Seperately derived system

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mikehughes8

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My limited understanding of what constitutes a seperately derived systems comes from the installation of a manual transfer switch. If it's a 4-pole transfer switch (nuetral is switched) that constitutes a seperately derived system. If it's a 3-pole transfer switch and the nuetrals are not switched, then it's not a seperately derived system. Is this the only requirement to be a considered a seperately derived system (no connection with the utility)?

thanks in adavance for the help
 
Well there is a bit more, you have to consider the proper connection of the XO/Neutral/grounded conductor. With the 4-pole ATS you must connect the XO/Neutral/grounded conductor to ground at the Main Service and at the Generator. With the 3-pole ATS (assuming a 3-phase 4-wire distribution) the XO/Neutral/grounded conductor is only connected to Ground at the Main Service the (usually installed) factory bonding jumper must be removed at the 3-pole ATS.
 
brian john said:
Well there is a bit more, you have to consider the proper connection of the XO/Neutral/grounded conductor. With the 4-pole ATS you must connect the XO/Neutral/grounded conductor to ground at the Main Service and at the Generator. With the 3-pole ATS (assuming a 3-phase 4-wire distribution) the XO/Neutral/grounded conductor is only connected to Ground at the Main Service the (usually installed) factory bonding jumper must be removed at the 3-pole ATS.
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Yeah, but which came first, the chicken or the egg :grin:

The issues you present have more to do with grounding and bonding than they do in determining whether the system is a separately derived one. You have to first determine whether it is a separately derived system before determining which grounding and bonding requirements must be met... not the other way around.
 
Smart $ said:
You have to first determine whether it is a separately derived system before determining which grounding and bonding requirements must be met... not the other way around.
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We get to decide if it will be SDS or not.

As we are talking about generators in this thread IMO the first step is deciding which system (SDS or Non-SDS) is the better choice for the installation at hand.

In general it costs less to install a generator as a non-SDS.

Going the non-SDS route eliminates a pole on the transfer switch, the grounding electrodes and an additional bonding jumper.

However if the generator is being connected to an electrical system that has GFP the choice will have to be SDS with it's switched neutral, electrodes and bonding jumper at the gen-set.
 
iwire said:
We get to decide if it will be SDS or not.
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If we are provided with a 3? genset and 3P ATS to connect into a 3? wye service, I don't see how we have the choice.

iwire said:
As we are talking about generators in this thread IMO the first step is deciding which system (SDS or Non-SDS) is the better choice for the installation at hand.

In general it costs less to install a generator as a non-SDS.

Going the non-SDS route eliminates a pole on the transfer switch, the grounding electrodes and an additional bonding jumper.

However if the generator is being connected to an electrical system that has GFP the choice will have to be SDS with it's switched neutral, electrodes and bonding jumper at the gen-set.
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Now you're getting into design considerations. The OP simply asked if "no connection to the utility" was the only requirement to be a considered a seperately derived system? The answer is yes. Grounding and bonding are a consequence of that condition... not a prerequisite
 
Smart $ said:
If we are provided with a 3? genset and 3P ATS to connect into a 3? wye service, I don't see how we have the choice.
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No kidding?

And if its outside it has to be RT.

If someone else is specifying the job material than no I don't decide they decided. That is obvious, the key is the NEC does not make the choice for us.

Now you're getting into design considerations.
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Of course because that is what it is.

The NEC does not mandate it either way.
 
iwire said:
We get to decide if it will be SDS or not.

As we are talking about generators in this thread IMO the first step is deciding which system (SDS or Non-SDS) is the better choice for the installation at hand.

In general it costs less to install a generator as a non-SDS.

Going the non-SDS route eliminates a pole on the transfer switch, the grounding electrodes and an additional bonding jumper.

However if the generator is being connected to an electrical system that has GFP the choice will have to be SDS with it's switched neutral, electrodes and bonding jumper at the gen-set.
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Bob not to over complicate the issue but the last sentence....
 
I think that the chicken and the egg might be confused here.

A "Separately Derived System" is a premises wiring system that has no direct electrical connection, including a solidly connected grounded circuit conductor, to supply conductors originating in another system. (summarized from the definition in article 100).

A separately derived system has to derive its power from some source; this source could be the secondary of an isolating transformer, the output coils of a generator, a battery with an inverter, etc. The point is that there is no 'galvanic connection' between the power source and anything else. There is no need for the utility to be present at all. You could have two generators and set things up such that each is a separately derived source relative to the other.

The wiring between multiple sources determines if they power separately derived systems or are both part of the same system.

If you have two sources with neutrals tied together, then you do not have separately derived systems. You simply have _one_ system that happens to have two power sources.

If you have two sources, but their neutrals are not in any way tied together, then you have two separate systems.

The question about SDS and transfer switches may be understood from the general requirement that each solidly grounded system have one, and only one, bond between neutral and ground. Once you have combined two sources with a transfer switch, you have created a single unified system. This system must have only one neutral-ground bond.

If you have ground-neutral bonds at both sources feeding the transfer switch, then you much switch the grounded conductor, so that only one ground-neutral bond is present in the system at any one time. If you have a transfer switch that doesn't transfer the neutral, then one of the sources must not have a neutral-ground bond, and you must depend upon the neutral-ground bond at the other source.

As Smart$ points out, it is often a design decision; you can generally tie or lift the ground-neutral bond at the generator, as suits the application and the particular transfer switch used.

As iwire points out, the particular design choice is often set by ground fault protection; if there is GFP then it _may_ force the location of the ground-neutral bond to be at the source in use. Note that I think that it would be possible to design GFP that would work with non separately derived systems.

-Jon
 
A generator that is supplying power to community fair, in a park is a SDS. You have your own utility, and then you must ground and bond per 250.20.
What this really means is a GES and a system bonding jumper.

The above rules are because there is no direct electrical connection to supply conductors of another system, including the neutral.

When you install a generator via a transfer switch you must still follow the above rules. What determines how the generator is wired for GES and a system bonding jumper is there a "direct electrical connection to supply conductors of another system, including the neutral."

To understand generator installations, read the definitions of SDS, including the FPN, and then the rules in 250.30.

The rules are modified a bit for portable generators, but only deleting the requirement for GES.

If you would remember that you must provide a path for fault current back the windings, without objectionable current you will be 95% there. The GES for a generator, has little value in my opinion. And if you understand that the GES is for lightning, if lightning hits a generator its toast anyway. The system bonding jumper is for bonding and fault current. Without it you are toast.
 
dcspector said:
Bob not to over complicate the issue but the last sentence....
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No problem. :)


winnie said:
As iwire points out, the particular design choice is often set by ground fault protection; if there is GFP then it _may_ force the location of the ground-neutral bond to be at the source in use.
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Generally the way GFP (Ground Fault Protection) is set up if you connect a generator to it with a solid neutral transfer switch you can have nuisance tripping of the GFP.
 
Here is a diagram from CAT power systems.

GFP_SDS.JPG


If you follow the arrows you can follow how current can sneak back to the neutral on the power company side of the GFP sensor. If the current is high enough it will cause a nuisance trip.

Edit, sorry that diagram shows a different problem than we are discussing.
 
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Here is a graphic from our old friend Ed MacLaren.

GenandGFP.gif


Roger
 
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