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Posted By: gfretwell 480 Delta - 01/06/18 07:09 AM
I was talking to a guy from Massachusetts (electronic engineer) and he says 480 delta is common up there for industrial applications. How is that generally grounded? He didn't know but he says he thought it was ungrounded. That did not sound right.
I have seen plenty of 480/277 wye but I have never seen a 480 delta down here.
Posted By: gfretwell Re: 480 Delta - 01/06/18 08:10 PM
Never mind, we determined that it was corner grounded delta. He just did not realize it.
Posted By: ghost307 Re: 480 Delta - 01/06/18 09:57 PM
We have several mission critical facilities that use an ungrounded delta system.
In the event of a ground fault the system stays energized until it can be corrected.
The only time that the system goes down is when there is a second ground fault before the first one gets cleared.

An ungrounded delta is kinda capacitively coupled to the Earth, so a ground detector device is used.
There are 3 lights wired from phase to a ground rod and all glow continuously at the same brightness.
If one of the phases has a ground fault that light will go out and the other 2 get much brighter.

That tells you which phase the ground fault is on...but finding where it is becomes very challenging.
One of our facilities has had a ground fault on one of the phases for several years...and the maintenance folks are still trying to chase it down.
I think there may be a ground in 2 places on the same phase; which is like trying to find the 2 bad bulbs in an old string of Christmas lights.
Posted By: Wiresmith Re: 480 Delta - 04/28/18 08:27 PM
Originally Posted by ghost307
We have several mission critical facilities that use an ungrounded delta system.


An ungrounded delta is kinda capacitively coupled to the Earth, so a ground detector device is used.
There are 3 lights wired from phase to a ground rod and all glow continuously at the same brightness.
If one of the phases has a ground fault that light will go out and the other 2 get much brighter.

.


are you sure that's not a high impedance grounded neutral system?

i was at a plant with a set-up like that and i can't remember the exact fault locator they had but we traced the ground fault with some type of frequency fault locator, even through metal conduit and enclosures, found it pretty quickly ( i think 1 hour)
Posted By: ghost307 Re: 480 Delta - 04/30/18 12:47 PM
Nope.
It's a true ungrounded 3 wire Delta.
Since the facilities are critical we can't shut them down unless we do a major upgrade so most of the existing Services are the original ones from the 1930s...or earlier.
One of them still has the oil insulated vertical drawout cast iron MV circuit breakers.
Major upgrades are designed with a conventional 3Ph4W system.
Posted By: Wiresmith Re: 480 Delta - 05/02/18 03:44 PM
Originally Posted by ghost307
Nope.
It's a true ungrounded 3 wire Delta.
Since the facilities are critical we can't shut them down unless we do a major upgrade so most of the existing Services are the original ones from the 1930s...or earlier.
One of them still has the oil insulated vertical drawout cast iron MV circuit breakers.
Major upgrades are designed with a conventional 3Ph4W system.



the reason i ask is you said this about the indicator lights

Originally Posted by ghost307
.
There are 3 lights wired from phase to a ground rod and all glow continuously at the same brightness.
If one of the phases has a ground fault that light will go out and the other 2 get much brighter.


i don't think the light would light up if it was completely ungrounded,

this light set-up is what is used on high-impedance grounded systems, a first ground fault will only be low current like 5 amps(i don't remember for sure). the lights do the same thing you are describing.
Posted By: ghost307 Re: 480 Delta - 05/02/18 07:42 PM
I may have misspoke, but it's been a while since I've had my hands inside one of these units (for obvious reasons).

The '3 lights' style of ground detector works on a true ungrounded system such as the ones installed back in the 30's because the phase conductors are only capacitively coupled to ground.
There is no physical connection between any energized conductor and ground in our systems.

http://www.ece.mtu.edu/faculty/bamork/EE5223/EE5223TermProj_Ex3.pdf

BTW - Our new installations take advantage of high impedance grounding to maintain system integrity while reducing the potential for catastrophic failure if a second ground fault should occur before the first can be tracked down and cleared.
Posted By: Wiresmith Re: 480 Delta - 05/05/18 03:06 PM
thanks for that link, looks like some good information. do the lights light up dimmly though without a ground fault on a true ungrounded system?
Posted By: ghost307 Re: 480 Delta - 05/05/18 11:29 PM
Yes. Under unfaulted conditions all 3 lights are dimly lit.
That also actually acts as a good safety check because if a light burns out you know it right away.
Generally they last forever because they are lit at only a fraction of the voltage that they are designed for.
Posted By: sparkyinak Re: 480 Delta - 05/21/18 01:16 AM
I'm curious. What you done so far to find the fault(s)? Any plans for a powering down the critql systems for maintenance?

I ask since I'm may be transferring soon and be working on ungrounded deltas. Always eager to learn something new
Posted By: sparkyinak Re: 480 Delta - 05/21/18 01:33 AM
I'm curious. What you done so far to find the fault(s)? Any plans for a powering down the critql systems for maintenance?

I ask since I'm may be transferring soon and be working on ungrounded deltas. Always eager to learn something new
Posted By: ghost307 Re: 480 Delta - 05/21/18 02:38 PM
One of the great benefits of an ungrounded delta is its ability to remain energized with a fault on any single line.
Unfortunately one of the great difficulties with an ungrounded delta is finding that fault.
A fault at any point in the system shows up exactly the same as another.

The only way that I have ever used successfully is to disconnect part of the system and see if the fault clears.
If not, reconnect that part and start again until you find what device clears the fault when it opens.
Now you know that the system is clear upstream of that point.
Now repeat that process further downstream of the source.
Keep repeating until you find the fault...it can take a while.
If you have more than 1 fault on the system...it can be a very long time since you only know that the phase is faulted or cleared; you'll never know if there is a single point of fault or more than one.
That's what doomed the ungrounded delta IMHO; it's very difficult to troubleshoot as the electrical system starts to grow.

I had a fault in a relatively simple system with 10 loads that was scheduled to be replaced as part of an upcoming renovation.
After trying to find the problem over the course of a week I finally gave up and replaced it in kind using EMT knowing that the whole thing would be in the dumpster in a few months.
In that particular case I not only got the process back online I saved a lot of troubleshooting time/expense.
The tough part was convincing the powers-that-be to spend some money on a temporary solution.
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