480V 3W delta service

Status
Not open for further replies.
tryinghard

tryinghard

Senior Member
Location
California
I have a 400A 480V 3PH 3W delta service to a service disconnect. This service disconnect is a 480V 4W ? A, B, C phases & EGC ? that feeds a transfer switch and an MCC each 480V 4W (A, B, C phases & EGC), the generator set is also 480V 4W.

I am concerned about not having a grounded conductor with the service; will a downstream ground fault be impeded without the service grounded conductor? In other words will a ground fault operate any breaker just a quick with or without the service grounded conductor?
 
Sounds like you have an ungrounded delta, if that is the case, a single down stream fault to ground will not activate an OCPD. Though you can intentionally ground a phase(corner grounding), it depends on your application as to weather or not you should.

Is there ground fault detection present? If not, and this is an ungrounded system, that would be cause for concern.

Sorry, I know nothing about transfer switches or gensets, but the many who do will be here shortly(I hope).

tryinghard said:
will a downstream ground fault be impeded without the service grounded conductor?
Click to expand...

Edit: Service grounded conductor? Thats for wye or 4 wire delta. Do you have a 3 wire wye, or mabe impedance grounded wye? The way it's written kinda throws me.
 
Last edited:
tryinghard said:
It's currently a 3W delta service
Click to expand...


I don't see a problem with the exception of it lacking a ground detection system. Sounds like a ungrounded 3W Delta system.

I wouldn't call the system beyond the service disconnect a 480 4W system. A 4 wire Delta system is a completely different animal.
 
The service is 3W ungrounded delta, without an EGC/grounded conductor (nothing to XO). The feeder going to an MCC is also 3W but it includes an EGC. I’m afraid this 3W service will disable the ability of a downstream ground fault to activate an OCPD.

If a ground fault happens isn’t it trying to return to source?

Will this system still have ground fault protection the same as if it were a 4W service?
 
Last edited:
tryinghard said:
If a ground fault happens isn?t it trying to return to source?

Will this system still have ground fault protection the same as if it were a 4W service?
Click to expand...

On an ungrounded system the FIRST connection to ground is not a true fault. There will be no fault current flowing as there is not a complete circuit back to the source. A SECOND connection to ground will create a line-line fault. This is why a ground dector should be used to advise when the first connection to ground occurs.

Ground fault protection really deals with current flowing on unintended paths, not necessarily ground. While it is technically possible to apply ground fault protection schemes to ungrounded systems it will not really function until a complete current path occurs either through (1) line being connected to ground in multiple places or through (2) lines being connected which results in a short circuit rather than just a ground fault.
 
coulter said:
Does this trransformer have an XO terminal?

carl
Click to expand...


Good question. If the POCO has a grounded conductor it has to be brought to the service.

tryinghard, very important to know if the poco is supplying a 480 delta, or just the phases from a 480Y...
 
jim dungar said:
On an ungrounded system the FIRST connection to ground is not a true fault. There will be no fault current flowing as there is not a complete circuit back to the source. A SECOND connection to ground will create a line-line fault. This is why a ground dector should be used to advise when the first connection to ground occurs...
Click to expand...

Jim or all,

So with a 3W ungrounded service do I have an effective ground fault path from the service disconnect (250-4(A)(5))?

When you say ?the FIRST connection to ground? what do you mean, do you mean ground fault or connection at a transformer?

I don?t want advise when ground fault happens but I do want short-circuit & ground-fault protection, instantaneously. In my case is the ground detector the remedy for the lack of an effective ground fault path or should I have a 4W service?

Will a GFI service disconnect breaker suffice?

The service is 3W ungrounded delta, no XO.
 
An ungrounded system will continue to operate when one of the three phases becomes grounded. It's not until a second phase becomes grounded that the OCPD will open. A ground detector will let you know that although the system is still functioning normally that there is a single phase ground fault. This will allow you time to find it while the system continues to operate. If you choose you can make a corner grounded system by intentionally grounding one of the phase conductors. IMO these are design considerations that should be considered by the designer not the installer.
 
infinity said:
An ungrounded system will continue to operate when one of the three phases becomes grounded. It's not until a second phase becomes grounded that the OCPD will open.
Click to expand...

Got it, and this is my concern won?t this happen even downstream on the branch or other feeders from the MCC, will each OCPD function correctly for ground fault?

infinity said:
A ground detector will let you know that although the system is still functioning normally that there is a single phase ground fault. This will allow you time to find it while the system continues to operate. If you choose you can make a corner grounded system by intentionally grounding one of the phase conductors. IMO these are design considerations that should be considered by the designer not the installer.
Click to expand...

I will not be the one choosing the design, but as the site electrician I can advise because the work we are installing has no electrical engineer involved. I know the agency I work for often uses non-electrical engineers to modify original installations. In this light I have concern. I cannot understand why a non-grounded service would be used for distribution of multiple feeders and branches. Is a 3W non grounded service correct for this application?
 
tryinghard said:
Jim or all,

So with a 3W ungrounded service do I have an effective ground fault path from the service disconnect (250-4(A)(5))?

When you say “the FIRST connection to ground” what do you mean, do you mean ground fault or connection at a transformer?

I don’t want advise when ground fault happens but I do want short-circuit & ground-fault protection, instantaneously. In my case is the ground detector the remedy for the lack of an effective ground fault path or should I have a 4W service?
Click to expand...

Efective ground fault path is all about EGC and proper bonding, no matter if the system is grounded. I'm not sure where you're going there. If I wanted simultaneous OCP for GF and short circuit, and it were an option I would choose a 4wire sys.


By first connection to ground I belive he meant of the unintentional varity(not to speak for him).
 
Last edited by a moderator:
The first connection can be intentional or unintentional.

If you want your system to respond immediately to a "ground fault" then you will need to intentionally ground one leg of your delta (this should be the B phase for new installations). All of your 480V equipment needs to be designed and rated for a "corner grounded" system - no fuses in the grounded phase except for those used for motor protection, circuit breakers need to be rated specifically for grounded B-phase systems (often special order or only with reduced AIC levels). Also, the grounded conductor should be white even though it is a phase conductor.
 
jim dungar said:
...If you want your system to respond immediately to a "ground fault" then you will need to intentionally ground one leg of your delta...Also, the grounded conductor should be white even though it is a phase conductor.
Click to expand...

That?s what I thought! I don?t even think a corner grounded delta system is correct for this application either.

How about a 4W wye?
 
tryinghard said:
... I don?t even think a corner grounded delta system is correct for this application either.

How about a 4W wye?
Click to expand...
The decision is usually based on what types of equipment the service is powering.
If it is industrial loads, mostly motors, lighting is from dedicated smaller transformers, then I would recommend impedance grounded 480Y. Same thing if continuity of service is needed.

If it's a commercial building where most of the loads are lighting and HVAC, I would recommend solidly grounded 480Y

If it's a dedicated load that needs an ungrounded supply, then I'd recommend an ungrounded 480D.

I can't think of any reason to recommend a corner grounded 480D.

carl
 
coulter said:
The decision is usually based on what types of equipment the service is powering.
If it is industrial loads, mostly motors, lighting is from dedicated smaller transformers, then I would recommend impedance grounded 480Y. Same thing if continuity of service is needed.carl
Click to expand...

It?s industrial and I can see the 480Y is needed is there a site or literature that I can access regarding transformers solidly grounded vs. impedance grounded? I would like to understand these more.
 
tryinghard said:
That?s what I thought! I don?t even think a corner grounded delta system is correct for this application either.

How about a 4W wye?
Click to expand...

You have repeataly told use that this is a delta transformer connection without an X0 bushing, but now you are talking about wanting it to be a wye connection. Your service will depend on what the POCO is willing to supply.
 
jim dungar said:
You have repeataly told use that this is a delta transformer connection without an X0 bushing, but now you are talking about wanting it to be a wye connection. Your service will depend on what the POCO is willing to supply.
Click to expand...

More than this I?ve been trying to stress the service is erroneous/inadequate (posts 11, 5, & 1). I work for an agency that is a utility company, we generate & transmit (we are the POCO), often our work is not designed by EE?s especially past new construction and this is my current case. Many installations are designed by people of authority and influence that understand how the power plant operates but do not necessarily understand how grounding works.

I find it impossible to believe a 3W ungrounded service is the correct application for an MCC that distributes maybe 15 feeders and 10 branch circuits! So I believe the 4W wye service is what?s needed based on distribution and it?s going to be my recommendation. There are three choices that I see: 1) leave it alone, 2) change the service, & 3) change the distribution.


coulter said:
I'm guessing this is after the service. 400A isn't much for industrial.carl
Click to expand...

It?s probably not and the powerhouse operators are probably relying on a switching order to manage it. I do not believe an EE calced or designed it.

Thanks guy?s you?ve been great help!
 
tryinghard said:
...I find it impossible to believe a 3W ungrounded service is the correct application for an MCC that distributes maybe 15 feeders and 10 branch circuits! So I believe the 4W wye service is what’s needed based on distribution and it’s going to be my recommendation. ...
Click to expand...
From what you have told us I would tend to agree. Un-grounded 480D is pretty special. My inclination is to not use it unless it is required by the equipment. There are issues with arcing ground faults that I don't completly understand. Jim can likely help out here.

The choice between solidly grounded 480Y or impedance grounded 480Y is generally based on continuity of service issues.

I'm looking for information sources regarding transformers solidly grounded vs. impedance grounded - but I haven't found anything yet. I'll let you know if I do.

You really have a tough one. I've noticed that occasionally POCOs don't always separate the generation and transmission from the hotel facilities loading - I know it's hard to believe.:rolleyes:

Let us know how it comes out.

carl
 
Last edited:
Status
Not open for further replies.
Top