High leg delta is where you have a 4-wire 240V Delta, where instead of all 3 phase carriers being 139V to ground like they would be in 240/139V wye, one of the 240V legs is center-tapped at the transformer, and the center-tap grounded and connected to the neutral. (+120 and -120, just like at a residential pole pig). The other delta- the "high leg" is now at a higher potential, 207V to ground.
So, it ends up where instead of having 3 phases, each 240V between them and 139V to ground, you end up with 3 phases- each of which still have 240V between them, but two of these are now 120V to ground, and the third is 207V to ground.
The advantage with this is that you can supply 3-phase loads and 120V 1-phase loads from a single distribution system without any additional transformers.
[This message has been edited by SteveFehr (edited 10-29-2006).]
