Definition of Separately Derived System

[eeee]

A colleague of mine has been referencing NEC 250.30. My colleague has stated that what I seem to understand is the exteranal pad mounted transformer and an external switch is not considered to be a separately derived system, but a MDP is seperately derived.

We have a configuration where utility power is connected to the external pad mounted transformer. The external pad mounted transformer is connected to an external switch. The external switch is connected to a existing switchboard on one end (a secondary run) and a new MDP (a secondary run) we are installing on the other end. The primary of the external switch is of course connected to the external pad mounted transformer.

My colleague has told me that the connection from the external pad mounted transformer to the external switch is not separately derived, so we can bond on the load side from the equipment ground and nuetral to the water pipes as well as ground to the grounding electrodes. But my colleague has said the MDP is separately derived so the nuetral can not be grounded on the load side per NEC 250.30(A), only my equipment ground is bonded to the water pipes and grounded to the grounding electrodes. The designer did the work this way. My nuetral is left floating on the load side of the new MDP we are installing.

 

[don_resqcapt19]

Re: Definition of Separately Derived System

Who owns the transformer?
Don

 

[charlie b]

Re: Definition of Separately Derived System

I think you have a few basic misconceptions to overcome. Start with this: The "derived" word within the phrase "Separately Derived System" (SDS) refers to the establishment of a new source of power. Here are a few examples:
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• <font size="2" face="Verdana, Helvetica, sans-serif">The utility's generating station is the beginning of one power system.</font>

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• <font size="2" face="Verdana, Helvetica, sans-serif">The utility supplies power to a transformer that serves your facility, and that transformer is the beginning of a new and separate power system.</font>

<font size="2" face="Verdana, Helvetica, sans-serif"></font>

• <font size="2" face="Verdana, Helvetica, sans-serif">Within one of your buildings, you install a step-down transformer to go from 480 volts to 120/208 volts, and that transformer is the beginning of yet another new and separate power system.</font>

<font size="2" face="Verdana, Helvetica, sans-serif"></font>

• <font size="2" face="Verdana, Helvetica, sans-serif">In order to provide a backup to the utility's power supply, you install a diesel driven electrical generator of your own.</font>

<font size="2" face="Verdana, Helvetica, sans-serif">The question of whether installation of a transformer creates a new SDS hinges on the manner of connecting the primary and secondary neutral conductors. The question of whether installation of an emergency or backup generator creates a new SDS hinges on the whether the automatic transfer switch includes the switching of the neutral (4-pole ATS) or only switches the phases (3-pole ATS).

I am willing to wager a small confectionary that your utility's transformer does establish a new SDS. From the secondary of that transformer, there has to be a main disconnecting means. It sounds like that would be your "external switch." At that point, and at no other point, the Neutral Bus and the Ground Bus are connected to each other. In addition, at that point the Ground Bus is connected to planet Earth.

A Main Distribution Panel (I presume that is what you mean by "MDP") does not create a new power source. It merely takes power from an upstream source and passes it along to downstream loads. An MDP is never, never an SDS.

 

[charlie b]

Re: Definition of Separately Derived System

Let me correct one aspect, based on Don's question (somehow he got his post in while I was typing).

At your "external switch," and at no other point further downstream, the Neutral Bus and the Ground Bus are connected to each other.

I added the "further downstream," because if the utility owns the transformer they will have their own rules concerning N-G bonding.

At your new MDP, the neutral should "float," using your term. By that I mean that it should not be connected to any ground rod or to any equipment grounding conductor.

 

[eeee]

Re: Definition of Separately Derived System

Thanks,

By the way, the transformer is ours. This relats to the previous response.

 

[ronaldrc]

Re: Definition of Separately Derived System

Sounds to me like another Buss exstension miss understanding? How about it Roger.

He said it was all bonded together on the load side which would be the secondary.I bet the bonding jumper between the ground and neutral is either at the transformer or main disc. where ever that is? And the MDP is being treated as a sub with buss exstensions.

Roger might explain.

 

[eeee]

Re: Definition of Separately Derived System

I need to clear some things up and note that I don't know what is meant by buss extensions for the previous comment, but thanks.

The electricity comes from the utility substation to our substation. It then goes to our pad mounted transformer. The it goes to our external switch. Then it goes to our existing switchboard on one path from the external switch and two our newly installed MDP on the other path from the external switch.

As I look at the designers' drawings, I note that I can't be for sure where the nuetral is bonded to the equipment ground (which in turn is bonded to the underground water pipes and grounded to the grounding electrodes) since it is off to the side on the drawing. I expect the nuetral is bonded to the equipment ground at our pad mounted transformer and probably is left floating at our external switch as shown on the first sectional cutout grounding drawing on the one-line. I know the nuetral is left floating at the newly installed MDP since the 2nd sectional cutout grounding drawing off to the left shows it as such for this part of the drawing. It is probably also left floating at the existing switchboard, which by the way has a transfer switch off to the side of it to connect with a diesel genset (which probably has the nuetral tied to the same ground the transformer's (a transformer that we own) nuetral is tied to.)

Note that the designers' drawing has two grounding cutouts off to the left of the drawing. One is next to our external switch and our pad mounted transformer. The other is off to the left of our newly installed MDP.

 

[charlie b]

Re: Definition of Separately Derived System

Originally posted by eeee:I expect the neutral is bonded to the equipment ground at our pad mounted transformer. . .

I think that is where the N-G bond should be.

Originally posted by eeee:(. . . the neutral) probably is left floating at our external switch (and) at the newly installed MDP . . . .

I think that is how it should be, although I suggest you stop calling it "floating." Neutrals don't float. They are grounded (but only at one point). In fact, the precise technical term for a "neutral" is "the grounded conductor."

Why do you suspect that there is a problem. Have you discussed this with "the designer"?

 

[eeee]

Re: Definition of Separately Derived System

I do not suspect there is a problem now that the designer has removed the nuetral from the ground rods, water pipes and equipment ground at the MDP, but left the nuetral bonded to the water pipes, equipment ground and ground rods at the transformer it appears. The external switch is now presumed to not have the nuetral connected to the equipment ground as you indicated in the last response.

I am having a discussion with another and separate colleague of mine in relation to getting our arms around the definition of a separately derived system as it relates to your comment that once the equipment ground is tied to the nuetral at the pad mounted transformer, the part of the circuit pointing towards the newly installed MDP is now considered a separately derived system.

We originally were thinking a separately derived system could only be a utility company, a diesel genset (with 4 pole switched transfer switch), or where our substation transforms utility power to our private distribution system. As an outside possibility we thought that a separately derived system could be defined where our pad mounted transformer converts our 13.2KV from our privately owned substation to 208/120volts (and has the nuetral bonded to equipment ground, water pipes and grounded to the grounding rods).

You indicate (as we interpret) the newly installed MDP is not a separately derived system, but the circuit that points to it is, since the transformer nuetral is grounded to EG, the water pipes and the grounding rods.

 

[charlie b]

Re: Definition of Separately Derived System

I think you are getting the idea. An SDS is most often created by a generator or a transformer, though an uninterruptible power supply can also create an SDS. But no manner of distribution or switching equipment can create an SDS. It has to be something that establishes a new source of power.

 

[ronaldrc]

Re: Definition of Separately Derived System

I was trying to give Roger a hard time,if he gets mad he'll get over it or at least thats what my better half always tells me.

The buss extensions are a joke forget about them.

Charlie explained it very well.

Ronald

 

[roger]

Re: Definition of Separately Derived System

Ronald, why do you think Buss Extensions are a joke?

BTW, unless the configuration of primary and secondary windings are only magnetically coupled, i.e. no bonding jumper to make a physical tie from the secondary Grounded Conductor to the MGN of the primary, the term Separately Derived System is incorrect, but that is a book from the days of past. (Think Ronald )


Roger

 

[ronaldrc]

Re: Definition of Separately Derived System

That grounding conductor is not supposed to be considered when speaking of a separate derived system.Technically they are separate or should I say the definition of separately derived? The only time it has current on it or supposed to is under a fault condition.

Besides the buss extensions we are speaking of are the long conductors that connect a main disconnect to the MDP off at a distance of which no one can agree on how this panel should be treated as a sub with a insulated neutral or a main with the neutral bonded to the can.

Both subjects are a big can of worms.

[ September 21, 2005, 09:12 PM: Message edited by: ronaldrc ]

 

[roger]

Re: Definition of Separately Derived System

Originally posted by ronaldrc:
That grounding conductor is not supposed to be considered when speaking of a separate derived system.Technically they are separate or should I say the definition of separately derived? The only time it has current on it or supposed to is under a fault condition.

Besides the buss extensions we are speaking of are the long conductors that connect a main disconnect to the MDP off at a distance of which no one can agree on how this panel should be treated as a sub with a insulated neutral or a main with the neutral bonded to the can.

Both subjects are a big can of worms.

Yes they are.


Roger

 

[ronaldrc]

Re: Definition of Separately Derived System

No they ain't