there will be two points of earth contact.

Single point grounding of a premises system is the earth contact point. There can be many terminating points in the premises grounding system but only one to earth.

[ September 02, 2003, 02:29 PM: Message edited by: bennie ]
 

bennie

Esteemed Member
Re: Separately derived source grounding

When the premises system is single point grounded the ground conductors in the premises are connected together to form the grounding system.

Connect a rod at the service and one at the transformer, with no low impedance bond, it is multi-point grounding just like a utility MGN system.

Draw a transformer in a wood constructed building with no water lines. Where do you, and how do you, make an earth connection?

The logical and only correct place is the building service ground electrode. Driving a rod for the transformer is the MGN system.

Ed's drawing is correct if in listed service equipment.
 

pierre

Senior Member
Re: Separately derived source grounding

When Schoffnerm posted this I was going to warn him, but I wanted to see where this went. It has gone just where I thought it would. ;)
My two cents

Ed I know the drawing is a schematic, but would it or could it be drawn to show the transformer GEC as going to a 'local' ground with a bond to the utility service?
My understanding is the transformer GEC is connected as close as possible, one reason is to maintain an equal potential between the new source and the surounding area (almost like a equipotential plane). At a separately derived system (do I even use that phrase)the connection has nothing to do with the overcurrent device or lightning discharge.

I am not sure why we need to bond the two systems together with a separate conductor when there is already a connection via the equipment grounding conductor to the transformer case (x/o).

Pierre
 

gwz2

Senior Member
Location
Indiana
Re: Separately derived source grounding

Slightly off the main subject on this thread, maybe a few notes added to Ed's drawing would contain;

1) The Service Equipment GEC may be a smaller size than the GEC from the Transformer.

2) The Bonding Jumper (BJ) from the Xo terminal to the Ground Bus in the transformer must be the same size as the GEC. 250.30(A)(1).

3) The GEC and the BJ must orginate from the same location, usually the Transformer or the Disconnect on the load side of the Transformer, not both.
 

Ed MacLaren

Senior Member
Re: Separately derived source grounding

Pierre, here is a sketch showing the alternative, as you suggested.

I am not sure why we need to bond the two systems together with a separate conductor when there is already a connection via the equipment grounding conductor to the transformer case (x/o).
I am not sure either. I suspect it's because lightning is very unpredictable, and the EGC needs to be dedicated to the more important function of ground-fault clearing.
Perhaps Don Ganiere will come to the rescue, re any NEC rules that apply. :)



Ed
 

don_resqcapt19

Moderator
Staff member
Location
Illinois
Occupation
retired electrician
Re: Separately derived source grounding

The only bonding together that is required is the bonding that results from the primary EGC and the secondary grounded conductor both being connected to the transformer enclosure. In both cases these connections are required to clear a fault that may occur within the transformer itself. I believe that the primary EGC satisfies the rule in 250.58.
Don
 

iwire

Moderator
Staff member
Location
Massachusetts
Re: Separately derived source grounding

Originally posted by brian john:
In posting a reply how do I include a quote from a previous poster.
Brian you can do it like dereck has pointed out or click on this
at the top of the post you want to quote from.

That will bring you to the posting page with the post you want to quote from already inserted with the "Originally posted by" wording all set for you.

Once there you can highlight the portions you don't want with your mouse and delete the unneeded portions.

Bob
 

bennie

Esteemed Member
Re: Separately derived source grounding

Now remove the wire between ground rods. There is a perfect ground loop. The system is exactly the same as a MGN system and very high in common mode current flow.

You can't bond(short) at the top, it must be at ground level.
 
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