Basically if you do not have a true Neutral - one that originates from your transformer - we do not recommend using our Omnimeters for voltages past 415V.  That being said, it technically may work to use Ground as Neutral, if you test and confirm that the voltage from Line to Ground/"Neutral" is what it should be and not something higher than this - for instance if it is a 240V line, then Line to Ground/"Neutral" should indeed be about 240V.  Again, we do not officially recommend/support this, however.  

As stated online, our Omnimeters go up to 480V 3 phase 4 wire, and 415V 3 phase 3 wire.

We are in the process of developing a meter that will be able to handle this voltage easily.  This will be a ways down the road from now, however.  Let us know if you want us to notify you when we have these new meters in stock in the future.

If at all possible, run true Neutral to the meter.  Then you can use our Omnimeter and 3 CTs: See Package Here.