Delta and wye power/ arc flash studies

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dm9289

Industrial Maintenance Electrician
Location
Pennsylvania
Occupation
Industrial process repair/ maintenance Electrician
The company I work for recently had an arc flash study and hired a comapany to update one line diagrams make fancy stickers etc. So I wanted to use this info to learn more about our overhead feeds etc. They were showing our overheads, feeds to various MCCs and induction fans etc as ungrouded Delta fed. I do not have much experience with power systems but it was my general understanding that on a 480v system for example in a wye you would have about 277v to ground which we do have in all the above examples. When I would measure inside the various areas we have 3 hots all reading 277v to ground. My delta understanding was 480v phase to phase and 0v to ground in an ungrounded delta. Or if the delta was grounded you would have 2 lower voltage legs and one high leg sometimes refered to as a bastard leg. Could you please undumb me or refer me to some good text I could purchase on these systems. I will recheck the post tonight as you can see by the time I work nightshift at my mill.

Thanks as always
Dave
 

jim dungar

Moderator
Staff member
Location
Wisconsin
Occupation
PE (Retired) - Power Systems
Depending on which type of meter you used, especially a digital one, it is extremely likely that a balanced 480V 3W ungrounded delta will read 277V to ground. This is often called a phantom voltage.

Arc flash providers model based on the number of conductors in a circuit, are you sure they show the transformers as 480V delta secondaries?

A high resistance grounded system will have voltages to ground just like a wye system does.
 

zog

Senior Member
Location
Charlotte, NC
In a ungrounded delta system there is no reference to ground, so when you measure phase to ground you should see anything from 200-300V, this will vary depending on the capacatince of the system. If you see 0V to ground you have a grounded phase (Either a big problem, or could be intentional if corner grounded).

If you read 277V to ground on each phase I would wonder if the labels from the study are correct. Just look at the transformer nameplate.
 

dm9289

Industrial Maintenance Electrician
Location
Pennsylvania
Occupation
Industrial process repair/ maintenance Electrician
The power company was going to come out and oen the transformers in our yard. Thanks but it appears i have a misunderstanding of the delta. I thought the 277v to ground was exclusive to the wye.
 

zog

Senior Member
Location
Charlotte, NC
The power company was going to come out and oen the transformers in our yard. Thanks but it appears i have a misunderstanding of the delta. I thought the 277v to ground was exclusive to the wye.

It is, if you get 277V to ground on a delta system it is by chance, not design.
 
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