Re: grounding of 240/480 delta transformer
OK, I give up on that this system is required by NEC to be grounded but, lets look at the application in this instance:
1. 240V Delta to 480V Delta
2. This system will serve two or more (separate) machines
3. Ground fault monitoring is not planned or economically feasible ( I added that part myself but, go with me here)
4. There may or may not be an EGC installed here to bond everything together, I hope there is.
The question was if it was acceptable to ground the case- presumably from a grounded 240 Delta system and let the 480V secondary float, ungrounded.
Since there are multiple applications there are multiple touchpoints. If say machine A faults on the B phase, and the system is not grounded or bonded, the OCPD will not open. Say machine B, sitting close by, same conditions, faults on the C phase, same thing- both machines are up and running, no stopage in the workplace, the front office is happy. However, if there is any connection between the two- communications shield, conduit, Joe the maintenance man- bad things are going to happen here.
Ungrounded systems are useful where there is only ONE phase to ground- Medical O.R. Isolation panels or where the nature of the process leads to a ground fault- arc furnace. They are not safer because you can touch them- sometimes- and not have the snot knocked out of you. Not a good idea in this instance, I think most would agree.
Now, if there was a way to monitor fault currents in this system, OK, let it float, it's allowed by NEC, if so equipped.
Otherwise, save the big bucks on the monitoring system, monthly maintenance and engineering. Ground a phase to deliberatly reference ground so that an unintentional fault will open the OCPD. The down side of this is that by going delta secondary instead of WYE you will have 480V to ground, which can be exciting also but, that's another thread.