| In the case of a commercial building served by 3 phase - closed delta | 240/120 Vservice, what is the best way to detect a ground fault on a | three phase motor circuit on phase C were it to come into contact with | the ground?
A GFI device? All 4 wires (A,B,C,N) would run through the CT together.
| Whereas the service transformer on phase AB has the only centered | tapped (neutral) windings 120V phase-to-neutral and 240V | phase-to-phase and phase C (not connected to the phase AB transformer) | is the wild phase.
It is 208 volts relative to ground.
| It seems to me that (without additional protective equipment), if | phase C accidently touches the ground there would be a voltage | imbalance, but the overcurrent protection would not necessarily trip | off. | | Am I wrong by assuming this?
That depends on how solid the ground path is. If the impedance is high to get back to the source, maybe not enough current will flow. That could easily be the case if it contacts earth. It could even be the case if it contacts a metal part of the building. But if the impedance is low enough, you get high current.
Still, if there is any risk of a ground fault, and especially one that could be contacted by a person as part of the ground path, then ground fault protection would be a good thing to have. A GFI device?
| Also, what, in general, are the pros and cons of a 120/240V 3 phase | closed delta connection?
If there is a loss of one phase in this system it can lead to trouble. Delta can have back-energizing issues as well as lopsided loading if a phase is lost. And 240D/120 would be quickly out of balance if the
120 volt loads predominate. Alternatively, a Scott-T version of that could help in limited 3-phase loading environments.
| Is this a good choice for a small commercial building with a 3 story | hydraulic elevator and mostly 120V. appliance and lighting loads?
208Y/120. Configure the elevator system for 208 volts. It is the more common electrical system in the USA, now. An elevator manufacturer not being able to do 208 volts is limit their market severely.