Separately Derived System

[Ed MacLaren]

I?ve noticed that the long-running controversy regarding the definition of an SDS has surfaced again in another thread. Statements were made about not ?confusing the new people in the trade?.
As an educator, this is also one of my concerns, and I offer the following in the interest of trying to resolve the controversy.

The term "separately derived system" is just a name used to identify a premise wiring system supplied from a source other than the main service transformer. When the use of step-down transformers and stand-by generators became common, a name was needed to identify the wiring systems that they supplied.

As some others have stated, the wording of the definition may not be perfect, but I think the intent is clear. The key is the interpretation of the words "electrical connection".

Referring to the sketch below, the 480/277 volt service (System 2) ends at the dry-type transformer. The 208/120 volt system is separately derived (System 3) and both are required to be grounded.

They are completely separate systems in terms of any current flow from one to the other, because they are connected to each other at a single point, the ground bus (A) in the dry-type transformer. There is a "connection" but not one over which current can flow.

Any load and/or fault current originating from either transformer is confined to that system, and can only flow back to it?s point of origin. Live-to-enclosure faults (aka ground faults) in both systems are illustrated.

Other than the problem of clarifying the definition of an SDS, there is no technical or safety issue.

Comments are welcome.
Ed

[ January 08, 2004, 10:13 AM: Message edited by: Ed MacLaren ]

 

[bennie]

Re: Separately Derived System

The schematic shows the MGN (multi-ground neutral) is electrically complete from the utility distribution system to the end user system. This excludes this premises from being a separately derived system.

Cut the interconnecting MGN and it will be a separate system but will be a violation of 250.24.B Grounded Conductor Brought to Service Equipment.

The only transformer source to a separately derived system is a premises wiring system (meaning entire building wiring, excluding the service equipment) that has no electrical connection to an exterior source (utility). Magnetic coupling is excluded as a direct electrical connection.

As I have stated before; A transformer that is the source for a separately derived system is an ungrounded secondary transmission line supplying a delta primary of a distribution transformer. The secondary of the distribution transformer supplies the premises wiring system and the system must be grounded as spelled out in 250.30.
This is due to the absence of the MGN.

Remember, the premises wiring system is the separately derived system, not the source of power.

 

[engy]

Re: Separately Derived System

Bennie, explain where any one neutral is grounded more than once.

This is not MGN

 

[don_resqcapt19]

Re: Separately Derived System

Bennie,

The only transformer source to a separately derived system is a premises wiring system (meaning entire building wiring, excluding the service equipment) that has no electrical connection to an exterior source (utility). Magnetic coupling is excluded as a direct electrical connection.

That is your definition, not the NEC's definition. The term is defined in the NEC and that is the only definition that can be used when applying the NEC. The code making panel has made a statement as to the meaning of the term SDS, that statement is the offical position of the NFPA and the only correct definition of the term as used in the NEC.
This no different from laws or contracts where the terms used within that document are defined in the document itself. When the term is defined in the document, that is the only definition that can be used for the interpretation of that document.
Don

 

[bennie]

Re: Separately Derived System

Before domestic electricity was installed to rural houses, many of the houses had light plants (generators). These premises systems were known as "Alternating Current Systems with no Exterior Connections". Section 2523 prior to 1956 Edition.
After 1956 the section was changed to "Isolated Systems". Then to "Separately derived systems"

None of the power sources mentioned in the definition are separately derived systems, they may, or may not be sources for separately derived systems.

Grounding was not necessary due to no lightning problem, and no high voltage lines.

When the REA was started in the 1930s, power lines were run to the rural property, and connected to the existing ungrounded premises wiring. Section 250.30 is illustrating how the grounding is to be performed. Once the premises is grounded it is no longer a separately derived system.

When the light plant (generator) is operated during a power failure, the premises should revert to a separately derived system and break contact with all the utility lines included the grounded neutral. During a storm the neutral often is crossed by a line conductor and smoked the farm house and its occupants.

I lived this part of history, this is why I am obssesed with history not being corrupted by uninformed individuals.

 

[engy]

Re: Separately Derived System

"When the REA was started in the 1930s, power lines were run to the rural property, and connected to the existing ungrounded premises wiring."
"During a storm the neutral often is crossed by a line conductor and smoked the farm house and its occupants."

Today we ground at the pole and at the service.
Do you feel we still have the same problem?

 

[bennie]

Re: Separately Derived System

engy: I count two on Eds drawing, there will be four grounds to the mile on the transmission line. Does that meet the definition of "multi"

Don: The code text is ok, it's the interpretation that confuses the issue.

Ask yourself, why disconnect the neutral on a generator and not on a transformer and still call them separate systems.

The neutrals, of generators, were switched long before GFCI's came along. That excuse is fabricated by the unknowing and uneducated.

A separately derived system is a "Premises Wiring System". Not a generator, transformer, or the other listed power sources.

A transformer with the X0 solidly connected to the X0 of the circuit conductor (X0) of the supply conductors originating in another system, is an MGN distribution system.

As I have challenged; prove to me and the world, that a conductor between two star points of transformers is not a circuit conductor and will not or can not carry current.

200.3 Electrically connected shall mean capable of carrying current, as distinguished from connection through magnetic induction.

Any conductor capable of carrying current is a circuit conductor.

 

[don_resqcapt19]

Re: Separately Derived System

Bennie,

Ask yourself, why disconnect the neutral on a generator and not on a transformer and still call them separate systems.

With the generator there is a direct physical connection of the grounded conductors, with the transformer there is not. The connection is via the grounding and bonding conductors. Again, I say that this is the NFPA's code and the only correct interpretation is their's.
Don

 

[engy]

Re: Separately Derived System

As far as MGN, you got me Bennie.
Yes it is MGN from utility up to the service.

I was thinking of only the Dry-Type, and in my haste didn't look close enough to the drawing before posting.

 

[bennie]

Re: Separately Derived System

Of course I agree it is the accepted definition by the members of the code panel, it is not the definition in the code book.

Ask why a transformer should be called anything but a transformer. The only country in the world that calls them a separately derived system is the US.

Ask why a generator is called a separately derived system when the utility neutral is disconnected. No country in the world does this.

This entire issue is a result of someone trying to make something of nothing.

 

[bennie]

Re: Separately Derived System

Thanks engy: A MGN system is definitely not a separately derived system. A premises transformer is definitely a MGN system.
I sure hope the NFPA takes up my challenge. I need an extra 10 grand.

 

[bennie]

Re: Separately Derived System

For the code panel to make such a stupid statement that when a neutral changes to an equipment ground conductor no current will flow, is grounds for a recall of the entire panel.

There is still McJob applications.

 

[don_resqcapt19]

Re: Separately Derived System

Bennie,
What conditions will cause current flow between XO of the 480/277 volt transfomer and XO of the 120/208 volt transformer shown in Ed's drawing? Why would current from an ungrounded conductor on the 208 volt system flow to the grounded conductor of the 480 volt system?
Don

 

[iwire]

Re: Separately Derived System

Bennie you will not win this one.

You are trying to bet against the house.

It is their code, their definition, how can you win?

I agree with you that it is a connection but I can live with their definition.

It works, it is not confusing to me, or the people I work with.

What happend in the 30, 40 & 50s makes little impact on those coming in the trade now, they do not know or care about the REA, "Alternating Current Systems with no Exterior Connections" or when the section was changed to "Isolated Systems".

When I talk to the electricians I work with and say wire that transformer as an SDS they know exactly what I mean.

Other than the fact this is a hair across your rear, what danger does it present?

Thanks Ed for starting this thread with a great sketch.

 

[Ed MacLaren]

Re: Separately Derived System

Bennie,
For current to flow, there must be a circuit connecting two points that have a difference of potential (voltage) between them.

A single connection at the ground bus (A) connects two separate systems, that have no difference of potential between them.

If you would, please point out the source, and destination, of any current that would flow through this connection, and the voltage that would cause it to flow.

Ed

 

[bennie]

Re: Separately Derived System

I'm sorry, I always thought a neutral/ground or MGN had a dual purpose, load current and a path for fault current.

Does fault current only flow on the equipment ground section? How does it get out? Are all of the equipment ground wires from one star point to another, full of trapped electrons?

If I paint this conductor white will current flow? The two star points will have a small difference in potential and will have current flow, therefore the equipment ground conductor is a circuit conductor.

A conductor can qualify as a circuit conductor with out being connected to a light bulb.

Draw a MGN system and a so called separately derived system and prove to me there is a difference.

Then I will make written apologizes to everyone I insulted.

 

[wyedelta]

Re: Separately Derived System

Hi Folks: Those people (panel who made the code)are expert in electrical (in various field) they consolidate their ideas and that is why have the code.Outside the code everything is illegal.The code is our basic law in this field of engineering.

As for the grounding conductors :All grounding conductors are subject for current carrying.It might be neutral or Ground conductor it must carry of a least 80% of the main conductor.Even equiptment grounding conductor are subjected for current capacity for it will take the fault current generated by the fault circuit.

ED : as an educator how would you convince your student about the subject matter. Student rely on instructor/professor.

 

[bennie]

Re: Separately Derived System

wyedelta: Check the makeup of the code panel members. Not many expert electricians. Mostly special interest officials.

 

[don_resqcapt19]

Re: Separately Derived System

Bennie,
Why would the fault current from the 208 volt system flow to XO on the 480 volt system?
Don

 

[bennie]

Re: Separately Derived System

An LG fault on the transmission line will appear on the premises neutral, unless the premises is a separately derived system.

An LG fault on the transmission line will smoke a generator that has a common neutral.

Yes, current will flow throughout a MGN system.