Discussion of Delta 3ph services. Open, Closed, etc

[bob8907]

I cannot seem to get a straight answer from most electricians and I think this is mostly out of their scope of expertise and I am not an EE so hoping to get something from the horses mouth.

When discussing Delta connections, some have indicated you may or may not have a high leg. I have NEVER seen a Delta system without a high leg. I am trying to gather some connection drawings of the different Deltas to better understand this. Per my understanding, the open delta uses on 2 transformers and 2 legs of input power to triangulate the 3 phase but still creates a high leg to the neutral reference. This load will largely be unbalanced so is only good for small installations. Closed delta uses 3 cans. I can see no way with either to eliminate the high leg while still offering A and B to neutral loading.

Can someone clarify this is laymen terms for me...??

 

[davidbeach]

Any delta with a center tap neutral will have a high leg. Any delta with a corner ground will not have a high leg. Any ungrounded delta will not have a high leg.

 

[bob8907]

Would you have any drawings or references on the different connections? I will assume the center tapped delta is most common in building installations where L-N loads are common and the corner ground delta might be used on dedicated installs like pumps?

 

[dpc]

Corner grounding is extremely rare, at least in the US. The center-tapped (wild-leg) delta is used mostly for small pump installations where the predominant load is three-phase with a small amount of 120 V. load. Both configurations are best avoided if possible. The open-delta center-tapped approach is sometimes used where there is a small amount of three-phase load (small motors) and more single-phase load (or where only V-phase primary is available.

When three-phase power is needed to a building, best options are 208/120V or 480/277V.



David Castor

http://www.cvoes.com