when is it a separately derived system?

[electrofelon]

With the advent of more and more devices that use power electronics, is anyone else unclear at times if we are creating a separately derived system? I got thinking about this because I have a job coming up where I need to power several three phase CNC machining centers off a single phase supply. I am considering using a central phase perfect phase converter to run a three phase distribution system in the building. How do I know if the three phase system is isolated from the single phase system? The question can also arise with inverters where some incorporate transformers and some do not, and to further confuse things, just because it has a transformer does not mean the source is isolated from the supply. How can one be sure that he/she is meeting the grounding requirements for separately derived systems?

 

[ActionDave]

I dunno. I suppose the ultimate test is to reach out and grab a live conductor. hmy:

 

[Smart $]

The definition says no direct connection of circuit conductors...

If you check resistance between all possible permutations of input to output, and none have an extremely low resistance (say <1 ohm over a short distance), then you have a separately derived system.

 

[GoldDigger]

The definition says no direct connection of circuit conductors...

If you check resistance between all possible permutations of input to output, and none have an extremely low resistance (say <1 ohm over a short distance), then you have a separately derived system.

Which, of course includes the grounded conductor if there is one. And the "direct" connection is important because the two grounded conductors will be indirectly connected via the ground bond.