delta secondary and bonding

[bert47]

I can only speak from industrial experience with delta secondaries. I've read where several have said that bonding one of the secondary phases was necessary to ensure operation of the ground fault protection equipment. My opinion is, that won't do it. All you've done is ground one phase (say C). Now your plant frame work is the C phase, relative to A and B. There is now a 480v difference between "ground-C" and phases A and B. Any short circuit between A, B, or "ground-C" will result in operating the OC protection (i.e. fuse or breaker) but not a standard GFI. Ungrounded delta systems were very popular in the past before GFI evolved and as GFI came of age a detection system was sold that used a "zig-zag" transformer to establish a high impedence grounded reference point for the system. It allowed a ground fault on any of the insulated A, B, C phases to flow through the GFI CT as it returned to the other phases and initiate a trip. This is the only way that I know of to effectively protect a delta system with GFI. There might be other ways I'm not aware of.