Unitentionally grounded ungrounded systems

[richwaskowitz]

When we install variable frequency drives in large industrial facilities where they are supposed to have an ungrounded 480 volt 3 phase distribution system, we are instructed by the drives manufacturers to disconnect a phase to ground jumper in the drive's front end that is there to connect the common point of the three incoming line MOV's to ground. If the system is grounded however, the jumper must be left installed.

The problem is that these ungrounded systems can and do become grounded unitentionally at times with no-one taking any steps to locate and correct the ground, sometimes for very long periods of time. How does one deal with this? This can't be all that uncommon of a situation.

 

[charlie b]

Re: Unitentionally grounded ungrounded systems

Originally posted by richwaskowitz: The problem is that these ungrounded systems can and do become grounded unintentionally at times with no-one taking any steps to locate and correct the ground, sometimes for very long periods of time.

That is a recipe for disaster. The whole intent of an ungrounded system relies on the timely detection and correction of any unintended ground fault. If you don?t correct the first, then the second will cause something to break, and perhaps cause someone to get hurt.

This can't be all that uncommon of a situation.

I truly hope you are wrong in this assessment.

 

[richwaskowitz]

Re: Unitentionally grounded ungrounded systems

Unfortunately not. Have you ever done much work in auto assembly plants?

 

[69boss302]

Re: Unitentionally grounded ungrounded systems

All to true that grounds are not addressed right away. You know Charlie b that these grounds were jumped on in the Navy and that Continuity of power was important.

Unfortunately like rich said it don't happen like that and not necessarily just in the auto plants. The steal mill I was in, didn't even have ground detectors on some systems and one of the ways they found out there was a ground was after about the third motor went poof in just as many day's somebody figured there must be a ground and we should find it. Oh, but you can't turn anything off to find it. "What do you mean you can't megger something hot?" There's to many bean counters that don't know anything about electricity not to mention safety.

The first ground is always hard to find but the second can show up in some rather spectacular display's . Is it safe NO!, Does it happen, Yes. I can't figure out why more people have not been hurt on the industrial end of this field. They are just plain lucky. And management gets to dodge bullets because production drives everything.

OK get me off my soap box. We have to do what needs to be done and in the safest manner we can. You wouldn't believe how many people even on the worker side don't even want to hear about NFPA 70E. (Well maybe you would by some of the comments on this site.)

 

[kiloamp7]

Re: Unitentionally grounded ungrounded systems

Another reason to use the good old solidly grounded wye.

Or if continuity of power is important enough,& skilled technical people are available, then the high-resistance grounded wye.

 

[petersonra]

Re: Unitentionally grounded ungrounded systems

The problem with using wye is that production is important. There is nothing inherently unsafe in using delta as long as the plant has ground detectors. Since this can consist of something as cheap as three lights, I don't see the ground detection issue as a major problem.

I wonder why plants don't just install the ground detect lights in the electrician's break room. Since there is always an electrician on break, a grounded phase would be discovered immediately.

[ April 08, 2005, 10:31 AM: Message edited by: petersonra ]

 

[mc5w]

Re: Unitentionally grounded ungrounded systems

A ground detector that uses 200 watt light bulbs essentially acts as a high resistance ground and will get rid of most capacitively coupled and resonant overvoltages.

One way to trace a ground fault is to connect a load from one of the 2 hot phases to ground and then amplitude shift key ( turn on and off ) that load. For 480 volts you could use a pair of 240 volt electric baseboard heaters in series. You could then follow the amplitude shift keyed ground fault current by looking for the characteristic wiggle on an analog ammeter.

I have found that high frequency current tracers are not what they are cracked up to be. They only get rid of 90% of the guesswork when trying to find the correct breaker to turn off.

 

[iwire]

Re: Unitentionally grounded ungrounded systems

Originally posted by petersonra:
I wonder why plants don't just install the ground detect lights in the electrician's break room. Since there is always an electrician on break, a grounded phase would be discovered immediately. LOL

However, discovering it and then leaving the break table to do something about it are separate issues. :roll:

 

[charlie tuna]

Re: Unitentionally grounded ungrounded systems

rich,
are you saying that assembly lines operate on ungrounded electrical systems??? in your post you say "supposidly" ungrounded systems---whats the deal??? i really think it is very important to have a return path to ground on the output of a variable speed drive due to it's high output voltage!!!

 

[don_resqcapt19]

Re: Unitentionally grounded ungrounded systems

i really think it is very important to have a return path to ground on the output of a variable speed drive due to it's high output voltage!!!

Is the output of a VFD a grounded system. I don't think so, unless there is an internal bonding connection in the drive.
Don

 

[charlie tuna]

Re: Unitentionally grounded ungrounded systems

don,
yes your correct -- the drive creates it's own variable d.c. frequency. but, from the damage i have come across inside the drives themselves, i would think they would be better off grounded. most of the damage occurs on voltage surges and blow out the igbt transistors. and i am curious that a assembly line would be run on an ungrounded system????

 

[iwire]

Re: Unitentionally grounded ungrounded systems

Charlie now you got me confused.

An assembly line is just the type of application I would expect to run an ungrounded system.

How does the lack of a grounded conductor at a 3 phase drive effect the drive?

 

[charlie tuna]

Re: Unitentionally grounded ungrounded systems

i'm thinking if the drive grounded out during a power surge and energized the enclosure from the drive's power source --- not the drive's output!

 

[marissa2]

Re: Unitentionally grounded ungrounded systems

Having a ground detector light is on thing, but in a large building finding the problem in another.
Lou

 

[69boss302]

Re: Unitentionally grounded ungrounded systems

I believe the largest source of power surge is from the output side actually. Regeneration when decelerating, or turning off the motor causes a large voltage kick back into the drive. This can be seen on the DC Bus and has a tendency to burn up IGBT's.

 

[petersonra]

Re: Unitentionally grounded ungrounded systems

Originally posted by 69boss302:
I believe the largest source of power surge is from the output side actually. Regeneration when decelerating, or turning off the motor causes a large voltage kick back into the drive. This can be seen on the DC Bus and has a tendency to burn up IGBT's.

Thats why braking resistors are added, or some kind of regeneration back to line. This has absoluteley NOTHING to do with grounded or not grounded power. It has to do with inappropriate selection of equipment.

 

[69boss302]

Re: Unitentionally grounded ungrounded systems

I know about breaking resisters and regen packages. I have also stood in front of the President of the company while he said, we don't have enough for all the extra stuff just get the basic drive and let's see how it does first. I even try to explain that you had better buy two right of the bat then. But what do I know I've only seen the problem to many times, not looked at the #'s. But if I did look at the #s wouldn't it make sense to spend the little more to make the thing work right, rather have to do it again with the right equipment the second time. Or even go Oh this didn't work we'll just go back to the cheap piece of s**t we had in the first place.

EDIT: You are right it has a lot to do with the choice of inappropriat equipment. But when an IGBT burns up it has a tendency to cause a ground on the supply end also.

[ April 18, 2005, 03:55 PM: Message edited by: 69boss302 ]

 

[iwire]

Re: Unitentionally grounded ungrounded systems

Originally posted by charlie tuna:
i'm thinking if the drive grounded out during a power surge and energized the enclosure from the drive's power source --- not the drive's output!

I am still confused.

The enclosure can not be energized.

The enclosure of the drive is always grounded, the system feeding it may not be, as in ungrounded delta.

A fault from one of the ungrounded delta conductors to the enclosure only results in a grounded delta system and should activate a ground fault indicator.

 

[mc5w]

Re: Unitentionally grounded ungrounded systems

Ungrounded systems can also suffer from ferroresonant overvoltages, particularly at light load. A 480 volt ungrounded system can have a phase to ground voltage of up 2,000 volts AC due to ferroresonance between the system capacitance to ground and steel core inductances such as transformers.

There is somebody over at www.eng-tips.com who says that at one plant where he worked they would get a ferroresonance condition every once in a while and it would cause 3 or 4 motors ( not connected to VFDs ) to burn out a week later. You can just imagine what this does to VFDs.