NEC handbook

[bennie]

Re: NEC handbook

Charlie: Excellent response.

I have traced the history of the separately derived system back to its origin, in 1914. The definition has never been changed for an AC system.

In the beginning, the term was "isolated system".
This was a premises supplied by a light plant, or generator. Some of the light plants operated at 24 volts, and were not grounded. The 120/240 volt generators were not grounded until 1914.

The term "isolated system" changed to "AC systems without external connections" in the 1930's. This was during the REA era, where rural farms were being electrified. Most of the farms were separately derived systems, powered by a generator, and ungrounded.

The utility companies would not connect to these "separately derived systems" until they were grounded. The section in 250 was created to illustrate the method for grounding the separately derived system.

The term "AC systems without external connections" changed to "Separately derived systems" in 1956.

This change was for editing purposes, by a staff member, for reasons of being more correct technically.

According to the curator of archives at NFPA headquarters.
There has never been any submittals for changing the definition of a separately derived system. The definition has changed by individuals applying their personal spin, and fabricating the purpose of a separately derived system without being aware of the true definition.

This incorrect perception is creating two ground points on a premises wiring system.

Just ask yourself...Why call a generator, or transformer, a separately derived system? Call them what they are.

By grounding a transformer under the instructions of 250.30 for a separately derived system, a ground loop is created. In many cases this is a second ground electrode system on the same premises. This is a hazard, and affects sensitive equipment.

The current flow, in the equipment ground conductor to the ground point of the transformer is common mode. The same net difference will appear on the service conductors.

The grounding method for a "separately derived system" is referring to the premises wiring system, not the source of power.

A generator, properly connected, will not interfere with the operation of GFI protection, as indicated in the handbooks. Disconnecting the utility neutral has nothing to do with GFI protection. Again, a fabricated explanation.

[ July 07, 2003, 11:58 AM: Message edited by: bennie ]

 

[charlie]

Re: NEC handbook

OK, let?s assume you have a standard SDS and it is feeding a panelboard in the middle of a plant. Let?s continue with the sensitive electronic equipment problems, what can we do and still meet Code?

Let?s use the provisions of 250.6(A). OK, that didn?t work, how about lifting the connection from the transformer to the building steel. What do you mean I can?t do that? 250.6(B)(4) says I can if the AHJ approves the modification.

Would it be safe? Let?s see, the case of the transformer would be grounded to the circuit that feeds it via the conduit or grounding conductor so it is safe. What about the panelboard . . . HMMM. Well the panelboard is connected to the feeder grounding conductor that runs from the transformer of this SDS. That means that the panelboard is at the same potential as the grounded conductor at the transformer . . . HMMM . . . that should be safe. Well how about the conduit system and the equipment it is connected to, is that safe? I think the answer is yes to that question. OK, how about the difference in potential between the equipment and the existing equipment that is served from the main service? Where is the return path to make a complete circuit . . . it does not exist since the transformer nor the panelboard have been ?grounded? to the original grounding electrode system.

Since the conduit system will be in contact with the building steel and other raceways, are you prepared to use a non-metallic system or keep it and all the equipment totally isolated from the building steel, other raceways, boxes, equipment, etc.? I think that would be the only way to make a system like this work. In other words, doing it the way the Code lays it out is the only way to be practical about the situation.

So, yes, you do set up two points on the grounding system for grounding the main service and then for the SDS. They are then tied together to make a single grounding electrode system. I think you still have to use the provisions of 250.6(A) and make enough modification to make the system work. Don?t loose sight of the fact that current prefers to return with the circuit conductors. That is not to say that it will not split at nodes to go back to the source in a multitude of ways.

 

[bennie]

Re: NEC handbook

Charlie: Why does 250.30 read...Grounding Separately Derived Alternating Current Systems?

And not..."Grounding transformers and generators". Transformers and generators are the only alternating current systems.

The reason is...Transformers and generators are not separately derived systems.

The premises wiring is the separately derived system. There is only one premises in question, so there can not be multiple separately derived systems.

 

[ryan_618]

Re: NEC handbook

Originally posted by pierre:
... To make a long story short, with guidance by many, I finally found the answer in the ROP (Report on Proposals). Does anyone else refer to these publications?

Pierre

Yes I use the ROP's quite a lot and have found them very benificial.

[ July 07, 2003, 06:02 PM: Message edited by: ryan_618 ]

 

[bennie]

Re: NEC handbook

Charlie: You are well versed in the MGN distribution system.
Would you call a delta/wye distribution transformer, a separately derived system?