Phase to Ground Voltage

[rhodel]

I have a RTU where phase to phase is 480V

Phase A to Grd - 813V
Phase B to Grd - 611V
Phase C to Grd - 317V

Rtu fed from 480V , 3P, 3W panelboard DP with an equipment groud

DP is fed from 500kva Delta 208V - 480VY step-up transformer. The feeder has (3) phase conductors and a ground.............No neutral

Ground voltages seem strange. What should they be? Where to look to correct? Seems like a grounding issue.

Thank you for your feed back

 

[texie]

I have a RTU where phase to phase is 480V

Phase A to Grd - 813V
Phase B to Grd - 611V
Phase C to Grd - 317V

Rtu fed from 480V , 3P, 3W panelboard DP with an equipment groud

DP is fed from 500kva Delta 208V - 480VY step-up transformer. The feeder has (3) phase conductors and a ground.............No neutral

Ground voltages seem strange. What should they be? Where to look to correct? Seems like a grounding issue.

Thank you for your feed back

Sounds like you have an ungrounded 480Y/277 secondary and the voltage to ground readings you are getting are meaningless as there is no grounded reference, while the phase to phase are valid. The code would generally require this derived system to be grounded. There are exceptions, but probably not best in your application. See 250.20.
Also, be aware of the other requirements for this such as disconnecting means, overcurrent protection on primary and secondary, Grounding Electrode System, etc.

 

[GoldDigger]

Also, be aware of the other requirements for this such as disconnecting means, overcurrent protection on primary and secondary, Grounding Electrode System, etc.

Plus, of course, ground fault detection if an ungrounded system is permitted!

 

[ActionDave]

DP is fed from 500kva Delta 208V - 480VY step-up transformer.....

This right here makes me believe that this is what you are dealing with-

Sounds like you have an ungrounded 480Y/277 secondary and the voltage to ground readings you are getting are meaningless as there is no grounded reference.........

 

[ptonsparky]

Sounds like design flaw, to be polite. Get out the instructions and and your code book. Bond the secondary (480v) as required.

 

[kwired]

I have a RTU where phase to phase is 480V

Phase A to Grd - 813V
Phase B to Grd - 611V
Phase C to Grd - 317V

Rtu fed from 480V , 3P, 3W panelboard DP with an equipment groud

DP is fed from 500kva Delta 208V - 480VY step-up transformer. The feeder has (3) phase conductors and a ground.............No neutral

Ground voltages seem strange. What should they be? Where to look to correct? Seems like a grounding issue.

Thank you for your feed back

As others have stated, sounds like the secondary is not bonded to ground. I bet if you connected any kind of load between any phase and ground you would change your readings, but there would not be enough power behind those voltages to drive the load. Those voltages are mostly the result of capacitive coupling. Testing with a low impedance meter will probably shunt enough current to have much lower readings.

 

[Besoeker]

I have a RTU where phase to phase is 480V

Phase A to Grd - 813V
Phase B to Grd - 611V
Phase C to Grd - 317V

Rtu fed from 480V , 3P, 3W panelboard DP with an equipment groud

DP is fed from 500kva Delta 208V - 480VY step-up transformer. The feeder has (3) phase conductors and a ground.............No neutral

Ground voltages seem strange. What should they be? Where to look to correct? Seems like a grounding issue.

Or the absence of one.
If the system isn't grounded at some point the readings with reference to ground could be almost anything and rather meaningless.

 

[don_resqcapt19]

The OP says it is a 3 wire system...there is no code requirement for that to be a grounded system. There is a code requirement for the ungrounded system to have a ground detector.

 

[Besoeker]

The OP says it is a 3 wire system...there is no code requirement for that to be a grounded system. There is a code requirement for the ungrounded system to have a ground detector.

No disagreement with that. I was simply making the point that voltage measurements wrt ground are likely to be indeterminate on a system that isn't grounded.

 

[don_resqcapt19]

No disagreement with that. I was simply making the point that voltage measurements wrt ground are likely to be indeterminate on a system that isn't grounded.

True. I was responding to the comments that suggested that this system is required to be a grounded system.

 

[Besoeker]

True. I was responding to the comments that suggested that this system is required to be a grounded system.

Understood.
I can't work out from post #1 whether the transformer is Y input and delta output or vice versa.

 

[don_resqcapt19]

Understood.
I can't work out from post #1 whether the transformer is Y input and delta output or vice versa.

It doesn't really matter as far as the code goes. Even if the output is wye, there is no code requirement to make a connection to the center point of the wye, unless you are using it as a circuit conductor.

 

[GoldDigger]

True. I was responding to the comments that suggested that this system is required to be a grounded system.

My concern is that if the secondary is wye there will be a neutral point at the source. Even though the system and loads may be 3 wire only, does the existence of a neutral point trigger the requirement that the neutral be grounded?

 

[Besoeker]

It doesn't really matter as far as the code goes. Even if the output is wye, there is no code requirement to make a connection to the center point of the wye, unless you are using it as a circuit conductor.

Even if you didn't need line to neutral voltage and code doesn't require it, wouldn't it be safer in most circumstances to ground the star point?

 

[don_resqcapt19]

My concern is that if the secondary is wye there will be a neutral point at the source. Even though the system and loads may be 3 wire only, does the existence of a neutral point trigger the requirement that the neutral be grounded?

Not for that voltage level. You have to ground it if it can be grounded so that the voltage to ground does not exceed 150 volts, so with a 480 volt system, it is not required to be a grounded system.

 

[don_resqcapt19]

Even if you didn't need line to neutral voltage and code doesn't require it, wouldn't it be safer in most circumstances to ground the star point?

I think it would be, but the code makes it optional.

 

[StarCat]

HVAC Common Failures

HVAC Common Failures

Ground faults are very " common " failures on HVAC machines. They happen often on fan motors, compressors, and yet still other ways.
I would be quite concerned about how the supply breaker will trip on phase to ground fault if the center tap on said transformer is not bonded to ground.
Even though the neutral point is not needed for service purposes, it would seem necessary to bond it in this case so the fault current returns to its source.
Please correct me if I am missing something here. This comes off like a potentially dangerous installation.

 

[GoldDigger]

I would be quite concerned about how the supply breaker will trip on phase to ground fault if the center tap on said transformer is not bonded to ground.

It would not, which is one reason that ungrounded delta systems are often used for critical industrial processes where an unplanned shutdown could be very expensive or dangerous. But for that same reason, an ungrounded system requires that the initial fault to ground be detected so that an orderly shutdown and repair can be done before a second fault occurs.

 

[StarCat]

10-4

10-4

OK very good.
I would say the attending Electrician raised a solid concern.
Now, while a Plant Electrician in such a setting wil likely be aware of the possibility of live machine cabinets, a typical HVAC Tech called out to service a downed unit on such a system might not be aware.

 

[don_resqcapt19]

OK very good.
I would say the attending Electrician raised a solid concern.
Now, while a Plant Electrician in such a setting wil likely be aware of the possibility of live machine cabinets, a typical HVAC Tech called out to service a downed unit on such a system might not be aware.

There is no more likely hood of a "live" cabinet on an ungrounded system than there is on a grounded system.