Separately Derived 3phase 3 wire System

[don_resqcapt19]

Re: Separately Derived 3phase 3 wire System

Bennie,

Connecting the X-O to two different ground points is in violation of the objectionable current code section.

XO of each system is connected to ground at only one point, that point is where the main bonding jumper is installed for the service and where the bonding jumper is installed for the secondary of the transformer. While it is true that the XOs of both systems are tied together, that is not the same as bonding XO at multiple points. Additional connections to earth on the grounding system on the earth side of the bonding jumper are electrcially the same as supplementary grounding electrodes as permitted by 250.54.
Don

 

[bennie]

Re: Separately Derived 3phase 3 wire System

Don: I know things have changed. My old manuals show a feed to two different earth locations as two ground points.
A single feed to an electrode, then to the others, makes one ground point, with all electrodes bonded together.

The instructions in 250.30 pertain to a separately derived system (premises wiring) that is not connected to a utility power source, or by a transformer with an ungrounded primary.
Check handbooks from 1984 and older. There is a clear schematic of a transformer supply to a premises wiring system.

The code is based on technology, many try to base technology on the code.

 

[don_resqcapt19]

Re: Separately Derived 3phase 3 wire System

Bennie,
Figure 250-9 in the 1984 NEC handbook shows exacty how SDS transformers are to be bonded, except that the required EGC from the primary source to the transformer case has been left out, but they have also left out any EGC on the secondary side.
Don

 

[bennie]

Re: Separately Derived 3phase 3 wire System

Don: That is exactly my point. The transformer shown is a distribution transformer with an ungrounded primary. This is a source for a separately derived system.

250.30 states the procedure for grounding the premises wiring, if required to be grounded.

The drawing indicates there is no solid electrical connection from the premises wiring (Secondary) to the supply originating in another system.

The transformer does not require an equipment ground. The neutral serves the purpose.

 

[dereckbc]

Re: Separately Derived 3phase 3 wire System

Bennie, any chance you can draw out what you are talking about and post it or send it to me? I would really like to see what you are talking about. I really do not see how bonding the Xo of a transformer to a GEC creates multi grounded neutral. You cannot run your primary without an EGC.

 

[bennie]

Re: Separately Derived 3phase 3 wire System

Dereck: A 480 ungrounded system, connected to the primary of the transformer, will not have an equipment ground.

Ungrounded utility secondaries supplying a distribution transformer will not have an equipment ground from the supply originating in another system.

The last correct handbook was when Joseph McPartland was the author. The author following him, drew in the green wire, making a MGN system.
Also drew in the green wire shunting the neutral on a transfer switch. Amateur mickey mouse engineering.

Email your address, I will send some drawings. palmerbenray@aol.com

 

[don_resqcapt19]

Re: Separately Derived 3phase 3 wire System

Bennie,
Look at Figure 250-1 in the '84 handbook. In this case they make it very clear that the buildign service is 480/277 wye and the secondary side of the transformer is 208/120 wye. The second paragraph above figure 250-1 says that the transformers that are on the load side of the service and the line side of the transfer switches are SDS. Also look at the wording in the comentary that preceeds figure 250-9. It says; "The requirements of Section 250-26 are most commonly applied to 480Y/277 V transformers which are used to transform a 480 V supply to a 208Y/120 V system to supply lighting and appliance loads."

The transformer does not require an equipment ground. The neutral serves the purpose.

There must be an equipment grounding conductor with the primary, even if it was an ungrounded system.
Don

 

[bennie]

Re: Separately Derived 3phase 3 wire System

Don: I know I have hammered this subject into the ground (pun intended)

The X-O is shown as being earthed at the transformer, making this a "source" for a separately derived system.

The neutral can be earthed at service switch. This is normally already the case. Additional earth points make this a MGN system.

MGN systems are fine outdoors. They contribute a lot of noise on premises wiring. Multiple ground points should be avoided.

The transformer grounding as shown on figure 250.26, 1984 Handbook is for earthing the premises wiring (SDS).

Explain to me... why earth the X-O to an additional electrode, when it is connected to the service ground system?

I have documented proof of my stand on this issue. The documents are on file at the NEC library.

 

[bennie]

Re: Separately Derived 3phase 3 wire System

Referring to a transformer. A source for a separately derived system, as a separately derived system, is the same as as calling a transformer (source) for a TV station, a TV.

 

[don_resqcapt19]

Re: Separately Derived 3phase 3 wire System

Bennie,
I don't under stand where the additonal earth points are. The service grounded conductor is connected to earth via the main bonding jumper, the grounding electrode conductor and the grounding electrode. The neutral of the transformer, which is serving as the source of a SDS, is bonded to earth via its bonding jumper, grounding electrode conductor and its grounding electrode system.
Are you saying when the EGCs of the service and SDS are tied together, that this is an additional earth point?
Don

 

[bennie]

Re: Separately Derived 3phase 3 wire System

Don: Hypothetical... A transformer 100 feet from main service and ground elctrode.

Equipment ground conductor installed and connected to X-O.

X-O is again grounded by driving a rod.

Paint the wire white and it is regrounding the neutral at two distinct locations.

Bonding electrodes is not accomplished from the feed end. It can only be done at the connection to earth point.

Bonding of electrodes is best outside of building.

 

[don_resqcapt19]

Re: Separately Derived 3phase 3 wire System

Bennie,
I fail to see any problem with that installation. And if there is a problem how cay it be corrected? The primary EGC must be connected to the transfomer case to provide a fault cearing path in the event of a primary coil to frame short, the secondary XO must be bonded to the frame to provide a fault clearing path for any secondary coil to frame short. What do you propose to make a safe installation and eliminate the problem that you see?

Don

 

[bennie]

Re: Separately Derived 3phase 3 wire System

Don: I just finished my daily therapy of oxygen. I call it hitting the bottle. Maybe my head is clear enough to make sense.

Stop regrounding the X-O. Ground it only at the service as it is presently being done.

The equipment ground conductor from the supply to the enclosure and X-O, is for clearing primary to ground, and winding to winding faults.

A secondary winding fault will likely burn clear before any fuse or breaker operates.

 

[don_resqcapt19]

Re: Separately Derived 3phase 3 wire System

Bennie,
What do you mean by "regrounding" XO? The XO terminal of the transformer that supplies the SDS is only grounded at the transformer. The service neutral is bonded at the utility transformer and at the service disconnect. Which of these connections do you want to eliminate?
Don

 

[bennie]

Re: Separately Derived 3phase 3 wire System

Don: Where does the equipment ground conductor terminate?

Answer... At the service ground/neutral buss and transformer X-O. The neutral/ground buss is connected to the earth. One point grounding.

The X-O is then earthed a second time by a ground electrode system that is not bonded to the service electrode. Two point grounding.

 

[bennie]

Re: Separately Derived 3phase 3 wire System

Imagine this setup in line with a SWER system. Part of the 8 amps will flow on the equipment ground even with power off.

 

[don_resqcapt19]

Re: Separately Derived 3phase 3 wire System

Bennie,
The bonding of the grounded conductor of the service occurs at the main bonding jumper. The bonding of the transformer XO occurs at the SDS bonding jumper. There is no reason why current would want to flow between these two grounded conductors even when their grounding electrode systems are tied together. The current from a hot on the secondary side of the transformer is only trying to get back to the XO terminal of that transformer. It won't try to go back to the service grounded conductor. Likewise, the current from a service hot will not return to the grounded conductor of the SDS.
I don't see anything here that indicates multipoint grounding. Multipoint grounding only occurs when the same grounded conductor is bonded to earth in more than one location. Multiple grounding electrode systems that are bonded together and connected to a grounded conductor at a single point is not multipoint grounding. This is exactly what we have when a SDS is installed per the NEC. The service and SDS grounding electrode systems are tied together but each respective grounded conductor is tied to this common grounding system ant only one point. The fact that the service grounded conductor is bonded at one physical location and the SDS grounded conductor is bonded at a second location will not cause current to flow on the grounding system. The grounding system is not connected in parallel with the grounded conductor.
A common example of multipoint grounding would be when the neutral bonding screw in installed in a subpanel. This does place the grounding and grounded conductors in parallel and causes objectionable current flow on the grounding conductors. This is not what happens when you install a SDS per the NEC.
Don

 

[bennie]

Re: Separately Derived 3phase 3 wire System

Don: Connect a wire from the neutral/ground buss of a 480/277 service, run it 100 feet and stick it in the dirt. Current will flow in proportion to neutral current and impedance.

Now go to the X-O of the power transformer, run a wire 100 feet and stick it in the dirt. Current will flow in proportion to neutral load current and impedance.

There is objectionable current flowing from both sources to ground electrodes.

You can not prevent the X-O current to ground, but with no earth connection at transformer, the total loop impedance is increased.

I agree a system connected by the NEC will operate good. Two ground points is not by the NEC.

There is no document in the NEC library that states a transformer is a separately derived system.

 

[Ed MacLaren]

Re: Separately Derived 3phase 3 wire System

Don,

The sketch below illustrates the way we would do the grounding/bonding of a typical installation here.(Canada.)

(Please excuse the color coding. We use green for all insulated system grounding, equipment grounding, and bonding conductors.)

Could you indicate if the grounding/bonding would be NEC compliant, and if not, what changes to make in the drawing?


Bennie,

Perhaps you could indicate where objectionable current would flow in the grounding/bonding conductors.

Ed

[ March 19, 2003, 09:46 AM: Message edited by: Ed MacLaren ]

 

[bennie]

Re: Separately Derived 3phase 3 wire System

Ed, well done. The only difference I can see in your schematic and the percieved method in the US, is the green wire from the power transformer back to the service ground point.

Disconnect this wire and stick it in the dirt, near the transformer. This method is wrong and not the intent of the NEC, it is commonly accepted by expert? interpretations.

Now draw a MGN, delta wye transmission system.

On second thought, use the same drawing, the systems are identical.

There is no separately derived system involved. This is a plain old transformer.

[ March 19, 2003, 09:37 AM: Message edited by: bennie ]