Ungrouned 480V Delta short

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binney

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I believe what I'm looking at is a 3ph 480v Delta ungrounded service. The reason I question it is because I'm not familiar with the voltages I'm getting.

A-B 480v
B-C 480v
C-A 480v

to ground
A-G 480v
B-G 480v
C-G 6v

This is in the factory I just started working for, but hese voltages don't seem right.
I believe the C phase is shorted to ground. If so, is it possible for this to cause stray voltage onto the ground of another service in thie same facility. Are there any suggestions on how to isolate the ground-fault.
 
binney said:
I believe what I'm looking at is a 3ph 480v Delta ungrounded service. The reason I question it is because I'm not familiar with the voltages I'm getting.

A-B 480v
B-C 480v
C-A 480v

to ground
A-G 480v
B-G 480v
C-G 6v

This is in the factory I just started working for, but hese voltages don't seem right.
I believe the C phase is shorted to ground. If so, is it possible for this to cause stray voltage onto the ground of another service in thie same facility. Are there any suggestions on how to isolate the ground-fault.
Click to expand...
You may have a corner grounded delta and if so this is how I look at it:
It is not "shorted to ground." The "C" phase been chosen to be grounded in your case where a grounded "B" phase is more common is a corner grounded delta.
Remember that a connection that is unintended can be considered to be a short circuit. What you have described is intended and not considered to be a short to ground.
Please understand that what I'm about to describe is only being used to illustrate what a "grounded conductor" is. Using 120/240v 1ph3w or a 208y/120 and a 480y/277 3ph4w as an example the neutral in each of these is commonly grounded. The neutrals, which are now grounded conductor, after it leaves the service entrance are to be kept insulated and isolated from the equipment grounding conductor(EGC). The neutral (N) is the grounded conductor and is one of the current carrying conductors.
The EGC provides a path back to the source should the be as line or phase to ground fault. The EGC provided a path back to the source such thar the over current protective device is able the clear the fault
In you case the "C" has been grounded and what you now should have is a both an "C" phase conductor that is "grounded" and a separate EGC. The "C" phase after it leaves the point where it is grounded should be insulated and isolated from the EGC just as the neutral would be. The EGC in a corner grounded delta provided a path for the fault current to flow back to the source such that an overcurrect protective device wuold be allowed to clear the fault.
 
If you actually have a corner grounded delta (and not an ungrounded delta with a fault), the NEC specifies that you should not put fuses in the grounded conductor (C leg) and any circuit breaker that interrupts the grounded wire must also open all the ungrounded wires at the same time. Is is possible to use just two pole (A and B leg) circuit breakers instead in a C grounded delta.
 
As noted, you have a grounded delta system, either intentionally or accidentally.
If it is intentionally grounded the above info on fuses and conductor ID should help you confirm.
If it is accidentally grounded, you can hunt for that ground by turning off feeders while monitoring the voltage. Once you turn off the feeder with the grounded conductor work your way down thru the branch circuits.

Note that if it is not intentionally grounded, present Codes require a ground detector to alert you when a phase goes to ground. Such detectors are helpful for everyday maintenance also.
 
The service is in an operating facility which makes it hard to safely get access or shut things down. Is there any way of confirming if I have a "corner grounded", or an "ungrounded" service with a short? The only place I see fuses is at the motor starters, which do have 3 fuses, but I believe that's legal, even if it's corner grouned. What other damage or harm can be caused if it's a non-grounded delta and the short remains?
 
binney said:
The service is in an operating facility which makes it hard to safely get access or shut things down. Is there any way of confirming if I have a "corner grounded", or an "ungrounded" service with a short? The only place I see fuses is at the motor starters, which do have 3 fuses, but I believe that's legal, even if it's corner grouned. What other damage or harm can be caused if it's a non-grounded delta and the short remains?
Click to expand...

If it is an ungrounded delta system, the clues are:

1) OCPD on all three phases.
2) Ground-fault detection system.

The issues with operating a faulted ungrounded system are:

1) Lack of protection from a second fault.
2) I have seen TVSS devices fail because they are connected from phase to ground and in your case would be expecting about 277V to ground and would trip at about 350V. The constant 480V can destroy the TVSS.

Mark
 
I think one concern if it is not an intentional ground would be when a 2nd phase goes to ground you are going to have a L-L short which, depending on the specifics, could create a major arcing event.
 
There are a lot of ungrounded delta systems out there with no ground detectors. It seems to be something that is often forgotten.

I think your best bet is to find the 1 line drawings of the power system and see what it says.

There are only a couple of places where the system can be intentionally grounded. Look there and see if there is an intentional ground.

I would also point out that it is not unusual for phase rotation to occur in older plants along the way. No one really cares as long as stuff still works.
 
binney said:
The service is in an operating facility which makes it hard to safely get access or shut things down. Is there any way of confirming if I have a "corner grounded", or an "ungrounded" service with a short? The only place I see fuses is at the motor starters, which do have 3 fuses, but I believe that's legal, even if it's corner grouned. What other damage or harm can be caused if it's a non-grounded delta and the short remains?
Click to expand...
If it is corner grounded there should not be fuses in the grounded conductor. Circuit breakers in the grounded conductor are OK as the associated grounded conductors can be simultaneously disconnected when it is operated or trips.

You really need to look for the presence of a system bonding jumper either at the source or at the first disconnecting means. The presence of such jumper will tell you what system conductor was intentionally grounded. No jumper means it is either installed wrong or is intended to be an ungrounded system, and leaves you with the question of where is the current ground located whether intentional or not.
 
I did electrical maintenance for several years at a waste water treatment plant with an ungrounded 480 delta system. This was before ground detectors were required. I assembled my own crude detector from 2 240 volt lamps in series from each phase to ground. Normally the lamps would burn at somewhat below normal brightness. A ground fault would cause one phases lamps to be dim or out and the others go to normal brightness.

I would shut down feeders till I found the grounded circuit. Most times the cause would be insulation failure in a water filled underground conduit or duct. All the underground wiring was RHW/USE.
 
I worked at an old, old wire cloth plant w/ a 480 delta ungrounded. I put together a trough with 3 lamps and 3 small transformers,one side of primary of trfmr connected to ground (building steel) other to line a, b c. if say "a"phase shorted to steel b and c would light etc etc nice set up we could track down the busway and determine which loom was shorted out.
 
As is common with most ground detectors, you have effectively converted the ungrounded delta to a high impedance grounded delta with a synthetic wye point, but without all of the calculations and fault trips that go with that configuration.
 
480volt ungrounded delta

480volt ungrounded delta

Yes, this is definitely a 3 phase, 480 volt delta service. Please keep in mind that 6 volts measured to ground does not necessarily indicate a grounded phase. I know this from personal experience. The only problenm with a grounded leg is when there are multiple delta services coming from this transformer. This is common when a utility company is feeding many businesses from one transformer set. If this three phase delta service is dedicated to one facility, then there really isn't a problem. The impedance of an electrical device or devices on one phase may be low, which is causing the lower than normal voltage reading to ground. The load of this system is connected from phase to phase, and therefore, does not rely on the phase to ground voltage.
binney said:
I believe what I'm looking at is a 3ph 480v Delta ungrounded service. The reason I question it is because I'm not familiar with the voltages I'm getting.

A-B 480v
B-C 480v
C-A 480v

to ground
A-G 480v
B-G 480v
C-G 6v

This is in the factory I just started working for, but hese voltages don't seem right.
I believe the C phase is shorted to ground. If so, is it possible for this to cause stray voltage onto the ground of another service in thie same facility. Are there any suggestions on how to isolate the ground-fault.
Click to expand...
 
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