Hi Paul,
What exactly does SDS stand for

Well it's supposed to stand for 'Seperatly Derived Systems' , which by NEC's definitions is something that creates a nuetral, as an X-former or Generator.

As with many of our NEC definitions, there are plenty of gray area's to dwell on.

On the 4-wire delta systems, why is the high-line defined as phase C in service equipment and phase B elsewhere? Come to that, what exactly do you define as "service" equipment?

hmmm, how does one nicely explain how our definitions go in circles ?
The 'Utility' and the NEC do not always see eye to eye, and many times make conflicting rules, as in the high-leg of a delta.

'Servive equipment' would be the main panel, shutoff , T-switch, whatever is first on the load end of the utilities incomming service entrance wires.

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