Separately Derived Systems

[solarEI]

I know that SDS are a touchy subject on these forums but i have a question that has been frustrating me. I understand the definition of SDS as well as the different grounding & bonding associated with them, I will try my bes to NOT make this a gronding vs bonding question. My question is:

Are there any codes (NEC) that directly or indirectly REQUIRE a SDS (in regards to generators), vs. just different design practices?

From what I can tell (and i have been wrong before) there aren't any codes (NEC) that actually require a SDS, they just tell you the definition and how to ground/bond when you do have a SDS... Am I missing something? Thanks in advance!!

-solareEI

 

[jwelectric]

Separately Derived System. A premises wiring system whose power is derived from a source of electric energy or equipment other than a service. Such systems have no direct electrical connection, including a solidly connected grounded circuit conductor, to supply conductors originating in another system.

Unless you are switching the neutral a generator is not a Separately Derived System.
See 701 for required standby systems

 

[bphgravity]

No. This is solely a design consideration. As indicated by you, if you CHOOSE to install a SDS or if the equipment installed is an SDS by design, the rules for SDS must be followed. But nothing in the NEC directs you for or against a SDS.

 

[dereckbc]

In reference to generators it is strictly a design issue. SDS and non-SDS both have advantages and disadvantages.

Typically you would choose an SDS system where ground fault realys are used and/or where sensitive electronics are operated. Otherwise when expense and non critical equipment is used, non-SDS is the natural choice.

 

[charlie b]

I agree. Not a code issue, but only a design choice.

 

[Gmack]

I would consider IPS's to fit the description of a SDS.

NFPA 99 also requires them in HCF, not all of course.

But the NEC does direct us to:

517.61 Wiring and Equipment
(A) Within Hazardous (Classified) Anesthetizing Locations
(1) Isolation Except as permitted in 517.160, each power circuit within, or partially within, a flammable anesthetizing location as referred to in 517.60 shall be isolated from any distribution system by the use of an isolated power system. [NFPA 99:Annex E, E.6.6.2]

There are more issues in patient "wet locations" and OR's etc that require a IPS .

 

[volt102]

As far as the generator is concerned, it doesn't care, its just happy to run.

SDS
If Main has GFP on it...480V 1000A
Design issues such as lightning protection.


Jim
New Hampshire

 

[Design Electric Inc.]

separately derived system

separately derived system

I believe that switching the Generators neutral qualifiy's a generator system, regardless of supplying 702,701 or 700 loads, as an SDS.
By using a S/N, the system now has a direct electrical connection: the grounded conductor is now supplying conductors of another system.
By utilizing a Xfr switch that opens the Neutral there is no direct electrical connection thus qualifying it as a SDS.
See 250.20(D) FPN NO:1
And also see Quote below.


The definition for a “separately derived system” was revised by eliminating the list of electric energy power sources that constitute a separately derived system. Actually, the original proposal was to add “fuel cells” to the list. With the list removed, there’s no need to keep updating the list of new power sources.

In addition, the definition was revised to clarify that a separately derived system could be supplied from “other than a source of electric energy.” For example, a photovoltaic system is a source of electric energy, but a transformer is not a source, it’s the equipment that provides the power. Small point, but you know every point counts!

Separately Derived System. A wiring system whose power is derived from a source of electric energy or equipment other than the electric utility service. Such systems have no direct electrical connection, including a solidly connected grounded circuit conductor, to supply conductors originating in another system. Figure 100-10

Author’s Comment: This includes a battery, a solar photovoltaic system, a transformer, or a converter winding, where there’s no direct electrical connection to the supply conductors of another system.

I believe the '05 code needed to clarify the qualifications of SDS to eliminate some ambiguities of interpitation.

Just don't forget that if you use a S/N , a Grounding Electrode System already exists at the normal power source. And we all know better than to have redundant and parallel GEC's.

See www.superiorpowersystems.com

 

[lile001]

Ground Fault Relays and SDS

Ground Fault Relays and SDS

In reference to generators it is strictly a design issue. SDS and non-SDS both have advantages and disadvantages.

Typically you would choose an SDS system where ground fault realys are used and/or where sensitive electronics are operated. Otherwise when expense and non critical equipment is used, non-SDS is the natural choice.

Can you explain more about why a non SDS is not recommended when you have ground fault relays?

 

[iwire]

Can you explain more about why a non SDS is not recommended when you have ground fault relays?

Under certain conditions a non-SDS can trip the ground fault relays.

 

[ramdiesel3500]

I find that a separately derived system is useful when using portable generator power if the generator has an internal neutral/ground bond. Otherwise, a properly installed non-SDS arrangement should work! However, most of the transfer switches we purchase on services over 1000A 480V will have a switched neutral in order to avoid nuisance trips resulting from stray current on the grounding system. Its probably easier for the manufacturers to do this than to have to deal with wiring problems that are not their fault! (fault??? hehehe)

 

[tom baker]

The issue of GFP relays and why a SDS is required is discussed in the IEEE Green Book.