480V 3W delta service

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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. .
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Just out of curiosity,in a nutshell, what type of Eq is this MCC feeding?
 
K2500 said:
Just out of curiosity,in a nutshell, what type of Eq is this MCC feeding?
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I don?t know but some of the circuits are a 3ph role up door motor, lighting?15 or so feeders and 10 or so branch, with the feeders that could be up to 50 circuits.

What kind of circuit would be best suited for a 480V 3W ungrounded delta?
 
tryinghard said:
What kind of circuit would be best suited for a 480V 3W ungrounded delta?
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I was'nt really thinking about the grounding, just that if they were all 480 loads you have no need for a neutral. Lighting single phase 480 ect. Seems like it would save a little money/time.
 
coulter said:
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.
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I don't know that 3W D is all that special. My experience is that it is more common than resistor grounded wye, but that may be just because of the things I run across. I honestly cannot recall ever dealing with a resistor grounded wye system, but it just may not have come up. Normally I don't really care except for a few cases with some VFDs and other devices that want it one way or the other.

I can't recall ever using the neutral of a 4 W wye system either.
 
petersonra said:
I don't know that 3W D is all that special. My experience is that it is more common than resistor grounded wye, ...
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Interesting. Hopefully you are limiting the conversation to ungrounded 480D. The only two cases I know of are where 480D is needed is Navy shipboard systems (continuity of service) and equipment that has an un-grounded service specified - I've only seen a few VFDs where it was specified. Other than that there really isn't any reason to use it. Restriking, arcing ground faults are a real problem with un-grounded 480D. The voltage surges do burn up equipment.

If continuity of service is an issue, then impedance grounded 480Y is a much better choice.

As for why you are seeing un-grounded 480D and not impedance grounded, I don't know. Maybe your area has the majority of weird 480 VFDs - or other equipment that specs un-grounded 480D. Or maybe, God forbid, poor design practices.

Now about the lack of a neutral:
Impedance grounded 480Y won't have one. It is not even taken to the first disconnect. Solidly grounded 480Y industrial usually does not have a neutral. It goes to the first disconnect as code specifies and stops there. unless there are 277V loads (lighting) there is no need for a neutral.

I'm wondering if all of these ungrounded 480D you are seeing are infact industrial grade solidly grounded 480Y with the neutral stopping at the first disconnect.

As for the lighting, you can't use ungrounded 480D for lighting. You can't even use 480v lighting on an impedance grounded 480Y. It's a code issue.

carl
 
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coulter said:
...I'm wondering if all of these ungrounded 480D you are seeing are infact industrial grade solidly grounded 480Y with the neutral stopping at the first disconnect...
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No these are actually 480V delta 3W ungrounded.

I understand for some specific circuits, that are usually critical and sensitive, the ungrounded system in used or wanted but I do not understand if it has other reasons like power factor or efficiency.
If there is a case fault it will not open the circuit and from what others are telling me these are in applications where the ground is the same potential therefore you?re like a bird on a wire and wont get electrocuted.
 
coulter said:
You can't even use 480v lighting on an impedance grounded 480Y. It's a code issue.

carl
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I cant seem to find the section, but if its a code issue than this plant has one heck of a violation. What section?

Edit:
BTW, with ungrounded delta vs impedance grounde wye, how does current flow differ along the ground path under fault conditions? Seems like impedance grounded would draw more current during fault, like it was adding a resitive single phase load to the circuit.
 
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K2500 said:
...with ungrounded delta vs impedance grounde wye, how does current flow differ along the ground path under fault conditions? ...
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I suspect you already know this - but just to clarify:
Impedance grounded Y has a grounding resistor between the neutral and the ground. There is no bonding jumper from the neutral and ground. For 1000kva, 480V xfmrs, the resistor is selected to limit the fault current to 5 - 10 A.

So if one of phases is grounded, the elevates the neutral to 277V. So to limit the GF current to 5A. the grounding resistance would have to be 55 ohms.

If this doesn't make any sense, I'll send a sketch tomorrow.

carl
 
k2500,
I cant seem to find the section, but if its a code issue than this plant has one heck of a violation. What section?
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See 210.6(D) and the definition of "voltage to ground" in Article 100.
Don
 
K2500 said:
... if its a code issue (480V lighting on impedance grounded 480Y) than this plant has one heck of a violation. ...
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It hasn't always been against the code. Maybe even okay by the 99 code. I don't have one to check.

carl
 
480 volt delta ground detection

480 volt delta ground detection

Hi guys- this is my first post.

I ran into a situation in a plant I maintain (an old Cargill plant by the way)
and it has a 450 KVA delta transformer (480 volt secondary), and I had odd voltages backfeeding on the plant system. After I began to isolate loads, I realized that it was a 480 V compressor's winding leads in the motor connection head that was shorting to the metal. This brought my phase to ground voltages to 140 V, 360 V ,and 360 V from each phase conductor to ground rather than each at 277 V or so. So now I can see how this old system can be hazardous.

I have been able to locate a ground detection system with alarm contacts
for the wye supply transformer, just not for this delta ( no reference to ground) system.

Any help locating a manufacturer for this purpose would be appreciated.
 
darhym,

Welcome! Do you know for sure your compressor motor had a ground fault? I've come a long ways since I first posted this topic because at that time I was fairly new to ungrounded systems.

On an ungrounded system you can get funky voltage readings between any phase to earth or frames but you wont get a wye type reading like 277 because that's a grounded system voltage and ungrounded has no referance to ground. You cannot have an ungrounded system fed from a grounded system because fault current will still route through bonding to source causing OCP to open on one fault rather than two, which circumvents the only purpose for an ungrounded system. Conversely you can have a grounded system fed from an ungrounded system but only through a separately derived system like a transformer, which would establish a ground bond. It is only the service that establishes an ungrounded system.

An ungrounded system voltage reading will only be accurate between phases because XO on the transformer is not grounded nor are any of the phases. Do you know for absolute sure the system you are referencing is an ungrounded system (no XO bond/no phase bond like corner grounded) at the serving source?
 
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