Grounded B phase (corner grounded) 480 question

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Brad7981

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Maintenance tech
Hello, I am a maintenance tech at a manufacturing company which has recently moved to a new shop with what I believe has a corner grounded (phase b grounded system).

There is a 480v 3 phase bus bar that serves all of the machinery connections. All of the disconnects off of the bus bar have 3 wires going to them.

The readings at the disconnects are as follows:

Phase to phase it all reads 480. There is no ground wire. Holding one probe to the side of the box and test each phase a = 480 b = 0 c = 480. From these readings I have assumed this is a grounded B system. Does that seem correct?

The bus bar is fed from a large panel board. The power goes from the panel board to a junction box and then to the bus bar. The junction box appears to have a ground (a wire attached to the junction box’s Sheetmetal)

Thats the end of the background. Assuming I am correct in identifying the system, I have one question.

What is the appropriate way to wire single phase grounded 480v from this setup?

Any response would be greatly appreciated. Thanks for taking the time to read my post.
 
You don’t want to hear this, but...
you don’t...

the corner grounded delta doesn’t support single phase loads.
the easiest thing to do is to get a 480/120/208 dry pack. Somewhere down the line 208 will be wanted...
 
Not enough editing time here...

think about single phasing from this bank just a moment...

you have to have a neutral for single phase installation, correct?
The corner grounded system doesn’t have a neutral, it has a grounded conductor. They are quite different.

also, balancing the system would be a nightmare.
 
ActionDave said:
There may be design reasons not to run single phase loads but it seems to me any single phase, line to line loads would run fine.

How would a line to line load know if was being fed from a corner ground delta or a center tap delta?
Click to expand...

I agree nothing wrong with using single phase off this which could be one ungrounded phase and the grounded phase. A corner grounded system looks just like a split phase system
 
Remember not to insert a fuse in the grounded phase.

Roger
 
In order to reduce shutdowns, a lot of manufacturing plants have ungrounded systems. It is possible the ground on your system is not intentional.
A quick check would be to turn off the feeder breakers and see it the voltage measurement changes.
(If the "0" voltage measurement changes, that's the feeder that has a "grounded" phase)
 
augie47 said:
In order to reduce shutdowns, a lot of manufacturing plants have ungrounded systems. It is possible the ground on your system is not intentional.
A quick check would be to turn off the feeder breakers and see it the voltage measurement changes.
(If the "0" voltage measurement changes, that's the feeder that has a "grounded" phase)
Click to expand...
If this is the case, ungrounded system, there should be some lights set up to indicate a grounded phase. Should be. I’ve only seen one of these systems in 40 years.
 
I see problems ahead with uneven loading and balancing..
It’s one of the reasons we don’t offer this bank any longer.
 
Brad7981 said:
Hello, I am a maintenance tech at a manufacturing company which has recently moved to a new shop with what I believe has a corner grounded (phase b grounded system).

There is a 480v 3 phase bus bar that serves all of the machinery connections. All of the disconnects off of the bus bar have 3 wires going to them.

The readings at the disconnects are as follows:

Phase to phase it all reads 480. There is no ground wire. Holding one probe to the side of the box and test each phase a = 480 b = 0 c = 480. From these readings I have assumed this is a grounded B system. Does that seem correct?

The bus bar is fed from a large panel board. The power goes from the panel board to a junction box and then to the bus bar. The junction box appears to have a ground (a wire attached to the junction box’s Sheetmetal)

Thats the end of the background. Assuming I am correct in identifying the system, I have one question.

What is the appropriate way to wire single phase grounded 480v from this setup?

Any response would be greatly appreciated. Thanks for taking the time to read my post.
Click to expand...

I would never assume what you have assumed.

There is only one way to know if you have a floating or solidly grounded, delta or Y, is to physically see the system bonding jumper and see if it is or was installed in a manner that looks proper or by intent.

I would want to see the upstream source transformer nameplate (delta or Y secondary) and then work back from there. The system bonding jumper can be, or will be, located somewhere between the transformer secondary and the first disconnect, usually at the transformer connections or at the first load side disconnect.

It will not be a wire to the sheetmetal of a J box. You (or I) would need to see the system bonding jumper and the required GEC terminating at the same point. You could have floating secondary with the first ground fault already present, or more commonly found, an intentionally grounded system done improperly and with no GEC. You need to physically see how the connections were done and both ends of the GEC.

If you are unfamiliar with what is written above, you would not want to do any wiring in the facility and to call in a master electrician who does know.

Line to line single phase 480 V loading is not a problem on a 480 V three phase system.
 
__dan said:
I would never assume what you have assumed.

There is only one way to know if you have a floating or solidly grounded, delta or Y, is to physically see the system bonding jumper and see if it is or was installed in a manner that looks proper or by intent.

I would want to see the upstream source transformer nameplate (delta or Y secondary) and then work back from there. The system bonding jumper can be, or will be, located somewhere between the transformer secondary and the first disconnect, usually at the transformer connections or at the first load side disconnect.

It will not be a wire to the sheetmetal of a J box. You (or I) would need to see the system bonding jumper and the required GEC terminating at the same point. You could have floating secondary with the first ground fault already present, or more commonly found, an intentionally grounded system done improperly and with no GEC. You need to physically see how the connections were done and both ends of the GEC.

If you are unfamiliar with what is written above, you would not want to do any wiring in the facility and to call in a master electrician who does know.

Line to line single phase 480 V loading is not a problem on a 480 V three phase system.
Click to expand...
Yeah I definitely agree one can't assume much with these As an example, a building with a 600v ungrounded Delta I worked on: metered like a corner grounded and had no ground detectors. Must be a corner grounded right? Wrong. It was actually ungrounded with a first fault and they just never put ground detectors in.
 
One thing to be careful of since you indicate that you have only tested this out on the shop floor: was this an UNGROUNDED delta system and you already have a ground fault on one phase, or is this a purposely corner grounded delta system? The way to check is to start at the transformer and look for the GEC on the B phase. If you go all the way from the transformer to the service panel and don’t find a GEC, you probably have a ground fault somewhere else.

And some free advice based on a question not asked...

If any of your equipment has VFDs, servos or UPS on it, you do NOT want to connect them to this system, you will want to get isolation transformers with a solidly grounded neutral to feed those loads. The concept of using delta power systems to maintain production came from an era before power electronics existed and is now outdated.
 
Jraef said:
One thing to be careful of since you indicate that you have only tested this out on the shop floor: was this an UNGROUNDED delta system and you already have a ground fault on one phase, or is this a purposely corner grounded delta system? The way to check is to start at the transformer and look for the GEC on the B phase. If you go all the way from the transformer to the service panel and don’t find a GEC, you probably have a ground fault somewhere else.

And some free advice based on a question not asked...

If any of your equipment has VFDs, servos or UPS on it, you do NOT want to connect them to this system, you will want to get isolation transformers with a solidly grounded neutral to feed those loads. The concept of using delta power systems to maintain production came from an era before power electronics existed and is now outdated.
Click to expand...

I confirmed with the previous owner of the building that it is a grounded B system.

In the previous factory, we had the control panel (yes, on servos and expensive pcbs) wired in from a standard delta 480. The wiring was phase a, phase b, and the ground wire (No neutral). No issues for ten years.

If using the a and c phase off of a grounded b system (to create 480 single), would that not create a similar quality power compared the standard grounded delta at our previous factory?
 
The single phase 480 V load, control circuit power or other single phase 480 V load, any combination of line to line phases should be identical or no different than another combination, A-C, A-B, B-C are all the same, 480 V single phase.

Again the statement from the owner is good, but I would never assume the owner knows electrical sufficiently, to know the difference from delta or Y, and floating or solidly grounded, (and any deficiencies that a power system survey may reveal). I would look it over if it were me. Talking to the owner is good, that he is there, but I have seen enough sites that the long term maintenance electricians had no clue.

An independent confirmation, I would have to see it for myself, and I would never rely on past practices or the owner's statement, until I became sufficiently familiar with their general trustworthiness and technical capacity.

Given statements so far, all I know is that I would make an independent confirming survey or spot check, to see if they know what they are talking about, especially if the system bonding jumper was a wire to the can. The older place new to you, might be in need of some updating and safety, code compliance improvements

Listen to the warning about, if you are connecting electronic equipment that has a three phase 480 V input, drives and UPS front end, anything that is a three phase input rectifier front end. Those are typically looking for a solidly grounded Y input and you may be smoking those if you are on a delta or floating system.

IMO you are still at square 1. The system needs to be looked over to see what it really is.
 
Brad7981 said:
I confirmed with the previous owner of the building that it is a grounded B system.

In the previous factory, we had the control panel (yes, on servos and expensive pcbs) wired in from a standard delta 480. The wiring was phase a, phase b, and the ground wire (No neutral). No issues for ten years.

If using the a and c phase off of a grounded b system (to create 480 single), would that not create a similar quality power compared the standard grounded delta at our previous factory?
Click to expand...
 
Brad7981 said:
I confirmed with the previous owner of the building that it is a grounded B system.

In the previous factory, we had the control panel (yes, on servos and expensive pcbs) wired in from a standard delta 480. The wiring was phase a, phase b, and the ground wire (No neutral). No issues for ten years.

If using the a and c phase off of a grounded b system (to create 480 single), would that not create a similar quality power compared the standard grounded delta at our previous factory?
Click to expand...
What do you mean by "standard delta"?

Standard delta is three corners of the delta and is either ungrounded or has one corner grounded. Other standard delta is one with mid point of one phase grounded - this leaves you with a high leg to ground on the opposite corner of the grounded point. For nominal 480 volts that high leg voltage is about 416 volts to ground, still 480 between all corners.

With the high leg system you do have 240 volts to ground from the other two leads and certainly can supply a 240 volt single phase load from it. Otherwise in all other cases you are limited to only 480 volts for any single phase loads. This mostly is convenient for small single phase transformers to obtain control voltages or maybe limited 120/240 single phase, or maybe some 480 volt lighting. Another possibility is smaller 480 volt heating loads, but any significant heating load will usually be designed to utilize all three phases.

On the corner ground system it doesn't matter if you connect a single phase 480 volt load to the grounded conductor or not, it is still 480 volts between any two leads of the system. If you have significant number of single phase loads you should try to balance them though.
 
Jraef said:
... The concept of using delta power systems to maintain production came from an era before power electronics existed and is now outdated.
Click to expand...
If I recall correctly, purposely corner-grounded systems were installed during WWII as an economy measure. Two-pole switches, contactors, motor starters, fuses and fuseblocks could be used, enabling us to get more stuff hooked up & running faster with the available production quantities.
 
I've heard that pocos like them because they can use smaller transformers than a wye. Industrial customers like them because of the higher phase to phase voltage. (240 volt deltas vs 208 volt wyes)
 
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