480 volt 3-phase

[Derek]

Im am sure that this is known by the Plant electricians and management as our services are checked by high voltage maintenance yearly.Every company I have worked for prior to this had relativity new facility's and I have just never seen it before.Now that I know what it is I will find out more information from them and why it is still used

You are saying it is not possible for a leg to have gone to ground since the last inspection?

 

[Brian123]

Corner grounded delta's do exist but they are very rare.

We have a corner grounded delta system at our business. We are located in a decommissioned ammunition manufacturing building. The entire depot was wired this way. I have heard that one reason for this was to save copper. With a corner grounded delta, you only need three wires. A wye would need four.

Brian

 

[leitmotif]

How old is your plant and how old is the wiring?

You folks need to find out what you have
Ie corner grounded or a floated delta.

IF you are corner grounded delta then you should be OK.

If floated you have a ground and that needs to get found and fixed. It is not much fun I admit - dealt with this for 4 years in Navy. The Navy reason for ungrounded was to prevent stray currents thru submarine hull and personnel safety.

The first ground supposedly is "no problem"
IT CAN BE A PERSONELL SAFETY ISSUE though
no one worries about it UNTIL a second piece grounds on a different phase. Then as stated things get real interesting.

The other thought that comes to mind is what if you are wye and it is not bonded to ground?

In my mind you guys need to determine
if wye or delta
then if wye what is matter with ground
and if delta whether floated or corner grounded.
Then if floated fix the ground.
Dan Bentler

 

[Jim Dungar]

There are three types of electrical systems used in the US. By far and away the most common type is the solidly grounded wye (i.e. 480Y/277). Too many people are only exposed to these systems and then assume anything else is not possible.

In many high power requirement systems (i.e. industrial plants and data centers) delta distribution systems are not uncommon. A delta system can be left intentionally ungrounded or it can be intentionally grounded through an impedance (most commonly a "high" resistor) or solidly grounded through a center tap of one delta winding (usually limited to 240/120V 3PH 4W systems). Delta systems did not disappear in the 60's, they are still being installed.

If you do not know what your system is troubleshooting can be very difficult. For example, on an intentionally ungrounded the system is actually grounded through the coupling capacitance of the power conductors. Depending on the input impedance of a voltmeter (among other factors), it is possible to read 277V to ground although it is also possible to read voltages close to 960V.

Do not try to troubleshoot systems that you are not qualified on. There are lots of things to learn so continue to ask questions. Because these systems are uncommon there is lots of false information available.

Prior to at least 2002, ground indicating systems on ungrounded power systems were not required by the NEC. However there has almost always been a requirement that they be restricted to installations with qualified maintenance personnel.

 

[monkeyhead]

Ok, let's see if I've got this (only ever dealt with delta-wye)...

Ungrounded Delta with a phase shorted to ground looks like a corner grounded delta. But it is dangerous because if a second phase shorts to ground you get a big boom.

Is corner grounded delta protected differently so that a phase to ground short (on one of the ungrounded lines) doesn't cause the big boom that will occur on the ungrounded delta with two phases shorted?

 

[DickDV]

I think you guys might be leaving a bit of a wrong impression about floating three phase systems. I would dearly love to agree that these are no longer being installed. Here in the lower peninsula of Michigan USA, I am seeing new construction with floating delta 480V systems at a disturbing rate. In the southwestern part of our state there is one contractor that routinely installs 480V three phase systems with a deliberate corner ground on pretty much all 480V work he gets.

These systems are potential killers for a number of reasons and I have been a vocal opponent of them ever since one almost killed me 12 years ago. It is an absolute travesty that they are not outlawed by the inspection authorities.

Bottom line: if you want a safer, predictable system, always specify grounded, wye derived, 480/277V service. And don't be seduced by the claims of high resistance grounding on wye systems either. They are almost as much of a hazard as the floaters. I'm not opposed to resistance grounding but the ground fault current MUST be high enough to clear the current limiting devices, absolutely!

 

[leitmotif]

Dick

I learned "high voltage" generation and distribution on nuc submarine. All AC 450 and (120 lighting) and 250 DC was ungrounded. The Navy claimed that was for personnel protection which in theory made sense but we could still get knocked for a loop or killed for sure.

We were very conscientious over fixing grounds and had 1 M criteria for AC and 50 K for DC distribution systems as acceptable minimums.

What I have never really understood is what are the supposed advantages of grounded vs ungrounded distribution?

If I had the design choice I would definitely go with wye distribution.

Dan Bentler

 

[DickDV]

Dan, the big selling point of floating delta 480VAC is that it will tolerate one ground fault without stopping your operations or clearing any fuses or breakers. Nevermind that someone might get killed in the meantime.

Of course, the really laughable part of that selling job is that, at the next downtime, you will go out and find the ground and clear it. Yah, right! That ground will stay there until the next ground fault occurs and shuts you down. Then, the maintenance budget will kick in and, well, lookie here! There's two grounds, not just one!

Wonder where that came from? And, is that maybe why that buzzer and those yellow lights are on in the substation? And have been on for months! Good grief! I see that all the time because AC drives and especially DC drives do not work properly on corner grounded delta power. So, I check it before I start up a new system and, if there is a corner ground, I report it and question it. You should see the blank stares I get. A corner ground? Really? What is that? Do we have to fix it now?!!!

Well, you get the picture. Sorry for the rant!

 

[leitmotif]

Dick

Well we all got our soft spots.

The last place I worked was where I wrote about funny voltages on a transformer (wye). when they tore it out to move I checked and it was not grounded there was 3" of yellow wire hanging off the star point terminal. The central theme with that outfit was "if it runs it is OK" UNSAID was we dont care about employee safety. I am so glad I am out of there.

SO the reason for grounding is better clearance of faults ie get a grounded phase conductor it basically becomes a short and blow fuse or breaker. Makes sense and is a simple answer - guess I knew that all along - was looking to get complicated answer
- I NEVER make things complicated
- wanna buy a bridge - got two of em -- make you a good deal on the Alaska Way Viaduct in Seattle - just come and get it.

Dan Bentler

 

[Jim Dungar]

There is no safe power distribution system with simple protection schemes. For example, the Line-Ground currents in many 480Y/277V systems are not sufficient to clear arcing faults, this is why these systems are required to have Ground Fault protection above 1000A, but how often are they disabled because their tripping is a nuisance.

If proper maintenance is not performed every system is extremely dangerous. It is a very simple concept - if there is a fault in the power system - fix it. In most facilities if there was a fault in the process it would be annunciated in/on SCADA alarm pages and reports. Why not monitor and report problems with the electrical needed to run the process, just like steam, air, and water supplies?

Many equipment manufacturers have decided that solidly connected wye systems are so common that their equipment is not built for use on other systems. This definitely causes problems, often resulting in violent failures. But this is usually because of the equipment design not the power system. Power is not much different than process, incompatible systems should not be directly connected.

 

[Roy Matson]

This is an interesting problem, I have never seen a system with one phase grounded, it sounds dangerous.
I second what Milldrone says, you may have a ground thats not supposed to be there. If thats the case you need to track it down by isolating each circuit.

 

[brucechase]

This is an interesting problem, I have never seen a system with one phase grounded, it sounds dangerous.
I second what Milldrone says, you may have a ground thats not supposed to be there. If thats the case you need to track it down by isolating each circuit.

It's not dangerous. I have seen and worked on many.

Look at your disconnects. If there is a slug (dummy fuse) in one phase, then it is a corner grounded delta. You can not fuse an intentionally ground leg of a power system.

 

[Vic]

If this is truly an ungrounded delta and one leg is grounded. The next ground will announce itself with a big boom! Like blow the doors off cabinets!

Let's be realistic here. A second ground on a ungrounded system will not "blow the doors off cabinets" any more than the first ground on a grounded wye system.

 

[leitmotif]

Let's be realistic here. A second ground on a ungrounded system will not "blow the doors off cabinets" any more than the first ground on a grounded wye system.

I dont think that two grounds on different phases of grounded or ungrounded delta would necessarily blow doors off cabinets. If they did that would be a blessing in a way - now you have found your ground problem (at least one)

The BIG problem is that the two grounds would result in current flowing thru components (structural steel, conduit, pipe, etc). If these were separated someone may be in for a surprise. I have heard of plumbers getting into trouble and getting bit when taking apart pipe runs.

Dan Bentler

 

[Derek]

I have heard of plumbers getting into trouble and getting bit when taking apart pipe runs.

Dan Bentler

Back when I was a young apprentice I learned a lifetime lesson
on this very subject....