SDS Generator

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ceb58

ceb58

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Location
Raeford, NC
The AHJ over what we do installing generators at communication sites are now wanting to see nothing but switched neutrials. This would make the gen. a separately derived system. Two questions
Can any one see any benefit from switching the neutrials in these cases? I cannot. They say it is for safety of the utility side as to not induce voltage on the utility during an outage. From what little I know if there is an outage and the gen. is running the return voltage will seek it source ( the gen) and flow very little or nothing on the utility.
The other thought. You have the main service disconnect, the neutral and the GEC are bonded. The phase conductors, neutral and an EGC go to the ATS. The neutral lands on the utility side of the pole it the switch. The EGC is bonded to the housing. Now the neutral and GEC are bonded in the generator, phase conductors, neutral and EGC come into the gen. disconnect. neutrals are isolated from can and EGC is bonded to can. From there they go into ATS. Neutral lands on E side of pole, EGC bonded to can. Now would the bond of these EGC in essence still tie the neutrals together through the housing? And if you were to have a line to ground fault on the gen. side you could,for a mill-sec., induce voltage on the utility side?
 
ceb58 said:
... Can any one see any benefit from switching the neutrials in these cases? ...
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In the 30+ years I've been working on multiple generator systems, I've never seen a 4 wire switch (three phase - of course)


ceb58 said:
... You have the main service disconnect, the neutral and the GEC are bonded. The phase conductors, neutral and an EGC go to the ATS. The neutral lands on the utility side of the pole it the switch. The EGC is bonded to the housing. Now the neutral and GEC are bonded in the generator, phase conductors, neutral and EGC come into the gen. disconnect. neutrals are isolated from can and EGC is bonded to can. From there they go into ATS. Neutral lands on E side of pole, EGC bonded to can. Now would the bond of these EGC in essence still tie the neutrals together through the housing? ...
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Absolutely. As you say, even if you put in a switched neutral ATS (single phase, 3 wire sw), the gen neutral is solidly tied to the gen EGC, which is tied solidly to the Building Grounding electrode system, which is tied solidly to the utility EGC, which is bonded solidly to the utility neutral.

ceb58 said:
... And if you were to have a line to ground fault on the gen. side you could,for a mill-sec., induce voltage on the utility side?
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Yes.

My recomendation is not put a bonding jumper in the gen.

cf
 
Unless there are specific reasons why one has to switch the neutral, its is always better to have a solid neutral. There are no benefits in the normal sense of the word and normal circumstances to voluntarily switch the neutral.

The only functional reason I've had advanced to me to switch four pole was with a dual supply / static switch application, where the engineers were convinced that if the primary supply lost its neutral the static switch would detect this condition and flip the installation over to the secondary supply, assuming (of course!) its neutral was solid.

I disageed their analysis, but still lost the battle. My argument was that given the neutral that could possibly be lost was a few of yards of cable and a busbar away from the transformers, I was of the opinion that static switch failure was more likely than neutral loss. Win some, loose some.
 
ceb58 said:
This would make the gen. a separately derived system.
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I do not agree.
switching the neutral in the transfer switch doesn't create an SDS. The neutral bonding (or unbonding) inside the generator is what determines it.
So, you could be creating a violation by switching the neutral, or, you may have to change the neutral bonding in the generator.
 
ty said:
I do not agree.
switching the neutral in the transfer switch doesn't create an SDS. The neutral bonding (or unbonding) inside the generator is what determines it.
So, you could be creating a violation by switching the neutral, or, you may have to change the neutral bonding in the generator.
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ty -
Not switching the neutral and no N/G bond at the gen go together

Switching the neutral and having an N/G bond at the gen go together.

I think ceb knows this.

cf
 
Cold Fusion said:
ty -
Not switching the neutral and no N/G bond at the gen go together

Switching the neutral and having an N/G bond at the gen go together.

I think ceb knows this.

cf
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I know HE probably does :)


But for the sake of symantics , and a few others...

if the generator is delivered and it has no N/G bond, you would have to get a transfer switch that does Not switch the neutral.
If you install a transfer switch that switches the Neutral, you would have to Bond the Neutral, or you would be in violation.

I just think it wrong to state that the transfer switch dictates wether it is an SDS or not. kind of putting the 'cart before the horse', that's all.
 
ty said:
...if the generator is delivered and it has no N/G bond, you would have to get a transfer switch that does Not switch the neutral.
If you install a transfer switch that switches the Neutral, you would have to Bond the Neutral, or you would be in violation.

I just think it wrong to state that the transfer switch dictates wether it is an SDS or not. kind of putting the 'cart before the horse', that's all.
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Not the way I would phrase it.

NEC definition of SDS is about if the neutral is to be solidly connected or not. That is based on a design decision. And that design decision also then specifies if the switch is 2wire or 3wire (single phase 120/240) After this is decided, then one either removes or installs the gen N/G bonding jumper to match.

Buying a gen with the wrong bonding jumper configuration is not a violation. Leaving it wrong is.

Minor note: None of gens I work with have a bonding jumper configuration that can not be changed. All have a removable jumper. Or a place to install a jumper - which could be at the first disconnect and not in the gen.

Note to the contrarians: Yes, I am sure that somewhere, someone has built a gen that has a non-removeable jumper, that I have not seen.

cf
 
Also notice 250.20(D) along with its FPN No. 1 ?An alternate ac power source such as an onsite generator is not a SDS if the grounded conductor is solidly interconnected to a service supplied system grounding conductor?example is were transfer switch does not include switching?the grounded conductor?. So in this light the transfer switch does determine as SDS or non-SDS.

I gota say though I?ve never noticed bonding jumper extraction or installation options on any gen-set. I believe they?re all manufactured as bonded because they are a supply.
 
tryinghard said:
Also notice 250.20(D) along with its FPN No. 1 ?An alternate ac power source such as an onsite generator is not a SDS if the grounded conductor is solidly interconnected to a service supplied system grounding conductor?example is were transfer switch does not include switching?the grounded conductor?.
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yes

So in this light the transfer switch does determine as SDS or non-SDS.
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i don't agree with your thinking. The reference above shows that the generator neutral bond (or non-bond)determines the system. Then it gives an example of the transfer switch for the type of system.

This is all symantics. I agree with you guys about which transfer switch goes with which system.
 
ty said:
yes

i don't agree with your thinking. The reference above shows that the generator neutral bond (or non-bond)determines the system. Then it gives an example of the transfer switch for the type of system.

This is all symantics. I agree with you guys about which transfer switch goes with which system.
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Yes, it is semantics. Point being, as CF stated, the transfer switch doesn't determine non-sds or sds. It is the application. Point being if I were to order a single phase gen. and a two pole transfer switch for a non-sds install and they screwed up and sent me a three pole instead. Then what am I to do. Send it back. No I will just tie the neutrials together and go on about my bussness and not use the third pole.
But we have got a little off base. We have no GF on the system. And I am looking for a GOOD reason why you would want to install a SDS system. The only thing the approval committee can come up with is it will be safer for the utility co. workers so it will not cause return current on the utility neutral. But I say even if you do switch the neutral they still are tied together through the EGC's
 
ty said:
yes

i don't agree with your thinking. The reference above shows that the generator neutral bond (or non-bond)determines the system. Then it gives an example of the transfer switch for the type of system.

This is all symantics. I agree with you guys about which transfer switch goes with which system.
Click to expand...

Todd, don't just trust my citing read 250.20(D) without the fine print note; the transfer switch type matters in such a way to qualify the system type and how the supply will be grounded.
 
ron said:
CF,


I can't give you a sketch, but there are loads of white papers and articles regarding when 4 pole transfers are needed and their relationship with GF detection. For example
http://ecmweb.com/power_quality/electric_groundfault_current_problems/
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I'm going to do my best to phrase this so that it does not sound like I an being demeaning to anyone. Please keep this in mind when you read this post. I have a different professional opinion from the author of the ecm article. If I translated the article correctly, Figures 1 and 2 are examples of the wrong way to connect a non-sds system - GF or not.

Both figure 1 and 2 show the gen having an N/G bond and the connection from the gen as 4W and they are using a 3pole transfer switch. Then they use this as an example of this causing a problem with the GF sensing.

They are right, when a gen is incorrectly connected with 4W (and N/G bond at the gen) instead of 5W (and no N/G bond at the gen), it causes problems. Has nothing to do with the GF. A 3pole transfer switch is just fine in thiis application. The gen needs to be connected 5W, with no N/G bond at the gen.

This is an excellent example of why I am not seeing a problem with 3W transfer switches for 3phase. I'm sure there are special cases, I just have not seen one yet.

cf
 
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CF,
The point in the first two figures of the article, is that a 3 pole transfer is not appropriate for some applications.
Ground fault is the major reason that the 3 pole transfer is not appropriate in some applications as it will sense incorrectly.
 
ron said:
CF,
The point in the first two figures of the article, is that a 3 pole transfer is not appropriate for some applications.
Ground fault is the major reason that the 3 pole transfer is not appropriate in some applications as it will sense incorrectly.
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Yes, I knew that was their point. Absolutely, clearly, I knew that.

My point is I just don't see how that their point is made when using a blatantly inccorrect connection. It doesn't show anything about three pole transfer switches being inappropriate for ground fault applictions.

The only thing it shows to me is: "The gfe may give the wrong indication if the gen and utility are incorrectly connected." :confused::confused: To which I would respond, "No kidding."

The other issue I have is that I have not seen a case where a 3pole transfer causes trouble. I'm not saying there isn't one, I just have not seen one. After reading the ecm article, I still haven't.

I am absolutely open to learn on this issue. I deal with multiple gen systems a lot. If there is something like this that I am not understanding, I want to know and understand about it.

cf
 
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