Sound Reinforcement - Forums for Live Sound Professionals - Your Displayed Name Must Be Your Real Full Name To Post In The Live Sound Forums > AC Power and Grounding

Open-Leg Delta

<< < (3/3)

Frank Koenig:
It seems to me that a lot of grief could be avoided if these things were just labeled better. Perhaps a nice (orange) sticker inside the door of the panel that announces that this a high-leg delta three-phase service. It could also have a little schematic diagram showing the secondaries of the transformers, how they are connected to the buses in the panel and what the voltage should read between various points. I know, I know, until code requires it this will never happen and likely not even then.

A point of terminology: Most problems arise because of the “HIGH-leg delta” topology. The fact that one pair of phases is not supplied by its own transformer winding – “OPEN-leg delta” – does not change the ways things can be screwed-up by an unaware user.

--Frank

Stephen Swaffer:

--- Quote from: Frank Koenig on August 25, 2023, 04:41:41 PM ---It seems to me that a lot of grief could be avoided if these things were just labeled better. Perhaps a nice (orange) sticker inside the door of the panel that announces that this a high-leg delta three-phase service. It could also have a little schematic diagram showing the secondaries of the transformers, how they are connected to the buses in the panel and what the voltage should read between various points. I know, I know, until code requires it this will never happen and likely not even then.

A point of terminology: Most problems arise because of the “HIGH-leg delta” topology. The fact that one pair of phases is not supplied by its own transformer winding – “OPEN-leg delta” – does not change the ways things can be screwed-up by an unaware user.

--Frank

--- End quote ---

Done:

NEC 110.15, 230.56, 408.3 (F) (1)  Unfortunately, code is rarely heeded retroactively and not always currently.  As always, a meter is your friend. Code also requires the high leg to be the "B" phase-the install I dealt with it was the "C" phase.

110.15 High-Leg Marking
On a 4-wire, delta-connected system where the midpoint of one phase winding is grounded, only the conductor or busbar having the higher phase voltage to ground shall be durably and permanently marked by an outer finish that is orange in color or by other effective means. Such identification shall be placed at each point on the system where a connection is made if the grounded conductor is also present.

Navigation

[0] Message Index

[*] Previous page

Go to full version