Transformer wierd voltages

...there is nothing wrong with having a corner grounded 480 volt system.

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Except that it means you will have the problems that you are having with the voltage. If your primary is corner-grounded, then I bet your secondary is not grounded, which presents a real hazard to personnel. There may not be enough short-circuit current to trip a breaker on the secondary. Another problem is that the secondary voltages-to-ground are now "floating" and may jump around.

In my opinion grounded delta 480 volt power systems present real safety hazards and should be prohibited. One phase is already shorted to ground. When a second phase faults, then there is a phase-to-phase fault, much more severe than the normal fault-to-ground. Phase-to-phase faults usually result in something getting burned up.

There is the case of an outside contractor coming in for a job. "Better check the voltage to make sure this circuit is off, Joe". "Moe, I checked to phase C and it shows 0 volts, so the main breaker must be off." Joe then sticks his hand in a box and gets fried.

According to NEC the 480 must be grounded either corner or with a center tapped phase (which gives 240/480) and the secondary must also be grounded (in this case wye) to give me 120 / 208.

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I must have missed the requirement for a corner-grounded Delta primary. Would you kindly point me to the NEC Article that requires that grounding method? I do not find that requirement in either Article 250 (Grounding) or 450 (Transformers).

I'm definitely with you on outlawing corner-grounded 480V systems. They are a hazard that is totally unnecessary. I have seen way to many older systems where the phase to ground insulation has very little leakage at 277V and unacceptably high leakage at 480V. At that's exactly what corner grounding does--it forces two of the three phases to 480V above ground.

Leaking phase-to-ground voltage on a corner-grounded 480V source almost killed me some years ago. I was checking out an old analog DC drive for upgrade to AC or digital DC. Having opened the disconnect and checking all the lights and meters for 0V, I pulled myself up on the machine with my left hand and reached into a control box to get drawings. As my right hand passed by the armature ammeter, POWWWWWW! 700VDC thru me, arm to arm to ground. Ends up the corner grounded floating 480V was leaking thru the old insulation and the drive rectifiers were rectifying the 480VAC to 700VDC just waiting for me to stick my hand in there.

I've been on a campaign against floating power networks since then.

DickDV said:

I'm definitely with you on outlawing corner-grounded 480V systems. They are a hazard that is totally unnecessary. I have seen way to many older systems where the phase to ground insulation has very little leakage at 277V and unacceptably high leakage at 480V. At that's exactly what corner grounding does--it forces two of the three phases to 480V above ground.

Leaking phase-to-ground voltage on a corner-grounded 480V source almost killed me some years ago. I was checking out an old analog DC drive for upgrade to AC or digital DC. Having opened the disconnect and checking all the lights and meters for 0V, I pulled myself up on the machine with my left hand and reached into a control box to get drawings. As my right hand passed by the armature ammeter, POWWWWWW! 700VDC thru me, arm to arm to ground. Ends up the corner grounded floating 480V was leaking thru the old insulation and the drive rectifiers were rectifying the 480VAC to 700VDC just waiting for me to stick my hand in there.

I've been on a campaign against floating power networks since then.

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DickDV - OK, I know I haven't worked on nowhere near as many motor systems as you, but I am very confused by this post. How is a corner grounded system "floating"? To me that is a contradiction of terms. Also what do you mean by "leaking voltage"? I've seen current leakage, but I don't know what voltage leakage means, or how 480 volts leaks to ground. Also, I don't understand how you could have a disconnect "open" and have any voltage anywhere. If you could please help me understand what you are talking about, I would appreciate it.

Lancie1 said:

Except that it means you will have the problems that you are having with the voltage. If your primary is corner-grounded, then I bet your secondary is not grounded, which presents a real hazard to personnel. There may not be enough short-circuit current to trip a breaker on the secondary. Another problem is that the secondary voltages-to-ground are now "floating" and may jump around.

In my opinion grounded delta 480 volt power systems present real safety hazards and should be prohibited. One phase is already shorted to ground. When a second phase faults, then there is a phase-to-phase fault, much more severe than the normal fault-to-ground. Phase-to-phase faults usually result in something getting burned up.

There is the case of an outside contractor coming in for a job. "Better check the voltage to make sure this circuit is off, Joe". "Moe, I checked to phase C and it shows 0 volts, so the main breaker must be off." Joe then sticks his hand in a box and gets fried.

I must have missed the requirement for a corner-grounded Delta primary. Would you kindly point me to the NEC Article that requires that grounding method? I do not find that requirement in either Article 250 (Grounding) or 450 (Transformers).

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Lancie
NEC requires 480 to be grounded at the main service does it not?
That only leaves you two choices corner or center tapped. So if you have a transformer fed from 480 to the primary and secondary of 208 120 then your primary is referenced to ground at the service.

Dan

Dan, 480V three phase does not need to be grounded at the service. That is the core issue with floating systems. You have a transformer with three 480V secondary coils, all three attached end to end to form a delta. There is no neutral and no ground, thus the term "floating.

brucechase, yes some of my post is not precisely correct. I'm sitting here with only half my brain working (got this miserable flu!). I should have said phase-to ground leakage CURRENT.

The leakage in this particular case occurred ahead of the disconnect, leaking into circuitry after the disconnect. A floating system is described above but, the problem is that a single ground fault can occure and the system keeps operating with one phase conductor grounded (corner grounded, its called). Of course, that's the big selling point---you can operate thru a single ground fault without clearing any fuses. Of course, as the story goes, at the next available maintenance window, the ground fault will be located and cleared (yeah, right, that fault will stay there until the second ground occurs and starts to clear fuses! At least, its that was in the facilities I visit!).

The whole point is that a "floating" system soon becomes a corner grounded system and stays that way. Dangerous!

Thanks Dick. I see I got some studying to do.

Dan

DickDV said:

There is no neutral and no ground, thus the term "floating.

brucechase, yes some of my post is not precisely correct. I'm sitting here with only half my brain working (got this miserable flu!). I should have said phase-to ground leakage CURRENT.

The whole point is that a "floating" system soon becomes a corner grounded system and stays that way. Dangerous!

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Okay, but where does anything mention a floating system. The OP stated he had a corner grounded delta. This would prevent exactly what you are describing with a floating system. If anything, it almost sounds like you would prefer a corner grounded delta to a floating delta. I personally prefer a 480 wye system with the neutral grounded but only pull the 3 phase conductors for all the motors (no neutral voltages). The point of the OP is that the primary is grounded and the secondary NEEDS to be grounded at the center point of the wye. The secondary is not grounded by the voltages shown. And as said many times, the fact that the primary is a corner grounded 480 delta has little bearing on the secondary. I'm not sure of your meanings in the 2 posts. If you want to outlaw 480 V corner grounded systems, do you want the floating system? Do you prefer a wye system over a delta? Just trying to understand.

You can wait until you get a little better if you need to before you straighten me out.

Dan, neither the 480 nor other voltages are required to be grounded by the NEC (except in residential). I've even worked on 2300V that were ungrounded for the exact reason stated by DickDV. In that situation, it was a big power system and the boiler ID and FD fans had to run to keep combustion gases out of the boiler.

There is nothing inherently unsafe from a corner grounded system any more than there is from a center tapped system. A corner grounded system is not already "shorted" to ground it is bonded. Ungrounded (floating) systems do have some safety issues.

Many people do not like corner grounded or ungrounded delta systems because they have no experience with them. I personally would not design a new service with either of these systems but that is my personal opinion and it is not based on any engineering reasoning. All delta systems may require non-common circuit breakers and newer VFD's are not usually compatible with them (primarily becuse of the way they are filtered). Many electricians are unfamiliar with them so these systems tend to cause issues during troubleshooting (as evidenced by this topic)

All systems must be grounded per the NEC. The NEC also requires most systems, not just residential, to be bonded to ground to provide a path for fault current to return to its source. If you do not understand the difference between grounding and bonding, and, grounded and grounding conductors you may have a hard time understanding the reasoning behind; ungrounded, corner grounded, high resistance grounded, and solidly grounded systems. For what its worth, I have dealt with all of these different systems since the begining of January and all were within 25 miles of my office.

In my view, a floating delta secondary from a substation or distribution transformer might as well be viewed as a corner-grounded system. When you least expect it and with only a trio of lights in the substation to give any warning, it can transition from floating to grounded.

Now, if all of your insulation is perfect and rated for 460V, corner grounding raises few safety issues. But, in the world I live in, that quality of insulation is rare and, further, is never tested for integrity after installation.

Any electrical equipment, especially VFD's, that have to do ground fault protection, cannot operate properly on off-balance grounded power sources. In my work with AC and DC drives, I will not start up any DC system that is fed by anything but a grounded wye source. Don't have it???---get me an isolation transformer and I'll be back to commission the system. The reason is simple. Consider that, as the SCR's fire in the DC drive, you are effectively yanking the motor armature from ground to 460VAC above ground at 120Hz. Not exactly what the motor was designed for. Consider the capacitance and inductance in that kind of circuit and the currents that must flow to charge and discharge at 120Hz. Personally, I won't be party to it. Get a transformer and I'll be back!

As for AC drives, I will commission them on floaters or corner grounded sources but all of the phase to ground surge suppressors must be disconnected. If the drive has a CE label (most do these days) the CE noise suppression network inside the drive must also be disconnected from ground. Then, the phase loss and ground fault protection to the motor must be deactivated in the software to stop nuisance faulting. Doesn't this sound like a lot of craziness?

Now, let's talk about small AC drives on floating systems. Generally, there is some potential for phase-to-ground leakage currents thru-out a facility. If there is no return path to the supply transformer, these currents are blocked and cannot flow. Now, introduce a small VFD into this system. It has an internal network to establish its own virtual ground so it can do its motor protection properly. The drive supply is fused at typically 10amps. Whatever leakage current there is in the whole facility immediately diverts to and thru the drive back to the transformer. If this current exceeds 10amps, the VFD fuses will clear as soon as you power up. NO, I didn't say "run the motor", just power up! The leakage currents are high enough to clear the fuses without the drive ever becoming active. Try to explain to your customer now just why each drive needs an isolation transformer to keep the fuses from blowing. The most common response is to be escorted to the exit with instructions to never set foot on the property again!

Sorry, but you can just feel my fond affection for floating power systems, can't you? Give me a grounded wye source, any day!

DickDV summed it up in my view.
In post 5 I asked about type of starter used for the reasons DickDV summed up.

After reading posts above, I am glad I am an Electrician in New Zealand where we use the earthed star (WYE) point on our 3 phase secondary's.

Sorry, but you can just feel my fond affection for floating power systems, can't you? Give me a grounded wye source, any day!

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I second the thought. Why install a new delta ungrounded or corner-grounded system that has known problems, presents a safety hazard to any unsuspsecting electrician or maintenance person, and will most certainly be brought up in wrongful-death court cases?

Many people do not like corner grounded or ungrounded delta systems because they have no experience with them.

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I worked for many years in a plant that had delta-configured secondary 480 volt transformers. I have worked as a contractor in serveral other places that had ungrounded 480 volt power. That is the main reason I have learned to hate and fear any power system that does not have a balanced 3-phase hard ground. That includes ungrounded delta, corner grounded, delta with one center-tapped phase, grounding transformers, and resistor-grounded transformers.

After much investigation and experience, I think these ungrounded delta systems were installed originally to duplicate how the electric power utility substations and transmission lines are designed.

The idea for transmission lines and transformer substations is that keeping the power on is very important. A tree leaning on one overhead line should not trip the line breaker (at least not for the first 3 or 4 times), and only highly trained and experienced people will be working on the equipment, using insulated bucket lifts, rubber gloves, hot sticks, and other protective tools. In a misguided effort to extend this reasoning into industrial plants, many delta-wired plant transformers were installed from 1900 to 1950 in the U.S. Inside plants, many untrained, poorly trained, or simply unsuspecting people HAVE come into contact with these ungrounded power systems.

Since then, most of us have learned a safer way, the good old delta-wye transformer with a grounded neutral on the secondary side only.

Dan, I would suggest a long-term program to get rid of this hazardous system. Is keeping the power on more important than any one person's life?

I worked for many years in a plant that had delta-configured secondary 480 volt transformers.

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It was an old converted war-munitions plant (U.S. Nitrate Plant #1, Muscle Shoals, AL), built originally in 1917 during World War I. During this war, equipment was valuable and irreplacable, down time was not allowed, and human bodies were considered relatively expendable. The delta ungrounded power system was used throughout, with underground feeders and manholes. During the construction of this plant, thousands of laborers were brought in to make and lay bricks. An influenza epidemic swept the country (no antibiotics or vaccines). Hundreds of unidentified men, living in tents at this construction site, died and were buried in mass graves with bulldozers.

Is keeping the power on more important than any one person's life?

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I think that in most non-military plants during peaceful times, life is now considered to be more valuable than production.

Dan, I would suggest a long-term program to get rid of this hazardous system. Is keeping the power on more important than any one person's life?

I hate to say it Lancie but I sure have doubts about how my boss would answer this.

My criteria for workmanship is
FIRST my safety
SECOND employee safety
THIRD quality of my work
FOURTH Production

We are moving. Planning on electrical is just starting. Building is relatively new (10 to 15 years old) So I would predict electrical will be 4 wire wye both 480 / 277 and 208 / 120.

The funny thing is I learned the trade (Navy) where everyting is ungrounded. Course as soon as we found a ground we isolated it and fixed it. Ground readings were taken hourly all year round.



Dan Bentler

DickDV said:

The leakage in this particular case occurred ahead of the disconnect, leaking into circuitry after the disconnect.
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The whole point is that a "floating" system soon becomes a corner grounded system and stays that way. Dangerous!

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I would be much more afraid of a system in which this happened. I don't see where a floating or corner ground would make much of a difference. If you say the insulation is great at 277 but poor at 480, then it is only a matter of time (a very small amount of time) that the insulation would be bad at 277. You are blaming the wrong thing in this scenario.

Lancie1 said:

There is the case of an outside contractor coming in for a job. "Better check the voltage to make sure this circuit is off, Joe". "Moe, I checked to phase C and it shows 0 volts, so the main breaker must be off." Joe then sticks his hand in a box and gets fried.

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Very poor example. What would be the difference of having one fuse blown with 2 other legs hot, in this situation. Any electrician who only checks 1 phase of a 3 phase system will eventually die. At a minimum that person is a danger to himself but can also kill others. Hope noone here thinks checking 1 phase is a safe way of doing business.

Dan, I would suggest a long-term program to get rid of this hazardous system. Is keeping the power on more important than any one person's life?

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I still don't see where a corner grounded delta is so dangerous. If you have a delta (either primary or secondary) you have 2 choices - Grounded or Ungrounded. To ground the delta you are mostly limited to a corner ground. OK, there is the center tapped one winding and impedance grounding but that is much rarer in industrial environments.

In my opinion grounded delta 480 volt power systems present real safety hazards and should be prohibited. One phase is already shorted to ground. When a second phase faults, then there is a phase-to-phase fault, much more severe than the normal fault-to-ground. Phase-to-phase faults usually result in something getting burned up.

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If the system is design like it is supposed to be (not installed by someone who THINKS they know what they are doing) then a phase to phase fault is no different than any other fault. The breakers and starter (and all others) HAVE to be designed to survive this service. Things only get 'burned up" when someone did not properly engineer the job. If that is the case, then all bets are off and things will get burned up on a phase to ground fault too.

Just to follow up and let you know I do not forget to take advice. We are moving and tore the transformer out. Looked at the connections - the clown who connected it NEVER tied the common on 208 wye secondary to ground. Granted we were only using it as source of 3 phase 208 but,,,,.
Now wonder I was getting funny readings trying to read phase to ground.

The other interesting thing I saw was same clown musta been out of green so just used another yellow for the ground conductor. Now I wonder where he terminated the other end of that yellow.

Scary place to be an electrician. Fuji foods is taking over the building - thinking of talking to them about job since I know where many of the skeletons are buried. On other hand leaving that building will be a joy.

The facility we are moving into will be wired plumbed etc etc all to code. Why it is never gonna rain there (near Seattle yet), there will be no taxes, poverty, and it is gonna be milk and honey and manna is gonna pour from heaven. That is what management says so it MUST be the truth. My middle ititial is J not G for gullible BTW

Dan Bentler