generator question SDS or Non-SDS

[wirenut1980]

I have been reading up on generators and their connections to transfer switches (and looking at some of Ed Maclaren's fabulous drawings) and was wondering if it is preferrable to have the generator set up and connected as an SDS or a non-SDS, and then thus whether or not the neutral is switched. In other words, what determines if one should install the generator as an SDS or non-SDS? Installer's preference? Voltage configuration (e.g. delta or wye)? Type of loads to be served?

 

[charlie b]

Re: generator question SDS or Non-SDS

Originally posted by wirenut1980:Installer's preference?

I hope this doesn't offend the Installers on this Forum, but I would rather it be a ?Designer?s preference.? If the Installer is also doing the design, that is fine by me. But if one person (i.e., the Designer) made the choice, the Installer should not go the other way, without at least discussing it with the Designer. I know that many things I show on the drawings will not be installed in the exact manner that I designed. I know that panel schedules are high on the list of ?Designer?s Fiction.? But I think the decision to use a 3-pole or a 4- pole ATS should be a design decision, and not a field decision.

That being said, I prefer to use a 4- pole ATS, and to make the generator the source of an SDS. I have no great reason and no really strong feelings on the topic, other than that it makes the relationship between the sources (i.e., separate and independent) easy to see. Also, there is no need to worry about removing the N-G bond that are installed by the generator manufacturers.

 

[wirenut1980]

Re: generator question SDS or Non-SDS

Charlie B, I thought about editing my post just for that reason right after I posted, but didn't. No offense to anyone on my part either.

 

[roger]

Re: generator question SDS or Non-SDS

From an EC's point of view, I'd also rather this be a designers decision, unless as mentioned, it is an installation with out a PE's input.

Roger

 

[kiloamp7]

Re: generator question SDS or Non-SDS

No question that it is the designer's responsibility to cleary show how the system(s) & the transfer switch are to be set up.

Personally, all my applications have been with emergency lighting systems (life-safety/legally required). Either 208Y/120V. or 480Y/277V.

I like the simplicity of 3-pole automatic transfer switches with non-switched neutral for these "plain Jane" applications.

4-pole transfer switches have their place but it is usually in systems with Equipment Ground Fault Protection, or "special" applications.

 

[benaround]

Re: generator question SDS or Non-SDS

Solidly grounded neutral,3 pole ATS, must remove the neutral/grounding jumper at the gen. set.
I hear that most of the gen. Reps. feel that the jumper should be left connected,that is where the problem starts.imo

frank

 

[kiloamp7]

Re: generator question SDS or Non-SDS

I have generally used nothing but 3-phase gensets & transfer switches. Have yet to see one come from factory with "X0" bonded to the frame.

Could equate these gensets to 3-phase xfmrs.
I have yet to see one of them come from factory with "X0" bonded to the frame. Sure, the iron core is bonded to the frame.

I do not know, but it would not surprise me too much if small, 1-phase, portable-type gensets, had the "neutral" bonded to the frame.

The installer should always check this out, whether it be genset or xfmr.

The concept of not switching the "neutral" is in keeping with one of the basic principles of electrical systems. - K.I.S.S. when possible.

 

[paul]

Re: generator question SDS or Non-SDS

Solidly grounded neutral,3 pole ATS, must remove the neutral/grounding jumper at the gen. set.
I hear that most of the gen. Reps. feel that the jumper should be left connected,that is where the problem starts.imo

frank

I have had to remove about a half-dozen jumpers after the fact, that were left in place by the original installer.

[ April 17, 2005, 05:45 PM: Message edited by: paul ]

 

[kiloamp7]

Re: generator question SDS or Non-SDS

If reference to my post of 3/31 in this same thread:

Well you never have seen everything - it finally happened. Today I saw my first 3-phase genset with the center of the wye factory-bonded to the frame.
This was disappointing to say the least, especially since it came as part of a package from the Kohler vendor that included a 3-pole auto transfer switch.
Was natural gas engine driven 19KW/24KVA with 480Y/277V. output.
Application was plain-Jane life safety emergency lighting.
We fixed it of course, while the Kohler start-up man waited a bit.

End result was a Non-SDS with non-bonded/non-grounded "neutral" on the genset side of the system, as was appropriate for the 3-pole ATS.

 

[dereckbc]

Re: generator question SDS or Non-SDS

Getting back to the original question at hand. If there is GFP involved, a 4-pole ATS is demanded. Otherwise you are just asking for big trouble with both the service and generator tripping off-line at the sametime.

I also prefer 4-pole as it is much easier to assure PQ issues like low EMF emissions, lowest possible ground fault impedance, common mode noise rejection, and minimum N-G voltage drop.