480/240 3w delta delta grounding

[chip]

WE HAVE 480 / 240 3W DELTA DELTA?STEP DOWN TRANSFORMER, WE TOOK THE GROUNDS TO THE TRANSFORMERS ENCLOSURE AND FRAME AND NOT TO ANY OF THE WINDINGS. IS THIS O.K. TO DO? I READ A LITTLE ABOUT THIS, AND I UNDERSTOOD IT TO SAY THAT IT WILL BE A FLOATING GROUND AND POSSIBLE THAT IF A SINGLE PHASE WAS TO SHORT NOTHING WOULD HAPPEN. THANKS, IF ANYONE COULD GIVE ME MORE INFORMATION ON THIS.

 

[charlie b]

Re: 480/240 3w delta delta grounding

Please turn off the ALL CAPS feature. It makes the post more difficult to read. It is also considered impolite, as though you were SHOUTING at us.

There are circumstances in which an ungrounded system is acceptable. Not many, but some. Look at NEC Article 250.21.

What is the application (i.e., type of facility, type of loads, etc.)?

 

[tom baker]

Re: 480/240 3w delta delta grounding

The NEC allows certain systems to operate ungrounded, see 250.20. However there is a risk involved.
If there is a line to case fault nothing happens.
If there is second line to case fault you have a line to line short.
If you want to operate undgrounded secondary then install ground detection system. Take a look at 250.36
Its critical you and your facility staff understand how this system works. Its more than we can go into here at the forum. Why not a standard delta wye transformer? Its much safer.

 

[templdl]

Re: 480/240 3w delta delta grounding

Yes, nothing may happens if one line goes to ground. But all hell breaks loose should a second line go to ground. Special attention must be paid to the application of ground fault detectors in this system. These systems are more apt to be used when there is an industrial process that is critical that can't be shut down and would lead to catastrophic results during the process because of a grounded line. If a line goes to ground the ground fault detector indicates this and the "qualified person" locates and identifies the problem and has an opportunity to isolate and/or repair it without shutting down the precess. Or, the process may be able to be shut down in an orderly fashion manually.
The trouble with an ungrounded system is that one line going to ground may not be bad but it's that second one that causes the grief. When 2 lines go to ground arcing fault voltages can actually exceed the L-L voltage. Also, if one line goes to ground in on part of a facility and a second line goes to ground in another part of the facility it is pot luck what path current will take between the two points where the lines have gone to ground.
If there is not good reason to have an ungrounded delta it is of my opinion the on line should be grounded, i.e. grounded 'B' phase.
An ungrounded delta system should be supervised by a qualified person who understands the system, recognizes the dangers of this configuration and knows what to do if there is a failure.

 

[chip]

Re: 480/240 3w delta delta grounding

Thanks for the information, I wanted addional feedback to back what I was getting out of all the information I found. If there is any other information about this please reply.