SDS Grounding

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don_resqcapt19

Moderator
Staff member
Location
Illinois
Re: SDS Grounding

Scott,
I see nothing in the NEC that says a SDS must be in the same building or structure as the primary. In my opinion, transformers on a campus or industrial distribution system that are not utility owned are SDS.
Any transformer that is on the load side of the service point is covered by the NEC and is a SDS.
Don
 

dereckbc

Moderator
Staff member
Location
Plano, TX
Re: SDS Grounding

Don, I agree with you 100%. I have designed several transformers that feed adjacent buildings and they are configured SDS. Some have the neutral bonded to ground electrodes twice. Once at the transformer Xo and again at the disconnecting device in adjacent building where no common ground fault path existed as permitted in 250.30(A)(1) exception #1.
 

lbudden

Member
Location
Colorado
Re: SDS Grounding

On a SDS, would you connect an IG to XO or to the frame of the transformer? I assume you would have 2 grounds in with the derived conductors---an EGC/bondng jumper to the secondary panel ground bar, and an EGC/IG that goes to an IG bar.
In Soares book of grounding under SDS, it shows a diagram of the secondary EGC or bonding jumper connected to the transformer frame. In the 2002 NEC handbook,it shows the secondary EGC or bonding jumper connected to XO.
Which of these are correct and do you put an Isolated Ground for the secondary panel on the same point as the EGC/bonding jumper? (XO or frame???)
 

dereckbc

Moderator
Staff member
Location
Plano, TX
Re: SDS Grounding

Ibudden, The IG can originate in either the transformer or the disconnect device. It depends on where the GEC is terminated.

If the GEC is brought to the transformer there are two ground cables that leave the transformer, the bonding jumper and IG. If GEC is terminated to the neutral buss at the disconnect device, then the IG originates there via bonding jumper to EGC bus.
 

scott

Member
Location
Colorado
Re: SDS Grounding

Fair opinion on the first part Don. I stated from the beginning that was my personal belief, and it was not directly backed by the NEC.

As for as the second part, why does the NEC not refer to an "outdoor transformer" as an SDS, if it is one? [see 250.24(A)(2)]

Why is there a specifice code section referring to SDS's (and are called such), that imply use inside of a building where existing electrical is present?

I fully agree that any transformer on the load side of the service point is covered by the NEC. And did not intend to imply otherwise, regardless of what it is called.

Enjoy your day!

[ March 04, 2003, 11:30 PM: Message edited by: scott ]
 

Nick

Senior Member
Re: SDS Grounding

The NEC does not specify that an isolated equipment grounding conductor has to terminate at the same location as the GEC. It only has to land at an equipment grounding terminal of the applicable separately derived system or service. 250.146(D)
It can terminate at a down stream panelboard or as far back as the bonding jumper for the system. It?s purely a design issue.
 

bennie

Esteemed Member
Re: SDS Grounding

I have been absent, from this forum, for some time. I am recovering from a near fatal event with lung disease. I went flat line two times in the ER. Getting old really sucks. :)

The correct definition, and purpose, of a "separately Derived System" has been of great interest to me.
While recouperating, I had numerous conversations and correspondance with the curator of the NFPA library.

Instead of starting a debate, I suggest anyone who has an interest in the correct technical definition and application of separately derived systems, to perform research on the past history and evolution of the term. Find out it's reason, and purpose, for being created.

Alternating Systems Without External Connections, was the orignal term. This changed to Separately Derived Systems in 1956.

Believe me, the description in the Soares book is incorrect, when applying to a transformer.

Regards: Bennie
 

bennie

Esteemed Member
Re: SDS Grounding

Thanks Roger and David.

The paddles, used to keep dirt from being thrown on me, also jogged my memory bank. :)

I have been hammering and getting hammered about my understanding of a separately derived system, for years.

I knew there was something being missed. I was in the trade for six years when the term "Separately Derived System" first appeared.

I now remember the critical point that is being mis-understood.

The definition of a Separately Derived System is very simple.
It is;
A "Premises Wiring System", with no solid ground conductor to the utility MGN system.

No transformer, generator, or any of the other power sources, in the NEC definition, are separately derived systems.
They are power supplies to a separately derived system.

Premises wiring grounded by the utility system, is therefore not a separately derived system.
 

dereckbc

Moderator
Staff member
Location
Plano, TX
Re: SDS Grounding

Nick if you read my post I never stated the NEC requirement the IG to originate from the GEC. It is my design preference to originate the IG from the N-G bond point to form a single point ground with the power source.

Bennie, great to hear from you again. Hope you get well soon.
 

don_resqcapt19

Moderator
Staff member
Location
Illinois
Re: SDS Grounding

Bennie,
Good to see you back.
We will have the official NEC postition on SDS in July when the ROP is published. I have sumitted a proposal that will force the panel to issue a statement on this issue.
Don
 

don_resqcapt19

Moderator
Staff member
Location
Illinois
Re: SDS Grounding

Scott,
The transformer in 250.24(A)(2) must be on the line side of the service as it "supplies the service". It appears that this transformer is not within the scope of the NEC and I don't know why the rule is in the code.
Don
 

bennie

Esteemed Member
Re: SDS Grounding

Don, you are doing great. I have been slowly reading all the past topics and posts.

I think we are getting a SDS definition nailed down.

A transformer, on the supply (line) side of a service must have an ungrounded primary secondary distribution source, to supply a separately derived system.
(Source; 1940 Edition of Arthur Abbott Handbook).
 

bennie

Esteemed Member
Re: SDS Grounding

I also discovered that there has never been any panel action concerning the changes in this subject from the beginning, when it was Section 2514 in code books prior to 1956. The section then changed to 250.26

All changes were by technical staff members. The changes and wording is correct in the NEC, the problem is the interpretation by expert authors, who have not done the home work.
 

scott

Member
Location
Colorado
Re: SDS Grounding

I see the point of contention, but I think that the transformer as described in 250.24(A)(2) is covered by the NEC (as long as it is on the load side of the service point). Why else would it be described in the NEC, and have a code covering it?

I am glad to hear that a proposal has been submitted, this may clarify things.

BTW - the application of a transformer as above happens more than occasionally around here, but not regularly. My utility calls it 'primary metered' and utility voltage is usually 13.2kv with customer owned and supplied outdoor transformers located around the property. This only occurs in large industrial or commercial lots that have on ground maintenance facilities.

Glad to see you back Benny. Your knowledge and experience are welcome (as if this is my site to welcome you back ;) ).

Enjoy your day!
 

bennie

Esteemed Member
Re: SDS Grounding

I traced the history of... Alternating-current Systems without Exterior Connections...Back to the 1920 era. This was through the Archives at the NFPA Library.

This system was later changed to Separately Derived Systems, to include the additional power sources mentioned in the definition. This change was by technical personnel, not by the code panel.

The original intent, of the code requirement, was to point out the premises wiring system required special grounding due to the MGN not being installed to the premises service.

A premises wiring system, with no solid electrical connection to the supply is a separately derived system.

The provide this feature, the primary distribution must be ungrounded.

This transformer can be by the utility or owner, it makes no difference.
 

chris white

Member
Location
Pennsylvania
Re: SDS Grounding

Bennie, I also want to welcome you back. I believe your absence has been very conspicuous. Although I'm not nearly as knowledgeable or experienced as many here, I take the electrical trade very seriously and try to read and understand as much as possible. Your posts have always been very thought provoking. I'm interested in the origin of electrical terms, concepts, code rules, etc., and it sounds like you've seen a lot of things develop that we take for granted now. Thanx for all your input.
 

bennie

Esteemed Member
Re: SDS Grounding

Thanks a lot Chris. I am glad to be back. I am glad to be anywhere :)

I am ill with chronic lung disease. I am surviving on large doses of inhaled cortizone steriods.

I don't want my years of experience to die with me, I am attempting to pass on as much as possible.

Right or wrong, I stir up a lot of thinking.

Thanks again: Bennie
 
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