Expert: Dave Nyce - 5/13/2003

Question
Hi Dave,  Thanks for all the quality answers you have given me in the past.  The question I have today is about power generation.  I was reading that when power is generated using a Delta configuration, often one corner is grounded.  How can this be done and still have 3 phase power?  Wouldn't this be the same as if someone grounded one of the three phase wires coming into the building.  Wouldn't a breaker trip since power would be draining to ground?  Thanks for any explanation you can give.  

Answer
If the 3 phases are already ground referenced, then a circuit breaker would open if you shorted a phase to ground. But, they are talking about a delta connection straight from the alternator, in which the phases are not ground referenced. They are isolated because the voltages come directly from the alternator coils with no connection to the earth or any common point. The delta phases are only referenced to each other. In that case, one phase can be grounded. Then you need to use a transformer if you want to regain isolation later.

Hope this helps!

Dave