Objectionable Current

[benray]

I have been in the electrical industry for 50 years. I am now engaged in performing commission inspections and related tests of new building electrical systems.
I am finding current on all equipment ground conductors to power transformers in the facilities.
This is due to the X-O of the secondary being effectively(earthed) at a point other than at the service ground electrode location.
I have been insisting the X-O ground conductor be routed back to the main service ground point.
The latest edition of the Soares Grounding Book,illustrates the many grounding methods that create two or more ground paths and points.

I would like some of the intellects on this forum to give me their opinion of Chapter 12, 8th Edition, of the Soares Book. This publication is now sponsored by the IAEI.

I feel this book is incorrect and creating many inferior designs of ground systems, with objectionable current flow from interior and exterior systems.

On another issue;

Generator separately derived system in same publication.

I can not see where the utility neutral is being disconnected by the transfer switch, when it is shunted by the ground conductor.

Need all the expert comments I can get. Thanks; Bennie

 

[dpc]

To jghrist,

I agree with your thoughts on wye-wye transformers. The NEC definition of a separately-derived system is based on whether or not the "grounded circuit conductor" (a.k.a the neutral), or any phase conductor, is solidly connected between the two systems. The " equipment grounding conductor" is not part of the definition, which makes sense, at least to me.

This is a common issue for standby generators. Four-pole transfer switches are often used to isolate the generator neutral from the system neutral.

The basic issue in the original question seems to be the distinction between the grounded conductor (neutral) and the grounding conductor (green wire). Although these may appear to be electrically common, their interconnection is strictly regulated by the NEC to ensure the proper functioning of the grounding system and the ground fault protection systems.

 

[benray]

I will try this another way;

Draw a distribution transformer, MGN system, as a supply to a building.

Now draw an end user transformer, connected as required in the NEC for a separately derived system.

What is the difference in electrical connections?

 

[jghrist]

Forget what I said about the ungrd wye - grd wye connection. The transformer primary would be floated and not connected to the primary neutral. An ungrd wye - grd wye transformer is not suitable for serving a 4-wire secondary system because the neutral is not stable. An unbalanced secondary load or a line-to-ground fault will cause the heavily loaded or faulted phase voltage to collapse.

There is an open in the zero-sequence equivalent circuit, with no short on one side like in a delta-wye. Zero-sequence current cannot flow in either the primary or secondary.

 

[dpc]

What does "MGN system" mean?

 

[busbar]

In utility medium-voltage distribution, typically a neutral conductor span is ran physically below distribution transformers—versus above the transformers for phase conductors. Then the neutral span is connected to ground electrodes every nth pole, and is common to primary and secondary windings and metal tanks of the overhead banks. A typical transformer looks something like the right photo on page 1 at

http://www.cooperpower.com/Library/pdf/20110.pdf

 

[benray]

On a MGN distribution system, the primaries and secondaries of all transformers, including the end user equipment, are electrically connected under the definition of NEC section 200.3.

The only transformer configuration that meets the definition of a separately derived system, is when the supply secondary is from an ungrounded delta or a floating wye.

 

[benray]

I was active in the industry when the term "separately derived system" was coined.

The term "separately derived system" never has appeared in the transformer article of the NEC.

"Separately derived system" is a procedure for connecting to ground, the equipment listed in the definition. The purpose is make it known the systems are not connected to ground at other supply systems.

What is the only common technical concept that all the items listed in the definition have in common?

The earth connection method is the only thing all the items have in common.

 

[dpc]

What does "MGN" stand for?

 

[dpc]

benray,

I'm not sure there is really an answer that is going to satisfy you, and it seems we've beaten this to death.

Let me just throw a little more gasoline on the fire and say that determination of a separately derived system has to do with the electrical isolation between the (normally)current-carrying conductors in the system. The equipment grounding conductors are not intended to be current-carrying and do not enter into the determination in any way. Article 110 defines separately-derived systems and, as you say, Article 250 deals with grounding and bonding of these systems.

If you disagree with the NEC requirements, you could propose a modification to the Article 250 committee.

But the requirements for grounding of separately-derived systems, especially transformers, haven't changed much that I can recall in the thirty plus years I have been doing electrical design work. Maybe I just have a bad memory - that's what my wife keeps telling me.

Good luck.

 

[benray]

Sorry dpc, MGN is Multi Ground Neutral. This is the most popular distribution system in use.
Through fault containment is provided at each transformer, by a shorting effect. Single bushing pots can be used. The neutral can serve as the third phase on open wye and delta banks.

This conductor extends to the end user service, and each branch circuit. This system is for both load and ground fault current.

 

[busbar]

Cows are often particularly skittish around common-neutral distribution systems. Half a volt in the wrong places can really get 'em p*ssed.

 

[benray]

A separately derived system will prevent the dairy cattle issue.

The normal load carrying conductor being electrically connected to other systems is not the defining issue, this is not even in the consideration.
The common connection of the ground fault conductor is the reason for a separately derived system.

The term "separately derived system" appears only in the grounding article of the NEC.

I am sure there is no documented standard declaring an equipment ground conductor as not being a circuit conductor.
NEC section 200.3 indicates otherwise.

 

[jghrist]

benray, back to your hypothetical service from utility MGN system and user's separately derived system. I'll assume a 4-wire 277/480 volt service and a 480-120/208 volt delta-wye user transformer.

The utility will provide four wires in the service. The neutral will serve as both neutral and grounding conductor, grounded at both ends to grounding electrodes. The user will have four or five wires in his distribution system: 3 phases, a grounded conductor (neutral) if he has any 277 volt loads, and an equipment grounding conductor. The 120/208 volt secondary of the user's transformer is a separately derived system. There will be five wires in the secondary: 3 phases, a grounded conductor (neutral), and an equipment grounding conductor.

The neutral, if any, of the 277/480 volt system will be grounded only at the service, not at the 480-120/208 volt transformer. The neutral of the 120/208 volt secondary will be grounded only at the transformer (or before the first disconnect). The two neutrals will not be connected together. The grounding conductor of the 480 volt system will be connected to the transformer case. The grounding conductor of the 120/208 volt system will be connected to the transformer case and to the nearest grounding electrode.

The 480 volt equipment grounding conductor will be connected to the 120/208 volt grounding conductor by virtue of the common connection to the transformer case.

Request:

If you contend that an equipment grounding conductor is a circuit conductor, give an example of a separately derived system. The equipment grounding conductors are always connected together.

 

[benray]

Draw the schematic for a user transformer. There is a solid connected conductor from the service 480/277 volt neutral to the neutral of the 120/208 secondary X-O. This electrical connection is complete to the utility MV MGN.

Please explain; where is the separation?

 

[busbar]

There is no separation, and it’s not obvious that it cannot be reasonably avoided with current practices and requirements. In a 480∆-208Y/120V unit, what is "separate" is not necessarily simultaneous current flow related to two the different winding sets—the hi-side equipment-grounding conductor(s) versus lo-side bonding jumper/grounding-electrode conductor/secondary-side equipment-grounding conductor.

Back to the original matter—is any of that objectionable current? I think if a person pokes around with an ammeter/voltmeter on any 60Hz transformer and related wiring they're bound to find ‘stray’ currents in various grounding/grounded conductors…but are those really objectionable? Do they cause singificant, practical problems? In a single residence served by a single overhead/underground transformer, there will be some current in the pole/pad ground rod/ring and the ground rod/metallic plumbing in the building—with some of it ultimately “in parallel.” The question is—except for cows, does that cause problems? An important aspect of installations as simple as described is, what is the open-circuit voltage if one ground-electrode conductor is opened? I think in most cases it would be roughly similar to ground-to-neutral voltage on a remote circuit—it’s there, but within limits, don’t we have to live with it?

 

[jghrist]

So because the 120/208 neutral is connected through grounding conductors back to the 480 service, you contend that the system is not separately derived? You may have found an inconsistency in the NEC definitions, but for the purposes intended, it is separately derived. Unbalanced load currents from the 120/208 system will not flow in any significant amount back to the 480 volt service through the grounding conductor because they have to go through the 120/208 neutral first to the transformer, which is the source of the current.

According to your strict interpretation of the definition of separately derived systems, they are non-existent.

 

[sgc]

Greetings,

I have a Topaz Line noise Ultra Isolator 15KVA 208Y/120 in, 208Y/120 output supplying our computer center. Some of the UPS's and surge protectors are indicating a ground fault condition. We are reading a voltage of 16 volts across the neutral and the equipment ground. This 16 volts are from some phase imballance and some electronic injection I am sure. If we install a jumper wire between the neutral and equipment grounding bus the fault condition lights go out and the voltage drops to .0 levels.
My question, is it acceptable to bond the equipment grounding bus and the neutral bus together in the breaker panel that is served from the isolation transformer?
This gets back to the question of connecting the equipment ground and the neutral buss bars together at other than the service entrance panel.
I am of the opinion that the isolation transformer is a separately derived system and is permissable to bond the equipment ground and the neutral busses together being as how it is the first panel to be served from the "new system". Am I interpreting this right and is this the intent of the NEC?

Thanks,
SGC

 

[peebee]

You are REQUIRED to bond the neutral to ground.

This does NOT get "back to the question of the . . . service entrance panel". This has nothing to do with the service entrance panel. This has everything to do with being a separately derived system, and all separately derived systems are required to be bonded to ground in accordance with NEC 250.20 & 250.30.

Your existing situation, unbonded, is a violation of code, is dangerous, and can lead to erratic operation or fried equipment.

 

[pennsy]

Benray said:
"The latest edition of the Soares Grounding Book,illustrates the many grounding methods that create two or more ground paths and points.

I feel this book is incorrect and creating many inferior designs of ground systems, with objectionable current flow from interior and exterior systems."

This is not correct, Soares is trusted in the trade,by inspectors, Engineers, and Electricians and is merely a descriptive commentary on article 250 of the National Electrode Code. As has been recently seen here, in posts, related to Grounding, it is obvious that grounding is not a completely understood subject, Soares Book on grounding is a bridge between the NEC and that fuzzy area of misunderstanding.
Everything found in the Book is taken from the NEC and explained and expanded on and repeated for redundancy , they do not invent new terms nor do they promote any method of grounding/bonding that is not approved by the NEC.

WmColt