240 volt ungrounded Delta

[JohnE]

Went to check out an old restaurant today to install a new piece of kitchen equipment and to begin an evaluation of the existing wiring in preparation of an addition. It has 2 services. A 120/240 single phase, and what appears to be a 240v ungrounded delta. Both very old and in bad shape. I don't have any experience with the ungrounded delta, but readings are 240V L-N on all phases. Main disconnect is connected to copper water main as it's grounding electrode. Readings from line to this ground are 240v on phases A & B and 0 on phase C. This doesn't make sense to me. Does it to anyone else? Everything is in such poor condition, I was afraid to open the main to take more readings for fear of it not resetting.

Any thoughts will be appreciated.

John

 

[RayS]

If you consider arc flash hazard reduction, it may be safer - no large fault currents unless there is a second phase grounded.

 

[don_resqcapt19]

John,

I don't have any experience with the ungrounded delta, but readings are 240V L-N on all phases. Main disconnect is connected to copper water main as it's grounding electrode. Readings from line to this ground are 240v on phases A & B and 0 on phase C.

There is no neutral on an ungrounded delta. The last set of readings sounds like a corner grounded delta.
Don

 

[JohnE]

John,

There is no neutral on an ungrounded delta. The last set of readings sounds like a corner grounded delta.
Don

Don,

I know that. It was a typo for L-L. Habit I guess.

I'm just revisiting this thread because we are about to begin the work on the restaurant. After rereading the thread and my post, I must have meant 240 V L-L on all 3 phases. There is definitely no neutral in this system. So if I have 240V L-G on 2 phases and 0V on the 3rd, then there is an existing ground fault.

We have not done any work on the 3 phase yet, but I did meet with the Poco to discuss this service as well as additional services for another building on the property. The Poco designer/engineer (whatever he is) told me that a 240V ungrounded service is common. That they still provide this today, and would not want to change it for a 120/208 or any other voltage for that matter. That surprised me.

I guess I'll be doing some troubleshooting real soon. There was a lengthy period of decision making with this project, and I completely forgot about my post regarding this until searching the forum.

 

[aja21]

John, you are correct. This sounds like an ungrounded delta with a ground fault which consequently will give you the same type readings as a corner tap delta. Can get confusing but they use these systems out here in Nebraska all over the place but mostly in industrial and ag systems to reduce downtime. They are cheaper for the poco because thats whats already there and they have lots of spare equip.
Once again I agree that a school or similar facility, even a restaraunt is not the place for it. But the poco rules.

 

[ghostbuster]

Another side issue to consider is the 120 volt single phase voltage required for most control systems.Usually a single phase step-down transformer is used 240/120v.If 1 of those 240 v. phases you are using goes to ground on an ungrounded delta system,you can loose all your 120 v.contol voltage.Contactors may drop out for eg. in the boiler room etc.Just something to think about.

 

[JohnE]

Another side issue to consider is the 120 volt single phase voltage required for most control systems.Usually a single phase step-down transformer is used 240/120v.If 1 of those 240 v. phases you are using goes to ground on an ungrounded delta system,you can loose all your 120 v.contol voltage.Contactors may drop out for eg. in the boiler room etc.Just something to think about.

Good point. I've never worked in a building with this system, so it's good to hear some of the issues that can arise.

This restaurant has a 200 amp 120/240 service as well as the 400 amp 3 phase delta. The 120/ 240 will be upgraded to a 400 amp. The 3 phase service stays, with little to no additional wiring. Just some rewiring of equipment already there.

 

[brian john]

I'm curious as to why someone would design a ungrounded system for a school? Seems like the purpose of using an ungrounded system does not fit into this application.

I am more than curious!

As stated in another post too many electricians are confused when they run into this type of distribution, besides which it is a school...School MAINTENANCE PERSONEL.......some are my friends, BUT NUFF SAID.

 

[aja21]

...School MAINTENANCE PERSONEL.......some are my friends, BUT NUFF SAID.[/QUOTE]I absolutely agree. This is not the place for this kind of system.