# Y Delta woes



## RePhase277 (Feb 5, 2008)

Need more info.


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## frenchelectrican (Mar 15, 2007)

chicken steve said:


> So i've a Y to delta 480.
> 
> But i'm not reading anything from phase to ground
> 
> ...


where did ya read the voltage ?

did ya read from line to line and line to ground and what the numbers ya hitting ?


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## chicken steve (Mar 22, 2011)

line/line is all 480

line to G is *0* , or a few residual volts French One

I'm thinking the secondary isn't make to XO

Thus it _can't _carry a fault.....am i wrong?

~CS~


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## chicken steve (Mar 22, 2011)

I have an _'ungrounded delta_' , (via others) that i'm making straight (no noodle) 480V branch circuits off of

>>>











Can anyone validate the grounding in art 250?

~CS~


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## frenchelectrican (Mar 15, 2007)

chicken steve said:


> line/line is all 480
> 
> line to G is *0* , or a few residual volts French One
> 
> ...





chicken steve said:


> I have an _'ungrounded delta_' , (via others) that i'm making straight (no noodle) 480V branch circuits off of
> 
> >>>
> 
> ...


Chicken Steve .,

Just be extra carefull with ungrounded delta system they will bite ya pretty hard if you are not aware of this .

Did the service is actually ungrounded delta ? if so they should have a ground fault indenticator lights or others means for ground fault.

Look up in art 250 but there few spots you will have to find it due it will be in at least 3 or 4 spots for sure plus I think it mention in art 310 but not sure due I havent look at nec code book recentally. so expect in few places.

Yes the ungrounded delta can carry single phase fault with out affect the system typically line to ground but line to line fault yes it will either blow the fuse or trip the breaker. 

Myself and few other guys in here have allready dealt with ungrounded system it is a pain in arse if not aware at the source.

but if you need single phase load., you will need SDS single phase transfomer and follow the SDS rules on that.

What this unground load is going to be feed for ? motor load or transfomer ? 

If you do see the transfomer then did you see how the secondary been hooked ? If so what type of transfomer it was ? typically I useally ran into are isolation or drive transfomer typically 1:1 ratio


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## chicken steve (Mar 22, 2011)

*CS*



frenchelectrican said:


> Chicken Steve .,
> 
> 
> 
> ...


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## splatz (May 23, 2015)

The ungrounded system has no _*grounded conductor*_. Since none of the conductors are grounded, there's no potential between ground and any of the L1 L2 L3 terminals. 

At the first ground fault, you basically have a corner grounded delta rather than an ungrounded delta. You need the ground fault detection to tell you something's wrong. 

When the second ground fault occurs, is when it gets ugly. 

https://iaeimagazine.org/magazine/2010/03/16/are-we-really-ungrounded/


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## frenchelectrican (Mar 15, 2007)

chicken steve said:


> frenchelectrican said:
> 
> 
> > Chicken Steve .,
> ...


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## chicken steve (Mar 22, 2011)

Spot on Mr Splatz.....


and yes it's this....










with ref to >>>
*250.4 General Requirements for Grounding and Bonding.
(B) Ungrounded Systems.
4>>>*



> (4) Path for Fault Current. Electrical equipment, wiring, and
> other electrically conductive material likely to become ener‐
> gized shall be installed in a manner that creates a low-
> impedance circuit from any point on the wiring system to the
> ...


BUT, there is no >>>
*250,21(B) Ground Detectors. Ground detectors shall be installed *

Mr Lofland's sign off got me>>>



> What are you waiting for?
> 
> Regardless of whether a grounded or ungrounded system is employed, both systems will be grounded. Don’t let the term “ungrounded” fool you. As we have discussed in this article, there is nothing “ungrounded” about an ungrounded system. *Relax and have another piece of cake.*


I think i've got a ligit complaint here.....

~CS~


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## chicken steve (Mar 22, 2011)

frenchelectrican said:


> chicken steve said:
> 
> 
> > If you did check the transfomer size I can tell you what is the correct OCPD it supposed to be.,
> ...


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## RePhase277 (Feb 5, 2008)

The person who put the transformer in originally didn't know what they were doing, most likely. If this were an industrial facility with a maintenance crew to watch the place, an ungrounded system might be OK. But with hippies and acid - dropping Vermont artists touching the equipment, it's probably best grounded.

Just ground one phase to the case and GEC. Use a Kenny clamp if think you are required to (you're not). Identify the grounded phase conductor white. You treat that phase just like it was a neutral, which means no fuses in that conductor.


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## MTW (Aug 28, 2013)

Call an electrician.


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## frenchelectrican (Mar 15, 2007)

RePhase277 said:


> The person who put the transformer in originally didn't know what they were doing, most likely. If this were an industrial facility with a maintenance crew to watch the place, an ungrounded system might be OK. But with hippies and acid - dropping Vermont artists touching the equipment, it's probably best grounded.
> 
> Just ground one phase to the case and GEC. Use a Kenny clamp if think you are required to (you're not). Identify the grounded phase conductor white. You treat that phase just like it was a neutral, which means no fuses in that conductor.


Chicken steve ., 

What both spatz and Rephase277 mention very clear and myself I feel what Rephase been talking about and it more common to corner grounded it and with true three phase loads like electric furance they dont affect the CG system at all. it been the same way with electric motors for years. 

I have a feeling that someone miss the crictial step when they install the upstep transformer.


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## chicken steve (Mar 22, 2011)

RePhase277 said:


> Just ground one phase to the case and GEC. Use a Kenny clamp if think you are required to (you're not). Identify the grounded phase conductor white. You treat that phase just like it was a neutral, which means no fuses in that conductor.


yup...

~CS~


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## chicken steve (Mar 22, 2011)

frenchelectrican said:


> > Chicken steve .,
> >
> > What both spatz and Rephase277 mention very clear and myself I feel what Rephase been talking about and it more common to corner grounded it and with true three phase loads like electric furance they dont affect the CG system at all. it been the same way with electric motors for years.
> 
> ...


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## Southeast Power (Jan 18, 2009)

Your equipment will never know you grounded a corner so, you are good there.
Also, you mentioned OX. Just to recap for visitors that might not read the entire thread. Don't connect anything to OX when you are using a delta wye transformer backwards.


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## chicken steve (Mar 22, 2011)

Southeast Power said:


> Your equipment will never know you grounded a corner so, you are good there.
> Also, you mentioned OX. Just to recap for visitors that might not read the entire thread. Don't connect anything to OX when you are using a delta wye transformer backwards.


Ok then i'm_ lost_ SP....:crying:~CS~


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## RePhase277 (Feb 5, 2008)

chicken steve said:


> yup...
> 
> ~CS~


So you're saying you already knew? Why ask then?


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## chicken steve (Mar 22, 2011)

No i don't know the best way to deal with this, yes i am asking

Isn't that what this place is for 277?

~CS~


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## Mulder (Sep 11, 2010)

Nothing is connected to XO on the primary when you are using a wye to delta transformer. Connecting the neutral on the primary can cause premature transformer failure. You can corner ground the secondary of the transformer without worrying about 480v to 120v control transformers. They only care that the have 480v between terminals on the primary, which they would. IMO the easiest fix is to pick a corner and ground it.


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## Mulder (Sep 11, 2010)

From a GE transformer document.

Grounding
When the secondary (wye) of a deltawye
transformer is energized instead
of the primary (delta), then the wye
side of the transformer is not a
separately derived service. As such,
the neutral should not be connected to
building ground nor should it be
bonded to the transformer enclosure.
The delta side of the transformer
becomes the output, which is the
separately derived system. The output
delta “B” phase should be tied to
ground unless the facility distribution
system utilizes a different grounding
scheme.


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## frenchelectrican (Mar 15, 2007)

chicken steve said:


> No i don't know the best way to deal with this, yes i am asking
> 
> Isn't that what this place is for 277?
> 
> ~CS~



Chicken steve .,

Just look back our comments about grounding the one leg of delta side of the transformer and it is been done like that.

I know it may spook ya at first about actually ground it on one leg of delta but if done properly it work very well with no side effect but the OCPD will work very well on this situation.

Once you properly grounded one phase then you have to mark the OCPD or disconnect switch to remind the next person whom will deal the CGD system they will know what to do next.

and it alot safer to run it in corner ground delta than just run floating ( ungrounded ) delta system especally if you dont have ground fault monitor system in place.

with corner grounded delta system the voltage will be little more stable but the reading will get ya cornfused for a second.

let me post a quick example .,

Alpha-Bravo 480
Bravo-Charle 480
Charle-Alpha 480

Now if you grounded B Phase .,

Alpha-Golf 480
Bravo-Golf 000
Charle-Golf 480 

this what it supposed to read if you done it correctly.


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## chicken steve (Mar 22, 2011)

Thx all.... 

so the way i'm reading this is, lift the Primary Wye off OX, connect Secondary Delta (shown here in A) to OX.... and i get>










source

~CS~


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## Southeast Power (Jan 18, 2009)

You nailed it CS .:smile:

I'm having this theory that it isn't really any safer to not ground one of the phases.
The first short to ground would be free, just like the intentionally grounded phase but, on both of them the second grounding short to ground would go boom.
I think I remember that it was considered optional.

If there are any Squid Electricians Mate on this forum, they might confirm for us that ship wiring uses a closed delta with ground monitors.

Just a little segue here.
Good thing about a closed delta is that if you lose a phase winding, you still have 57% of the transformer capacity.


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## chicken steve (Mar 22, 2011)

Which is _safer_?

I honestly _don't _know....

According to Mr Splatz' article we're to either ground a phase, _or_ introduce a phase monitor for compliance










I expect my AHJ will require a decision , go either way....

The thing about any given monitor is, it can _sing, quack_ or _whatever _they do down the hall in a closed MDP room for a month of sunday's until these artists turn the reggae down to investigate.

~CS~


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## splatz (May 23, 2015)

This certainly isn't something I really know enough about but



Southeast Power said:


> I'm having this theory that it isn't really any safer to not ground one of the phases.


I thought that corner grounded was considered generally safer than ungrounded, but the ungrounded was more fault tolerant - the equipment doesn't go down on the first ground fault. 

I don't get the safety part because, as you say below, the first ground fault is free. Is the second ground fault in an ungrounded system any worse than the first ground fault in a grounded system? 

Also, wouldn't ungrounded be safer with respect to unintentional contact with a live conductor, terminal, etc.? That is, you'd rather stand in a pan of water and stick a coat hanger in a pin and sleeve connector on an ungrounded system than on a grounded system. 



> The first short to ground would be free, just like the intentionally grounded phase but, on both of them the second grounding short to ground would go boom.


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## splatz (May 23, 2015)

chicken steve said:


> Which is _safer_?
> 
> I honestly _don't _know....
> 
> According to Mr Splatz' article we're to either ground a phase, _or_ introduce a phase monitor for compliance


Not just that article, you have to have the ground fault monitor for an ungrounded system. 



chicken steve said:


> I expect my AHJ will require a decision , go either way....


It's not surprising to me that it wasn't caught, let's be perfectly honest, people "out of their depth" work on this stuff, and inspect it. 

I am lucky, I know an older EE that's on the ball and charges fair prices. I'd get him involved in a situation like this. 

As in my last post, I don't quite understand the whole thing - it seems like the first ground fault is free and the second one costs the same as the first in a grounded system. (Confirming that I am out of my depth / over my head.) 



chicken steve said:


> The thing about any given monitor is, it can _sing, quack_ or _whatever _they do down the hall in a closed MDP room for a month of sunday's until these artists turn the reggae down to investigate.


I agree - is a monitoring system really meaningful without qualified people on site to watch it and respond appropriately to alarms?


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## chicken steve (Mar 22, 2011)

I can't do biz w/out _resources_ Splatz. 

We did a 4000A480V3PH service 5 miles from me , the poco had to provide a custom vault , _big_ doings for my podunk existence out here in dogpatch. 

The poco had a crack *EE* whom i solicited on that one , we checked, _double_ checked and _triple _checked _everything_, and i _still_ donned my entire arc flash suit to turn it on (_yeah i know, all for show_) 

Our state inspectors also have EE access , as well as a gaggle of other area's of expertise engineer level resources. 

The ground_ing_ gets me every time, guess i should attend another Soares course, if they ever blow on through here again (_Our IAEI chapter literally bribes J Sargent with maple syrup to speak_) 

~CS~


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## chicken steve (Mar 22, 2011)

splatz said:


> It's not surprising to me that it wasn't caught, let's be perfectly honest, people "out of their depth" work on this stuff, and inspect it.


Worse is....

I'm seeing what _may_ be un permitted work 

Yup, i've really _stepped in it_ on this one.....

~CS~


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## Mike_kilroy (Sep 2, 2016)

*so the way i'm reading this is, lift the Primary Wye off OX, connect Secondary Delta (shown here in A) to OX.... and i get>

NO NO NO NO!!!!!!!!!! 

*I suspect the above statement was just a typo as your jpg shows phase A grounded to ground, and you called it OA (phase A - I dont know easily how to make omega symbol).

BUT DO NOT TIE YOUR GROUNDED LEG TO OX!!!!!! LEAVE OX TERMINAL OPEN! TAPE IT OFF SO NO ONE EVER TRIES TO WIRE TO IT IF NEED BE!

Now, in YOUR case, you would not have blown anything up if you indeed tied your output phase to 0X terminal, but it would put lots of noise on your 480v secondary, plus there would be some currents flowing from any imbalance in the incoming 208V; That unconnected OX neutral on primary side will be a 0 of all three phases: if the input phases are not dead nuts identical voltages, then 0X will be offset from 0V. Tying it to the POCO source will then circulate that small offset, tying your output to it will do the same and as the incoming 208 voltages change slightly with changes on the 208 POCO side, that will be noise and you would be stuffing it right into the 480 loads. The heating elements in the kilns wont care, but I suspect there may be some electronics in those same kilns. More on the grounded corner issue at the end of this....

Whew. OK. Other than that, ok. '

I will STRONGLY suggest you follow normal rules of thumb and do NOT ground phase A but instead ground phase B - just to let anyone else ever looking at it not question your competency. Any phase grounded is the same, BUT common is B - even GE spec quoted above said B.





splatz said:


> Not just that article, you have to have the ground fault monitor for an ungrounded system.
> 
> As in my last post, I don't quite understand the whole thing - it seems like the first ground fault is free and the second one costs the same as the first in a grounded system. (Confirming that I am out of my depth / over my head.)
> 
> I agree - is a monitoring system really meaningful without qualified people on site to watch it and respond appropriately to alarms?


All good points here except I do not believe code anywhere requires a ground or ground fault monitor on delta systems. I may be wrong but I run into ungrounded deltas in industrial facilities a few times each year.

LAST COMMENT TO NOTE CS: Corner grounding introduces phase to ground voltage of 480 NOT 277v: MOST VFDs today have MOVs on their inputs... when they have MOV ph-ph, ok, but when they add MOV ps-ground, these are usually rated lower than 480V peak, and they WILL BLOW. Normally on 480v they will just pop - sometimmes as loud as a shotgun, but then blow open and so stink but the VFD is still ok. But it can pop and short, thus causing the VFD to appear broken...

In any case, I again assume at least some of those Kilns may have small VFDs for blowing air/heat around: if so, that grounded delta may blow some. Just be prepared for that. NOt sure if you want to be so proactive as putting a big note on wall saying "remove any MOVs phase-ground in equipment attached here" or "call equipment mfgr to check how to disconnect any MOV to ground units". OR, since you will be their electrician, you might save the day when/if equip blows up - just open the cover, find the blackened blown remains of the MOV and clip it out. Problem probably solved and he look like a hero.


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## frenchelectrican (Mar 15, 2007)

Chicken steve .,

As I mention before make sure if you going to grounded one phase on delta secondary.,, make sure you land it on Bravo phase not the Alpha phase as you see on diagram.

I always grounded at Bravo all the time but Alaha or Charlie phase not so .,, due majorty of manufacters always mention Bravo phase grounded. 

Now Mike Kilroy mention VSD that is a dang bat good reminder on that espcially a true full delta system ( you will know when you read phase to ground ) that can be a game changer if you are not aware of it.


Note : Mike there is a code about ground monitor but I dont recall where it was but I know it mention it at least once but I am not sure how well it was enforced on that part. 

Chicken Steve ., ya you can use the ground monitor but the issue is the customer will understand it ? if they have it own mantenance personal in there it may be ok but IDK .,,


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## Southeast Power (Jan 18, 2009)

Mike_kilroy said:


> *so the way i'm reading this is, lift the Primary Wye off OX, connect Secondary Delta (shown here in A) to OX.... and i get>
> 
> NO NO NO NO!!!!!!!!!!
> 
> ...




I missed that. :sad:

Im sure it was a typo or to state to remove the conductor from OX, keep it terminated to ground, and connect it to B Phase.


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## splatz (May 23, 2015)

Mike_kilroy said:


> All good points here except I do not believe code anywhere requires a ground or ground fault monitor on delta systems. I may be wrong but I run into ungrounded deltas in industrial facilities a few times each year.


I was just going by



> *250.21 Alternating-Current Systems of 50 Volts to 1000 Volts Not Required to Be Grounded.*
> 
> (A) *General.* The following ac systems of 50 volts to 1000 volts shall be permitted to be grounded but shall not be required to be grounded:
> 
> (1) Electrical systems used exclusively to supply industrial furnaces for melting refining tempering and the like


The kilns would appear to fit... 

But 



> (B) *Ground Detectors. * Ungrounded alternating current systems as permitted in 250.21(A) operating at not less than 120 volts and not exceeding 1000 volts shall have ground detectors installed on the system.


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## Southeast Power (Jan 18, 2009)

I think it would be easy to make a ground detector out of some 277 volt rated lamps connected phase to ground.
Im thinking they would be dim backfeeding through the other lamps and full bright when a fault occurs.


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## frenchelectrican (Mar 15, 2007)

Southeast Power said:


> I think it would be easy to make a ground detector out of some 277 volt rated lamps connected phase to ground.
> Im thinking they would be dim backfeeding through the other lamps and full bright when a fault occurs.


That is most simple old school trick I done that for many years. 

And I used that when I am checking the ground fault on floating delta.


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## Mike_kilroy (Sep 2, 2016)

Southeast Power said:


> I think it would be easy to make a ground detector out of some 277 volt rated lamps connected phase to ground.
> Im thinking they would be dim backfeeding through the other lamps and full bright when a fault occurs.


Right. 480v bulbs tho. 3. 1 ea ph to ground. None lit = good. Two lit gf on the unlit ph.

Sent from my SM-G900V using Tapatalk


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## paulengr (Oct 8, 2017)

I think I remember something in the Code about supervised or qualified personnel with ungrounded systems so it can be a Code violation based on who watches it. Insurance companies also have strict rules on these things. You have to treat ground faults like a fire alarm. As to fault monitors Bender is the best for electronic but much simpler and most common is phase lights. Just hang 3 480 V rated pilot lights (like a CH/AB/SqD 800 series industrial panel light) phase to ground. All 3 dim = good. Two bright, one dim=bad.

The THEORY with ungrounded deltas is it's more reliable on bolted faults and this is absolutely true if it's maintained properly. In fact if it's not it's far worse than 'boom'. You get a line-ground-line fault and since ground resistance is usually not that low the overcurrent protection never sees it so we get burn down..not boom, more like COOK with lots of damage. Insurance claim will then require getting rid of that ungrounded crap.

BUT the reality is even worse. In an ungrounded arcing fault, it charges the system capacitance and turns it into a voltage multiplier. Voltage increases ABOVE line voltage until the insulation fails at the weakest point typically in motors where insulation is only rated for about 1200 V surges compared to "600 V" wiring which is tested to over 1800 V. In ungrounded deltas operations expect about 4 times the failure rate on your electrical equipment because that's what the Navy failure rate is doing this stuff where it's Code for them. Actual measured voltage usually goes to about 600-800% according to IEEE if something doesn't fail first and the failure point can be anywhere, usually not at the fault. IEEE grounding book mentions one case where over 40 motors were destroyed one by one over several hours before anyone figured out what was going on.

The explanation is a little hard to follow. Take my word for it or read this:

https://www.google.com/url?sa=t&sou...FjAAegQIExAB&usg=AOvVaw2__vN0bJ1jTKjyDNkTy5xh

The best solution is a high resistance ground which gives you both high reliability and freedom from transient issues. It costs a little more up front but saves a lot of money long term because arcing faults basically don't happen and equipment damage in a fault is usually so minor it can be hard to find. I mean in a 480 system we can limit ground faults to say 15 A...#16 wire can be shorted to ground and not even smoke the insulation. In a new install just order the transformer delta-wye. In your case you buy a zigzag or a delta-wye transformer and connect a power resistor and ground fault alarm which is really just an overcurrent alarm on the transformer wye. Transformer rating of 10 kVA is plenty for this and a cheap Carlos Gavazzi single phase current relay with a trip setting of about 60% of the resistor size so say 9 A on a 15 A resistor (recommend Post Glover...That's who everybody sells) with a 2-10 Second delay is plenty. You can get it all in a box for a couple thousand from Federal Pacific or some others. iGard is WAY overpriced and GE doesn't know what they are doing. Don't buy that pulser crap either. Works less than 50% of the time no matter how good the sales pitch sounds.

PM me if you want to go this way. I have decades of experience with grounding in mining and chemical plants which have all kinds of screwy grounds.

Sent from my SM-T350 using Tapatalk


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## chicken steve (Mar 22, 2011)

well now i'm all confused and intimidated :crying:, is there a pix or schematic showing what i need to do with OX? I thought i was creating an SDS, with GEC's etc.....?:surprise:~CS~:surprise:


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## paulengr (Oct 8, 2017)

Southeast Power said:


> I missed that. :sad:
> 
> Im sure it was a typo or to state to remove the conductor from OX, keep it terminated to ground, and connect it to B Phase.


That makes it an extension of the primary side so primary side faults affect the secondary side and vice versa.

Grounding the primary side is a good idea. It helps clear faults on that side but it does set up a multigrounded system. If it's utility, leave it grounded. If it's yours, I'd decide to ground or not based on distance.

On the secondary side for anything other than very small loads, give it it's own ground rod and system bonding jumper. This makes it a separately derived system. The impedance through ground faults to the primary side is infinite. This avoids utility faults affecting your system and vice versa. On small control power transformers only they are often bonded and jumpered to the primary side to avoid the extra cost of a separately derived system. Wye-wye transformers are the only common ones with no ground fault isolation. Too bad utilities love them.

An "ungrounded" deltas by the way isn't ungrounded. It's capacitively grounded through the insulation, which is what causes all the damage to them.

Sent from my SM-T350 using Tapatalk


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## Mulder (Sep 11, 2010)

chicken steve said:


> well now i'm all confused and intimidated :crying:, is there a pix or schematic showing what i need to do with OX? I thought i was creating an SDS, with GEC's etc.....?:surprise:~CS~:surprise:


For a wye-delta nothing gets connected to the primary XO terminal. The secondary does not have an XO.


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## Mike_kilroy (Sep 2, 2016)

Mulder said:


> For a wye-delta nothing gets connected to the primary XO terminal. The secondary does not have an XO.



Absolutely correct. Period


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## Mike_kilroy (Sep 2, 2016)

paulengr said:


> 1) That makes it an extension of the primary side so primary side faults affect the secondary side and vice versa.
> 
> 2) Grounding the primary side is a good idea. It helps clear faults on that side but it does set up a multigrounded system. If it's utility, leave it grounded. If it's yours, I'd decide to ground or not based on distance.
> 
> ...


Ok guys, ban me if you want, but I must object to bad untrue comments. I will take each separately. I too am an engineer, with now 43 years industrial experience.

1) Total nonsense. If you tie ONE wire to primary side, how do you flow any current to the secondary side? this statement is flat wrong.

2) Dangerously wrong! Tying the primary wye Xo to POCO ground is flat wrong! Xo is the vector 0 of all 3 input phases on the wye transformer. POCO gives you their zero to use. If any voltage on the 208 input is different than another, this transformer's Xo will be at something OTHER than POCO zero volts supplied on their ground. Tie this xfmr Xo to POCO ground and you GUARANTEE a voltage difference, and thus current flow. Enough, and you burn up your transformer.

3) Who said the secondary had to be a "separately derived system?" This customer has kilns to power. That is all.

3a) Nonsense! Stating that driving a ground rod and attaching to secondary does NOT connect to primary side ground rods negates everything everyone knows about electricity! Of course they are LOW IMPEDANCE connections between both!!!

4) Of course everything has capacitance to ground - even the space station miles overhead has capacitance to ground. To state an ungrounded delta is not ungrounded does injustice to electricity!

I will also add a comment about "ungrounded" deltas. Not too many years ago many European VSD drive manufacturers decided to sell their wares in USA. ALL FAILED their first 1 - 2 years with blown up drives and they could not understand why! Took them YEARS to figure out that we had a lot of manufacturing plants on ungrounded delta POCO sources: ALL Europe is 380 grounded wye. The result was ALL these VSD mfgrs blew up drives like popcorn! If it was not a lightening strike 4 miles away, it was welding equipment, or VSD PWM switching noise, or other equipment starting on the ungrounded delta power source. These things cause huge voltage spikes since the delta was NOT grounded. ALL these VDS mfgrs blew input diodes and PCB traces since these spikes cause ALL THREE PHASE VOLTAGES TO SPIKE 2000, 3000, 7000V above ground. Even though the ph-ph voltage remained 480, the ph-ground spikes since it was simply capacitively coupled to ground. It usually took these European mfgrs 1 year each to figure it out and either 1) tell USA customers NO UNGROUNDED DELTA ALLOWED, or B) increase their PIV ratings on diodes by a factor of 3-4 and increase pcb trace spacing to allow for 3-4000v between traces. So the moral of the story is at least ground ph B! then all 3 phases are referenced with LOW impedance to ground and you do not have to have 4x normal amount failures like our Navy. Some people never learn.

OK, soapbox off. Now ban me.


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## MTW (Aug 28, 2013)

chicken steve said:


> [
> I'm unsure how, i get no readings H to G , so i'm lost as to how the fault path occurs back to XO , especially if there is no connection to it


It doesn't. That's the whole point of an ungrounded system.


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## paulengr (Oct 8, 2017)

Mike I think we're closer than you realize on some of this.



Mike_kilroy said:


> 1) Total nonsense. If you tie ONE wire to primary side, how do you flow any current to the secondary side? this statement is flat wrong.


This is a bit confusing here. In a wye-delta where the neutral is on the primary side, fault current through that connection flows through the primary side, not the secondary. So it influences conditions on the primary side but not the secondary side.

Before I get to the other points, let's clear a couple misconceptions about grounding and transformers up that explain my position and some of yours.

In three phase power, ground faults do NOT flow through a delta-wye transformer, period. There is a sixty degree phase shift so ground current cancels out. In a wye-wye transformer, they flow straight through as if the neutrals are tied together. Whether the neutrals are physically tied together or not is immaterial...electrically, they are connected together. There is a nice chart showing this in the IEEE Green book (grounding book). PM me and I'll see if I can dig it out if you need it.

Second, what is the resistance between ground rods? You stated that it is "low". This statement is only true some of the time. Soil resistivity varies all over the place but a basic formula that is commonly repeated in almost every 3 point ground tester is R=rho/(2*pi*a) where rho is the soil resistivity in ohm-meters and a is the distance between ground rods in meters. Close in the soil resistivity dominates the resistance so we get kilo-ohms of resistance. The grounding electrode resistance might be very low (under 25 ohms) but the soil resistance itself is pretty high. As the distance between grounding electodes increases, "a" takes effect and at around 3,000-5,000 feet in my experience the soil resistance is much less than a ground wire of the same length. So your statement is true at long distances (utility distances) but is not true for typical commercial, residential, and industrial sites where distances are measured in feet instead of miles. At residential distances, ground rod resistance to another ground rod is several kilo ohms. At utility distances, the resistance between ground rods is almost zero, limited only by the resistance of the ground rod connection to the soil itself, and they typically follow IEEE Standard 80 for substations which recommends a resistance of less than 1 ohm. So for utilities there really is no need or point in running a 4 wire wye distribution system..a ground rod is a better source of grounding/bonding. The 4th wire would be wasted copper.

Third, there is something called a multigrounded system and a single point grounding system. The general design for utilization systems (industrial, commercial, residential) is usually single point grounding. It also more or less obeys the (incorrect) phrase that electricity follows the path of least resistance. This phrase is obviously incorrect because it follows ALL paths proportional to their conductances (inverse of resistance) but as long as one path is much lower in resistance it gives the illusion that it actually follows this path. So considering the conditions when it comes to grounding for typical industrial/residential/commercial systems where ground rod spacing is not measured in miles, they are effectively isolated so the concept of "follows the path of least resistance" and "single point ground" appears to be true. But at pole line and substation distances, effectively Earth is a very low impedance conductor and fault flows through multiple grounds simultaneously. Thus utilities use multigrounded systems. Code (NESC) specifies that every pole is grounded, generally static lines are used to minimize voltage differences as you described, and there is a lightning arrester at least every 5 poles. Since most transformers are wye connected and grounded, they perform as a ground and share the fault current load with every pole in the system.



> 2) Dangerously wrong! Tying the primary wye Xo to POCO ground is flat wrong! Xo is the vector 0 of all 3 input phases on the wye transformer. POCO gives you their zero to use. If any voltage on the 208 input is different than another, this transformer's Xo will be at something OTHER than POCO zero volts supplied on their ground. Tie this xfmr Xo to POCO ground and you GUARANTEE a voltage difference, and thus current flow. Enough, and you burn up your transformer.


This is simply not true for three reasons. The first one is that this makes no sense at all. If there is a voltage imbalance wouldn't this lead to a current imbalance when you connect your wye transformer connection and force the purported voltage differences across it to zero? It forces the ground-neutral voltage to zero but GPR is an entirely separate issue mostly caused by capacitive coupling.

Second, If I connect X0 to a ground, I guarantee a voltage difference? What? I've shorted them together...that's the definition of NO voltage difference. Current flow on the other hand I can't disagree with but this assumes that on the utility system, your transformer is the one and only path between ground and line voltage. Again with the assumption that electricity follows "the path of least resistance", or impedance in this case. So each of our phase conductors is separated from Earth and from each other by insulation whether of the solid kind or just plain air, right? And the Earth is a conductor, right? That's the definition of a CAPACITOR. Ungrounded systems are capacitively coupled. In fact you argue that point later on. The lines, static lines, pole grounds, other transformers, all make up part of a multigrounded system. If what you were saying is true then utilities would routinely blow every transformer to pieces in every substation and on every pole transformer. It doesn't happen though because even though for instance ground potential rise is a real thing, it's purely a local effect. In a truly "ungrounded" (capacitively grounded) system, you can outright tie a phase to ground making a corner grounded system either intentionally or through a fault. When you do this, you will see the system charging current which can vary from almost nothing to several amps of current. Even if you float X0 this current still exists and you can see it at the source transformer metering, but it is distributed throughout the system. There are even ways of estimating how much it is, but most of it exists because of system capacitance. In high resistance systems this is even more obviously because you will actually see it as a reading at the protective relay that measures ground fault current. It is hidden in ungrounded systems but it still exists.I work on medium voltage equipment all the time. I've seen the system capacitance sparks fly. It's very real but I've never seen it get high enough to destroy transformers. A few amps just isn't going to do it, and the problem is also proportional to voltage so at 208 VAC, it's probably not even noticeable.

In addition there has been an ongoing debate amongst engineers for years arguing for a single point grounding system vs. multigrounding. In theory we want a single point grounding system back to the system bonding jumper because that way we control where the current goes. That works just fine when dealing with distances less than 1,000 feet. But at utility distances, resistance through Earth is less than through copper wire. At that point multigrounding applies because for all practical purposes, single point grounding theory no longer works. And as stated earlier, just stick a ground rod anywhere you need one. No bonding needed. This isn't an argument against not using static lines though...those are there for an entirely different purpose just as the ground rod is not the same as bonding.



> 3) Who said the secondary had to be a "separately derived system?" This customer has kilns to power. That is all.


You just argued for isolation from the POCO then argued to skip isolation. Huh? If you tie the bonding on the secondary side of the transformer into the primary side, it's not a separately derived system. If you isolate them...that is provide your own ground rod and tie the bonding on the secondary side to that, it is a separately derived system. There isn't an obvious physical connection to the neutral on the secondary side because it's an ungrounded system except that this is illegal so normally phase lights or a Bender style fault relay is used. A connection still exists though...it's just capacitive. Leaving it as a separately derived system still entails proper bonding. It's just that there's no connection on the secondary side because there is no "neutral" as such.



> 3a) Nonsense! Stating that driving a ground rod and attaching to secondary does NOT connect to primary side ground rods negates everything everyone knows about electricity! Of course they are LOW IMPEDANCE connections between both!!!


It's basic electricity. Let's pretend we have two voltage sources and two loads connected in series so I have two circuits. Now let's connect them together. What current flows across that connection? Nothing. They are not connected together. Each one appears as a separate system. The resistance between them can be zero but it doesn't matter because we're not completing the circuit.

Now moving on to transformers, with a delta-wye transformer there is a 60 degree phase shift from one side to the other. That's important because that means that there is no ground fault path between them. If we have ground faults, they are the same as the "two circuits" example above. I'd argue that the resistance between two ground rods in relatively close proximity (more than say one ground rod apart) is very high and that if they are very close together or very far apart, or even electrically tied together, it really doesn't matter because they are two separate systems. Current flow is zero, period. BUT if they are two wye-wye transformers then there is a direct neutral connection since the phase shift is zero. Now the ground impedance does matter because it completes the circuit.



> 4) Of course everything has capacitance to ground - even the space station miles overhead has capacitance to ground. To state an ungrounded delta is not ungrounded does injustice to electricity!


I think you meant to state that it is "ungrounded" since obviously it's not. It's just grounded via capacitance. NEC recognizes it too. Bonding doesn't go away just because there isn't a jumper to X0 since there is no X0 to jumper to.



> I will also add a comment about "ungrounded" deltas. Not too many years ago many European VSD drive manufacturers decided to sell their wares in USA. ALL FAILED their first 1 - 2 years with blown up drives and they could not understand why! Took them YEARS to figure out that we had a lot of manufacturing plants on ungrounded delta POCO sources: ALL Europe is 380 grounded wye. The result was ALL these VSD mfgrs blew up drives like popcorn! If it was not a lightening strike 4 miles away, it was welding equipment, or VSD PWM switching noise, or other equipment starting on the ungrounded delta power source. These things cause huge voltage spikes since the delta was NOT grounded. ALL these VDS mfgrs blew input diodes and PCB traces since these spikes cause ALL THREE PHASE VOLTAGES TO SPIKE 2000, 3000, 7000V above ground. Even though the ph-ph voltage remained 480, the ph-ground spikes since it was simply capacitively coupled to ground. It usually took these European mfgrs 1 year each to figure it out and either 1) tell USA customers NO UNGROUNDED DELTA ALLOWED, or B) increase their PIV ratings on diodes by a factor of 3-4 and increase pcb trace spacing to allow for 3-4000v between traces. So the moral of the story is at least ground ph B! then all 3 phases are referenced with LOW impedance to ground and you do not have to have 4x normal amount failures like our Navy. Some people never learn.
> 
> OK, soapbox off. Now ban me.


No disagreement there. Insurance companies hate them for good reason.

But the VFD situation is a little worse than described.. PIV doesn't really matter although what you are describing (transients due to intermittent ground faults) is a big problem for everyone that is arguing for ungrounded systems as being more reliable when they are just the opposite.

The semiconductor paths are all basically line-to-line but there is a nasty little property of semiconductors used in VFD's called self-commutation. When the voltage gets over a certain level which is not very far from normal operating voltages (and can't ever really be much higher), the semiconductor will fire REGARDLESS of whether or not voltage is applied to the gate. This is the reason for all the warnings about not using a megger on a VFD. If the semiconductor goes into self commutation then we get a current flow straight through the drive (dead shorted) and very quickly within seconds, the semiconductor grows magnetically propelled rocket legs and ejects as shrapnel. The solution of course is to install surge protection within the drive to avoid any chance of this ever happening since obviously we can't just fault/trip the drive since it conducts with no power on it. In a solidly grounded system we can use a MOV set for about 300 VAC connected from line to ground. If however we're on an ungrounded delta or a high resistance grounded system, we need to make line-to-line connections and increase to at least 500 VAC although transients can get as high as 140% of line voltage so a safe bet is 700 VAC. And by the way, the 300 VAC MOV went up in smoke in seconds of being installed if there is ever any line imbalance for any reason (like a fault). And if there are no LC/RC snubbers to protect the drive from "itself" (such as on very cheap diode front end pump/fan drives), it will destroy itself in short order after that. These days the drives I see either have only line-to-line MOV's or they have a jumper and both sets so you remove the jumper if you don't have a solidly grounded wye distribution. When you are wiring up a drive one of the first things to look for is some kind of screw or other "secret" jumper if you have an ungrounded system. On some cheap drives you actually have to take a piece of dikes and physically cut the MOV's off the board. If you're solidly grounded then don't worry about it.

In a diode it's called the avalanche point and it works essentially the same way in reverse bias conditions as it does with SCR's, MOSFET's, and IGBT's but the result is the same...total destruction of the semiconductor. The big difference is that diode PIV's are multiples of the working voltage where in the other devices, it's not that high and never will be. It's inherent in the device. So the antiparallel and rectifiers diodes are not really an issue at all...it's the other devices that cause all the fuss.


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## chicken steve (Mar 22, 2011)

:surrender:*<* the ET symbol for having your _azz handed to you_ from an EE....~C:vs_OMG:S~


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## chicken steve (Mar 22, 2011)

and i'm 3 cups of joe into theroyville reading you guys, along with my infantile understanding of earthing systems wondering wtf i'm going to do to pass when my ahj blows through.....big dude, pointy teeth, only smiles when he has gas..... ~CS~


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## chicken steve (Mar 22, 2011)

On the bright side, i'm starting to_ possibly_ grasp why the welding shops i've roped up have had mysterious troubles......one even smoked a pole pig ~CS~


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## RePhase277 (Feb 5, 2008)

Why are you concerned about the AHJ? There have been several suggestions in this thread about what course you should take to make this right.

I don't believe leaving this ungrounded and using a detector is an option due to the lack of qualified supervision. Either corner ground it or rip it out and replace the transformer with a delta-wye.


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## MTW (Aug 28, 2013)

RePhase277 said:


> Why are you concerned about the AHJ?


Agreed. Most AHJ's here wouldn't even have a clue about the principles of an ungrounded system. 

Secondly, isn't an ungrounded system _preferable _here? I assume that's why they installed it in the first place. Kiln systems can't suffer a catastrophic loss due to an inadvertent shutdown with an ungrounded system.


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## chicken steve (Mar 22, 2011)

RePhase277 said:


> Why are you concerned about the AHJ? There have been several suggestions in this thread about what course you should take to make this right.
> 
> .


Liability 277.

~CS~


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## RePhase277 (Feb 5, 2008)

MTW said:


> Agreed. Most AHJ's here wouldn't even have a clue about the principles of an ungrounded system.
> 
> Secondly, isn't an ungrounded system _preferable _here? I assume that's why they installed it in the first place. Kiln systems can't suffer a catastrophic loss due to an inadvertent shutdown with an ungrounded system.


In an industrial steel mill I would agree. But that place would have a supervisory maintenance crew. Steve's job is a bunch of hippie artists. I don't think they should even get into smelling distance of an ungrounded 480 volt system.


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## RePhase277 (Feb 5, 2008)

chicken steve said:


> Liability 277.
> 
> ~CS~


What liability do you have for grounding this to code?


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## MTW (Aug 28, 2013)

RePhase277 said:


> Steve's job is a bunch of hippie artists. I don't think they should even get into smelling distance of an ungrounded 480 volt system.



Neither should Steve.


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## chicken steve (Mar 22, 2011)

RePhase277 said:


> What liability do you have for grounding this to code?


I'll be extending 4803ph to a number of places 277

Those feeders need to be properly grounded to code(s) mentioned by fellow posters here to pass inspection.

A '_final_' protects EC's , at least in the sense of '_that's how it was left_'

what they do to or with it afterwards (and they will) will not concern nor include my company

~CS~


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## chicken steve (Mar 22, 2011)

So i _finally_ made it back to the job>
208 / 120 3 ph Wye into an 800A main, big MBJ block in it fans N&G out everywhere, including a 2/o cu to the sprinkler main
this is the main switchgear/ xfomer>









It all goes into a Buss duct>









The Xformer itself has no XO, It's bonded everywhere. 
Square D cat # 150T64H>>
http://static.schneider-electric.us/assets/sup-obs-digest/transformers-sd.pdf










here's a quick cartoon of the 480>










~CS~


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## chicken steve (Mar 22, 2011)

It would be greatly appreciated if anyone knows of a SqD rep up on corner grounding this particular model. Also I am confused the end result of corner grounding in that this is all straight 3 ph loads. But what about single ph loads? Is it a phase, or a neutral ? Y/N, Both? :vs_OMG:~CS~:vs_OMG:


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## Wiresmith (Feb 9, 2013)

chicken steve said:


> Yeah, been looking , but just can't quite put my finger on the 'letter of code' here.....:crying:



i know im a little late but 250.21(a)(1)

if its exclusively for melting furnaces it can be ungrounded with ground detectors(250.21(b))



chicken steve said:


> It would be greatly appreciated if anyone knows of a SqD rep up on corner grounding this particular model. Also I am confused the end result of corner grounding in that this is all straight 3 ph loads. But what about single ph loads? Is it a phase, or a neutral ? Y/N, Both? :vs_OMG:~CS~:vs_OMG:




its a grounded phase, its neutral to earth, its a grounded conductor


something to keep in mind
one problem i have seen when a high impedance grounded system gets a ground fault (corner grounds) is with vfds seeing a ground fault


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## Wiresmith (Feb 9, 2013)

on a system like a melting furnace, sometimes you do not want a ground fault to shut down the furnace because it can ruin the product, so you have a detector and when the process is complete you can orderly shut it down for repair


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