grounded phase

[plt]

I have been asked about a grounded phase conductor on a 240/3 phase delta.I know that one phase will read 0volts to ground and that phase with any other phase will read 240 as will anyother phase combination.I have never seen anything that would indicate that there is a color code for that grounded phase to identify that that third phase goes to ground. This whole topic of a grounded phase has illuded me. no one that i ask seems to know anything about it. If anyone can tell me more about it:it's purpose and theory behind it, please do so. this is extremely interesting to me.
Thank you
Richard

 

[bphgravity]

Re: grounded phase

What you are describing is a "corner-grounded delta" connection. This system provides three phase power at 240-volts or 480-volts. 1 leg is grounded and should be identified as a grounded conductor per Article 200.6. The other two legs should be fused and identified as ungrounded conductors.

 

[charlie]

Re: grounded phase

The grounded phase wye is a bit strange also. If you have a 480 or 600 volt wye that does not have the center point grounded or brought out as a circuit conductor, you are permitted to either operate it as an ungrounded or corner grounded delta system.

With the above system, once you are outside the transformer enclosure, you can not tell that it is wye connected regardless of the instrumentation you use.

 

[sandsnow]

Re: grounded phase

What is the application for using a wye xfmr as a 3 phase 3 wire secondary grounded or not ? It just never occurred to me you could do that?

It makes me curious about all our rules for grounding and just how many systems the code permits to operate ungrounded. Strange

 

[bennie]

Re: grounded phase

A delta is a motor load,only. a 277 is a commercial lighting load. and a motor load.

A delta 240 volts is also a motor load.

It all depends on what works best for the situation.

 

[don_resqcapt19]

Re: grounded phase

Larry,
In general systems that operate above 150 volts to ground and do not use a "common" (neutral) as a circuit conductor are not required by the code to be grounded.
Don

 

[sandsnow]

Re: grounded phase

Thanks Bennie & Don


My first question had to with the wye system operating as a 3phase 3wire. Let me kind of rewrite it. I was more interested if there systems like this actually in use? If so, why did they use a wye instead of the more common (at least here) delta for 3phase 3wire.

 

[dereckbc]

Re: grounded phase

I will take a shot at this that may help. Both Delta and Wye have pros and cons.

Delta systems are used widely in industrial applications like an extruder where outages cannot be tolerated from line-to-ground faults. The loads are 3-phase like motors and heaters. A line-to-ground fault will not trip the system off line. It will set off alarms to alert maintenance personnel to take corrective action. The down side is over current detection systems are more complex, therefore more expensive. Since Delta's are not solidly referenced to ground, that leaves capacitive coupling which add stress to cable insulation and instability. Therefore the system requires more maintenance and shorter life span in theory.

Wye systems or solidly grounded systems use simple over current protection (fuses and breakers). System voltages are stabile with respect to earth minimizing stress on cable insulation. Down side is the are designed to fail causing service interruptions from line-to-ground faults. Some can argue wye systems are more dangerous to human life from the possibility of comming in contact with live conductors.

The third alternative is a resistive or impedance grounded system, which take advantage of both systems.

Hope that helps.

 

[sandsnow]

Re: grounded phase

Thanks Derek, but that's not what I was looking for either. I apologize, but I'm really not trying to be difficult here.
We have 3phase 3 wire delta here with and without ground detectors.
What I'm interested in is the
3 PHASE 3 WIRE WYE UNGROUNDED system
that Charlie mentioned. I understand the code allows this. What I would like to know is if this system is actually in use anywhere. I've never heard of it and thought it might be particular to a different part of the country.

 

[arcblast]

Re: grounded phase

I had to replace a 480 v transformer the other day on a large saw in our plant.
the transformer was a 480-120 stepdown....but what was odd to me was, when i hooked l1,l2,l3 to x1,x2,x3 to the transformer and then hooked the oil sump to the 120v side of the transformer, the pump wouldn't run....then I was told I had to ground a phase on the primary side to the chasis !!!....it works fine but what the hell is that all about ?...like this topic starter....i am very interested in knowing more !!!

Mike

 

[plt]

Re: grounded phase

Thank you all. You have been very informative and the other spins on this topic have helped me and some of my co-workers understand more about this system, aswering questions before they need to be asked. Thanks again I will keep an eye on this post to see if any thing new is added and watch the responses.