480 volt DS

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DARUSA

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New York City
A 480 volt disconnect switch that supply power to a elevator transformer had this voltage: phase A to phase B 480 volts !phase B to phase C 480 volts, phase C to phase A 480volts,now phase A to ground 451, phase B 0 voltage and phase C to ground 447 volt . How it can happen? Delta corner grounded transformer or what?
 
I guess with those readings it can't be 480Y/277, so I'm sticking to a 480 delta 3 ph,

ungrounded system with a fault to ground in the 'B' phase.
 
It's a Delta with either an intentionally-grounded phase or an unintentionally-grounded phase.

My guess is the first one.
 
LarryFine said:
It's a Delta with either an intentionally-grounded phase or an unintentionally-grounded phase.

My guess is the first one.
Click to expand...
Quite likely, but why the discrepancy... with B phase to ground at 0V, and A and C phases to B phase at 480V, then A and C phases to ground should also be 480V.
 
Smart $ said:
Quite likely, but why the discrepancy... with B phase to ground at 0V, and A and C phases to B phase at 480V, then A and C phases to ground should also be 480V.
Click to expand...

"B" phase is physically tied to ground, thus the 0 volts, If you disconnected this connection to ground, you would have 0 volts to ground between A,B or C. Someone will post a diagram of this pretty soon. It is a long misunderstood concept that taking a "hot" to ground always produces a short circuit. It all depends on where the ground is placed in the system, if connected at all. The ground is not necessacary for the system to operate.
 
hillybilly I hear what you are saying but smart knows how it works. :)

hillbilly1 said:
"B" phase is physically tied to ground, thus the 0 volts,
Click to expand...

Correct, which is what the OP says he has.

  • A to Ground 451 volts
  • B to Ground 0 volts.
  • C to Ground 447 volts
  • A to B 480 volts
  • B to C 480 volts
  • C to A 480 volts

Now do you see the problem?
 
Dar,

Are the line-to-line voltages all precisely 480, or more likely was there some variance? When you started measuring could you have read something lower than 480 and just considered it 480 as many of us would do for quick and rough measuring and just never went back to the L-L voltages when the L-G issues came up?
 
Volta said:
Dar,

Are the line-to-line voltages all precisely 480, or more likely was there some variance? When you started measuring could you have read something lower than 480 and just considered it 480 as many of us would do for quick and rough measuring and just never went back to the L-L voltages when the L-G issues came up?
Click to expand...

Actually yes, it was 471,479,475 at certain moments!!!
 
DARUSA,

I am curious, the secondary of the transformer is 240v or 208v. Delta/Delta or Delta/Wye?

Also, was this a new install or a trouble call?
 
benaround said:
DARUSA,

I am curious, the secondary of the transformer is 240v or 208v. Delta/Delta or Delta/Wye?

Also, was this a new install or a trouble call?
Click to expand...

The disconnect was in the open position when I took the voltage reading!!!!!
Elevator transformer. 480 delta prim to 120/208 Y secu.
 
I wish I knew more about how this works.

At the service disconnect phase B is intentionally grounded and all metal parts of the

electrical system are bonded together and to phase B. At the elevator transformer this

same B phase, by way of an EGC, connects to the XO of the secondary. So, B phase or the

intentionally grounded conductor is connected to both sides of the transformer. On the

480v side as B phase and 'ground' and on the 208v side as XO to 'ground', or, not to XO

and the elevator power has no 'ground', and no ground fault path. So, what's the problem?
 
Ground is nothing special it is simply dirt and things that touch it. Because it is so prevalent we have chosen to referene our electrical systems to ground by purposely connecting them together.

When we choose to bond a conductor our only choices are to use a 'phase' conductor or a 'neutral' conductor. This first bonding connection is a single current path, therefore it does not create a short circuit regardless which conductor is chosen.

A secondary of a isolation transformer creates a Separately Derived System, so any bonding on the primary has no impact on the bonding of the secondary.

Based on the OP voltages, I would guess that the system has an unintentional L-G fault on what is supposed to be an ungrounded system. An ungrounded power system actually contains some amount of coupling capacitance so we can measure 'phantom' Line-Ground voltages. These phantom voltages may or may not be equal, in fact it is possible, although unlikely, they could even be greater than the nominal L-L voltage.
 
iwire said:
hillybilly I hear what you are saying but smart knows how it works. :)



Correct, which is what the OP says he has.

  • A to Ground 451 volts
  • B to Ground 0 volts.
  • C to Ground 447 volts
  • A to B 480 volts
  • B to C 480 volts
  • C to A 480 volts

Now do you see the problem?
Click to expand...

Sorry, I missed that, didn't read it close enough, that is strange!
 
Could it be an impedance type grounding system, I have not seen one, but I have heard of it being used to reduce the amount of available fault current if a fault happens.
 
hillbilly1 said:
Could it be an impedance type grounding system, I have not seen one, but I have heard of it being used to reduce the amount of available fault current if a fault happens.
Click to expand...
A high resistance grounded system creates a 480Y/277 system in which no neutral loads may be used.

If A->B = 480V
and
B->G = 0V
Then
A->G = 0V

If not, we have an error in measurement or we are not really measuring what we think we are. I leaning towards the OP system is not reference to ground in the manner most of us are accustomed to.
 
jim dungar said:
A high resistance grounded system creates a 480Y/277 system in which no neutral loads may be used.

If A->B = 480V
and
B->G = 0V
Then
A->G = 0V

If not, we have an error in measurement or we are not really measuring what we think we are. I leaning towards the OP system is not reference to ground in the manner most of us are accustomed to.
Click to expand...

I'm thinking I have seen a diagram using it with a corner grounded delta too, could be wrong, that was about 20-25 years ago:)
 
hillbilly1 said:
I'm thinking I have seen a diagram using it with a corner grounded delta too, could be wrong, that was about 20-25 years ago:)
Click to expand...
I guess anything is possible, but, I have never heard of one in 30 years.:cool:

Regardless, it would not explain the OP readings.
 
jim dungar said:
A high resistance grounded system creates a 480Y/277 system in which no neutral loads may be used.

If A->B = 480V
and
B->G = 0V
Then
A->G = 0V

If not, we have an error in measurement or we are not really measuring what we think we are. I leaning towards the OP system is not reference to ground in the manner most of us are accustomed to.
Click to expand...
Is that a typo? Shouldn't it be A->G = 480V
 
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