480V delta strange voltages

[Julius Right]

Julius,
Thanks for the refresher course in vectors. Couple of points:
In the diagram shouldn't the CG be CN since there is no gnd?
Also, if you use a ruler and measure AN and compare it to CN, CN is longer but the numbers show different (306/294) Why is that?
Can you show the math or formula used to arrive at these numbers?
Tks

You are right: no CG. It has to be CN. The sketch is not perfect- once again you are right.:ashamed1:
The calculation- in Excel- it is very simple. I took at first a=b=60^o and I changed the numbers up and down until I got the result.

 

[Julius Right]

Corrections [following GoldDigger last post]:
1) The connection is Y
2) The neutral could be grounded.
3) If IA [B, C]N it is the current through transformer A[B or C] and ZA[B or C]N it is the impedance of these transformers then: |IAN*ZAN|=306V;|IBN*ZBN|=281V;|ICN*ZCN|=294V.
That means the currents or the impedances or both are unbalanced but the supply voltages are balanced.:weeping:

 

[don_resqcapt19]

Hummm....

I recall my dad talking about two overlaods being the norm during the 1940s. This, of course, would be the one with "no relevance as to wye or corner / ungrounded delta". no, I'm not old enough to have seen those.

I recall seeing only two overloads in Corner Grounded 480D (only) during the 1960s. I am old enough to have seen and worked that.

However, if you are dead certain, and have researched the applicable age group standards and codes, I'll believe you over my recollection from 50+ years ago.

ice

The requirement for 3 overloads first appeared in the 1968 code. The plant that I do a lot of work at was built under the previous codes and all of the original motor starters only have two overloads. The system is 480Y/277.

 

[iceworm]

The requirement for 3 overloads first appeared in the 1968 code. The plant that I do a lot of work at was built under the previous codes and all of the original motor starters only have two overloads. The system is 480Y/277.

Hummmm ... the thought just occured to me the only (or vast majority) 480V systems I was around were corner grounded. Could easily be the reason I thought that was all there was

Okay, cancel my line of reasoning.

ice

 

[mjmike]

Update: The utility is a 480/277V grounded Y service. Looking to see where the unbalance occurs. Original prints had it as a 3-wire system so it must have changed at some point.

 

[kwired]

Update: The utility is a 480/277V grounded Y service. Looking to see where the unbalance occurs. Original prints had it as a 3-wire system so it must have changed at some point.

Is there any 277 volt loads?

Either way, I believe you have an open neutral somewhere, but in the absence of any 277 volt loads I'd still expect closer to 277 volts being read on all three phases unless there is no bonding jumper at the source.

Do you have voltage readings of more then just a few volts from neutral to a remote ground probe?

 

[mjmike]

Is there any 277 volt loads?

Either way, I believe you have an open neutral somewhere, but in the absence of any 277 volt loads I'd still expect closer to 277 volts being read on all three phases unless there is no bonding jumper at the source.

Do you have voltage readings of more then just a few volts from neutral to a remote ground probe?

There is something going on. The Utility Co is checking it on their end. No 277V loads that I know of. I also think there is a neutral/gnd issue. Didn't take a reading per your last question.

 

[Smart $]

Is the MCC the service equipment? That is, does the service go directly to a section of the MCC or is there a remote service disconnecting means?

Not sure if bringing the grounded conductor to the service disconnecting means (SDM) was required back in the 60's, but I suspect it was. If not, and the grounded conductor was not brought to the SDM, that could be the reason for readings.

If the MCC is not the SDM, a 3-wire supply is commonplace and completely compliant where there are no L-N loads.

 

[Jraef]

Two overloads were common for 3 phase systems years ago and have no relevance as to wye or corner / ungrounded delta.

Correct. In fact the third overload heater was not a requirement in the NEC until 1971, so prior to that, most starter mfrs only used two simply because it wasn't required and it was cheaper to build. An option was available for the 3rd OL relay, but it wasn't standard. The 3rd pole option was available for when you DID have a corner grounded delta system by the way, not for when you did not. The problem was that if your system B phase was grounded, and your B phase inside of the motor or the leads went to ground with high resistance (arcing GF), not enough was was different as far as the other two legs were concerned, so the starter would not sense it fast enough and the motor damage was worse. So if you DID have a corner grounded delta system, you added the OL relay to the center (B) phase of the motor starter because it would sense the current flow in that wire, not rely on the current flow in the others.

That changed in 1971 and the mfrs were given time to change all of their products, so the practice continued on for a few more years after that. If you look through the varios NEMA motor controller history lines, you see a major change in product designs right at that time, ie Allen Bradley changed from the 709 to the 509 starters, Sq D changed to the 8536, etc. etc.

None of this however relates to the OPs question. I agree, the info posted indicates an UN-grounded delta system, and he might not be aware of the ground detection system in place. I have been in plenty of facilities where I see ground lights hooked up and two of them are burned out, or the bulbs are just missing. The old timers who knew what they were for were all laid off at one point and the young bucks who replaced them for lower wages had never seen one before and had no idea what they were. I can't even count the number of times I have sat down and explained it to maintenance crews in older facilities, even where some have been there for over a decade. It's just too uncommon now for new hires to have had any exposure to it.

 

[Julius Right]

It is possible to be also an ungrounded Delta connection, of course.:slaphead:
Since I never measured the voltage to ground in a healthy Delta system, I wonder if one could well measure the voltage to ground since the current flowing through insulation resistance and parallel capacitance to ground is very low [I think a few microamperes].In this case the error of a multimeter measuring is very high, I think. The actual voltage to Ground could be any other. You?ll better test the insulation to ground [or even the current from live terminal to ground through a high?parallel-resistance]:angel:.

 

[don_resqcapt19]

It is possible to be also an ungrounded Delta connection, of course.:slaphead:
Since I never measured the voltage to ground in a healthy Delta system, I wonder if one could well measure the voltage to ground since the current flowing through insulation resistance and parallel capacitance to ground is very low [I think a few microamperes].In this case the error of a multimeter measuring is very high, I think. The actual voltage to Ground could be any other. You?ll better test the insulation to ground [or even the current from live terminal to ground through a high?parallel-resistance]:angel:.

Larger ungrounded systems, without a ground, often have enough current to pull in a solenoid voltage tester, when testing from hot to earth...I think that is about 20mA.

Using a meter, if everything is perfect you will get the same voltage readings to ground from an ungrounded delta as you would from a grounded wye, but most of the time there is more capacitance on one leg and you will not get the 277.

 

[Julius Right]

Thank you, Don!