Corner grounded
- Thread starter kimrichi
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Most of the corner grounded systems I have seen are in "heavy" industry where they have chosen that system so as to reduce downtime from electrical "faults".
Locally most are 480 volt, but 240 volt are sometimes used.
Locally most are 480 volt, but 240 volt are sometimes used.
kwired
Electron manager
- Location
- NE Nebraska
Sorry to rain on your parade, but there is no more or less downtime from faults. If there is a fault, the overcurrent devices still open and the equipment still stops operation.
Ungrounded or impedance grounded systems are intended for that purpose and allow for a fault to happen and indication of the fault happens but equipment continues to run, as long as there is not a second fault on another phase, then shutdown is going to happen anyway.
Corner grounded systems have some advantage in material cost because there is no fourth conductor or neutral conductor. The loads supplied are going to be loads that do not need any such neutral conductor. If you do have loads that need the neutral then you will spend more anyway on transformers and other necessary equipment to separately derive a system with a neutral.
Service supplied corner grounded systems are a thing of the past here and possibly in most places, with only a few existing services still in place. They are however pretty common in the case of backfeeding a delta-wye transformer say from 208 volts to derive 480 volts. In that case there is no choice but to ground a phase of the transformer output as there is no neutral to ground.
chris kennedy
Senior Member
- Location
- Miami Fla.
- Occupation
- 60 yr old tool twisting electrician
You sure about that?
kwired
Electron manager
- Location
- NE Nebraska
If you are going to ground something you have no choice but to ground a phase. Your only other option is to install as an ungrounded system, which means to do properly you must have a ground detection system installed.
I just knew someone would challenge that after I posted it.
chris kennedy
Senior Member
- Location
- Miami Fla.
- Occupation
- 60 yr old tool twisting electrician
jumper
Senior Member
- Location
- 3 Hr 2 Min from Winged Horses
Ummm... it reduces downtime if the fault is in the grounded phase. Corner grounded systems are installed with ocp only on the two ungrounded phases. If there is a fault to ground on the grounded phase, there is no ocp to trip. Thus the potential for down time resulting from a phase to ground fault is reduced by 1/3rd.
kwired
Electron manager
- Location
- NE Nebraska
This is no different than having a neutral- ground fault on a system with a grounded neutral.
What are chances of having a grounded conductor to equipment grounding conductor fault, other than physical abuse or poor installation practices?
At same time voltage to ground from the other two phases is higher and it may take less foreign interference to develop a fault.
Yes, it is different. The difference is the number of wires. With a 3? grounded neutral system you have 4 wires. With a 3? corner grounded system, you only have 3 wires. In the former case, you can have, potentially, 3 line-to-ground faults. In the latter case, you can only have 2 line-to-ground faults... thus a reduction by 1/3rd.
ptonsparky
Tom
- Occupation
- EC - retired
I guess it kind of depends on the practice of the area. I see 3 or 4 wire from the meter for corner grounded systems. Ag.
Once we have that first bond, it is 4 wire thereafter and that moves us back up to 3 current carrying conductors to ground fault possibilities.
Once we have that first bond, it is 4 wire thereafter and that moves us back up to 3 current carrying conductors to ground fault possibilities.
Corner grounded is a 3W system. How do you get 4W? Are you counting the EGC?
ptonsparky
Tom
- Occupation
- EC - retired
Yes, I did.
Well in the future, don't... please?!!
ptonsparky
Tom
- Occupation
- EC - retired
What I am trying to say is that we can have 3 line-to-ground faults on a Grounded Delta. I repair several a year. You may not have a noticable flash or a circuit breaker trip, but the grounded phase can and does fault to ground. Most of my repairs happen to be "to earth", but once you leave the service disconnect any connection (intended or not) of the Grounded conductor to the EG (or Earth) is a ground fault.
kwired
Electron manager
- Location
- NE Nebraska
Ok, I'll give you that one. But the result of having the fault is no different than a neutral fault to ground, nothing immediately noticeable happens in many cases. Now if you have resistance in the EGC, you will will start to have stray voltage issues.
If the goal is to not shut down a process when a fault occurs an ungrounded or impedance grounded system is much better choice, but you already know that.
The corner ground system is not chosen for reliability during fault conditions, the fault has to be on the grounded phase or it is no different than many other systems. It is chosen because a neutral is not needed and some conductor costs can be saved.
texie
Senior Member
- Location
- Fort Collins, Colorado
- Occupation
- Electrician, Contractor, Inspector
All this talk about ungrounded delta and corner grounded delta makes me wonder. Are there any POCO's out there that will still furnish such service? I'm not aware of any that will supply anything other than grounded, center tapped single phase or grounded Y. Anything other than a Y is pretty much a thing of the past, even on large services.
ptonsparky
Tom
- Occupation
- EC - retired
Getting more rare for new, but plenty of existing to the South & East of me. Mostly 240.
ptonsparky
Tom
- Occupation
- EC - retired
Deleted.
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