ungrounded Delta

[dm9289]

I work in a 1950s factory which is suppied by 2- 3000a 480v ungrounded Delta services and have been trying to increase my knowledge, no one in this small factory understands it very well and I am trying to understand it better.


What would be the advantages of a ungrounded Delta?



I hooked up many motors to available 480v will motors run on wye or Delta same hook up or is it motor specific ( most ones I see list high or low V)?



Arc flash company is the people who came up with the statement our system is 480V ungrounded Delta, through power company transformers etc. But if you go to most places in the facility you will read anywhere from 250-270v line to ground is this normal seems to me it is behaving as wye?





Anyone have some good suggestions for resources that helped them understand these delta/ wye systems?

 

[templdl]

I work in a 1950s factory which is suppied by 2- 3000a 480v ungrounded Delta services and have been trying to increase my knowledge, no one in this small factory understands it very well and I am trying to understand it better.

What would be the advantages of a ungrounded Delta?

I hooked up many motors to available 480v will motors run on wye or Delta same hook up or is it motor specific ( most ones I see list high or low V)?

Arc flash company is the people who came up with the statement our system is 480V ungrounded Delta, through power company transformers etc. But if you go to most places in the facility you will read anywhere from 250-270v line to ground is this normal seems to me it is behaving as wye?

Anyone have some good suggestions for resources that helped them understand these delta/ wye systems?

Well, it looks as if your post has sat here for a bit so I'll step up to the plate and start a discussion.
Personally, I am of the opinion that corner grounding is preferred. It is done be simply grounding one phase. That phase would be a "grounded" conductor. Instead of it being a neutral it is a phase conductor. As such it can't be fused but you could used a breaker only if the other 2 phases are opened at the same time.
At the point where that phase is grounded is the point where you connect you EGC which is brought out as you would the EGC for a grounded eye system.
With a corner grounded delta should either one of the ungrounded phases goes to ground the is a path back to the source to trip an OCPD.
If left ungrounded should 1ph go to ground there is no path back to the source. The issue becomes very serious matter should a second phase goes to ground. From the point at which the first phase is grounded to where the second phase is grounded should provide some excitement.
And arcing faults are a much greater issue because arcing voltage can greatly exceed 480v in 480v ungrounded delta system
The advantages oh an ungrounded delta I the capability of preventing an unplanned shut down because of a ground fault that could prove to be a disaster with a continuous manufacturing process. Through the use of properly applied ground fault detectors with qualified people present should a ground fault detector sense a ground fault shutting down the process in an orderly fashion is possible as well as being able to locate an isolate the could to prevent the fault from escalating and taking down the entire manufacturing process.

I trust that this will jump start some discussion.

 

[hurk27]

As Temp pointed out, ungrounded deltas was the preferred choice in plants that had production lines the would be dangerous or costly if a fault brought down the whole place, as an ungrounded delta even an arc flash has one less path to occur on as there is no fault path until the second phase goes to ground, first fault will only make it corner grounded.

The problem you will have is that back in the day many didn't use ground fault detectors and relied upon a maintenance worker to do a daily or shift check to make sure everything was good, but as these plants were closed and or others move in, this daily/shift check was never done by the new company that moved in, hence to why ground detectors were most likely added to the code, which if kept as a ungrounded service ground detectors will be a must.

Problem two, if you decide to corner ground these systems you may open up a can of worms, this is because both are well above the 1k amps that the code will require ground fault protection, if a fault was to occur on these systems after they have been changed to a corner ground the grounding paths might not be able to handle the magnitude of this available fault current and grounding paths such as conduit couplings and box connectors in EMT or other raceways could fail in a catastrophic arc flash in un expected areas where people would not be waring protective arc flash gear, that could cause injuries or death, other types of grounding paths might also fail such as cable trays, and or building bonds if they are in the path of the fault current, this is why a GFP's should be installed if you were to chose to ground these systems, these would also have to be coordinated with down stream breakers and fuses as you would not want a fault on a downstream circuit to take out the whole buildings power.

Third problem, most VFD drives do not like ungrounded systems, but can be accommodated by using a local 480v delta to 480/277 WYE and re-grounding the secondary X0 and connecting to building steel.

So these are some of the things that should be discussed when whom ever it be that makes the decision as to which way to go.

As far as motors go, they don't care if the supply is a delta is a WYE or delta, they only care about line to line voltage and how balance the voltage is, like I said some VFD's will not like the ungrounded delta and some drive manufactures will not warranty the drive is such is found to supply the drive, but that is just a matter of installing a local transformer for the drive to provide a ground reference.

As far as the voltage you are seeing to ground these could be just static voltages, and if balanced enough they will seek a neutral form of the supply, this could also indicate you could have a high impedance grounded system where each line is grounded through a resistor bank to the GEC's which will also give you close to the 277 volt reading but the 277 volts current is un usable as it is floating through the resistors, but this will limit your voltage to ground to around the 277 volt range.

I hope this clears up the questions you needed to know, if you have anymore just ask away