Transformer Delta Secondary phase grounded

[ksavoie]

I'm doing a project in an old plant where there is a 13.2kv to 480v, 1500kva, delta - delta transformer. The 'B' phase on the secondary is grounded. What advantage or disadvantates of this phase being grouned? Why not just have a floating ground?

 

[davidbeach]

Do a search on this forum for "ungrounded" and you should find lots.

 

[apowerengr]

What is a floating ground? It's either grounded or it's not. I agree fully with davidbeach, we continue to discuss grounding methods in many threads.

 

[TestBeforeTouch]

Up through the 1950's many industrial power systems were ungrounded. This allowed the system to continue to run on a single fault. Unfortunately, ungrounded systems had problems:
> Arcing to ground could cause fires because the fuse would not blow
> It could be difficult to locate the fault so it would stay on a long time.
> Arcing could cause overvoltages (3x or more)causing other equipment to fail not associated with the first problem.

Rather than replacing their transformers or buying a zig-zag transformer many industrials decided to ground the B phase because it was much cheaper and easier. However there are some differences with B phase grounding that people need to be aware of:
> The voltage of phases A and C to ground are the line to line voltage (e.g. 480) not the "normal" line to neutral voltage so there is a greater shock hazard to people working on the equipment
> Some equipment is not rated for B phase grounding because of the higher voltage to ground
> B phase fuses must be removed and a dummy fuse or bus bar installed in its place, because the NEC in Article 240.22 does not allow a fuse in a grounded conductor. This is because if the B phase fuse were to open and a ground fault existed at the load, current that normally would flow in the B Phase could flow over over the much smaller grounding wire (e.g. a 200 amp load requires a 4/0 phase conductor but only a #6 grounding conductor)
> One advantage of B phase grounding is that because the voltage to ground is higher a ground fault on a large piece of equipment is more likely to clear quickly, especially in an older facilty where they used just conduit and cable tray as the ground fault return path.

 

[jghrist]

See

http://www.i-gard.com/pdf/publications/EB_Grounding_Overview.pdf

for a discussion of the problems with ungrounded systems and advantages of high resistance grounding.