3-phase corner-grounded delta questions

When wiring up electrical service configured as 3-phase corner-grounded delta, it would seem that the appropriate way to connect overcurrent
protection would be to make the 2 ungrounded wired breakable, and pass the grounded wire straight through. That would imply using equipment that is wired the same way as center-tapped 1-phase. NEC states that a 2-pole breaker must not be used for such service unless it is marked as being rated for both 1-phase and 3-phase (240.85). I would presume that is because of the higher hot-to-ground voltage and the effect it could have on interruption capability (as in NEC note in 240.85).
Is a 2 pole plus neutral breaker panel what is typically used for 3-phase corner-grounded delta? I can't imagine using a 3-phase panel for this.
Are such breakers readily available?
Does the higher hot-to-neutral voltage decrease the interruption current rating, considering that the neutral wire isn't passing through the breaker (disregarding GFCI breakers where the neutral wire is involved).
Would these or other issue vary whether the configuration is closed-delta or open-delta ("V")?
Are corner-grounded delta configurations ever used at voltage levels above 240 volts (in the US), such as maybe 480 volts or 600 volts? Have utilities ever used it for primary distribution?
FYI, I'm not referring to delta configurations where one of the windings is center-tapped to get 240/120 volts for conventional single phase compatible lighting voltage.
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On 2/21/04 4:46 AM, in article snipped-for-privacy@enews3.newsguy.com,

Some of the power suppliers use corner ground 3๘ 480 for irrigation well hookups in southeast Nebraska. We run 4 wires to the wells or pivots. The grounded phase is treated just like the ungrounded conductors except we use a slug in the grounded phase instead of fuses. I've never checked to see if some sort of 480 volt breaker would work. We were told breakers aren't as reliable as fuses in outdoor applications. Pump panels all seem to be set up for fuses instead of breakers.
Dean
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breaker
utilities
is
compatible
Molded-case breakers for this application must be rated 240 or 480 (or 600) volts. In particular *slash-rated* devices are a serious misapplication, like with 120/240- or 480/277-rated breakers. Likewise, on the fuse side, parts rated 125V or 300V can not safely be used on corner-grounded, midtap-grounded, high-resistance-grounded or ungrounded 240 and 480V systems .
Some utilities will provide corner-grounded, midtap-grounded, high-resistance-grounded or ungrounded 480V systems to allow skirting NEC 230-95 ground-fault protection required for >1000A 480Y systems,
Another circuit-beaker rating that gets overlooked for corner-grounded, midtap-grounded, high-resistance-grounded or ungrounded 240 and 480V systems is that molded-case breakers' *single-pole* interrupting ratings must be applied, which for up to 1200A frame size is 8.66 or 12.1kA.
UL White Book: "Two-pole circuit breakers which are suitable for controlling three-phase, corner grounded delta circuits are marked "1๘ - 3๘" to indicate their suitability. Three-pole circuit breakers are suitable for use only on three-phase systems unless marked to indicate otherwise."
3-pole overcurrent protection is required {i,e,. in the neutral conductor, too} would be for running overload protection of 3๘ motors. This applies to time-delay fuses and motor-overload relays in starters.
-s falke
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| Molded-case breakers for this application must be rated 240 or 480 (or 600) | volts. In particular *slash-rated* devices are a serious misapplication, like | with 120/240- or 480/277-rated breakers. Likewise, on the fuse side, parts | rated 125V or 300V can not safely be used on corner-grounded, midtap-grounded, | high-resistance-grounded or ungrounded 240 and 480V systems .
Why can't a 300V fuse be used on a 240V delta system? Is it because the system might somehow revert to being a wye and give you 416 volts? I just want to know what the specific issue is ... e.g. the potential failure mode.
| Another circuit-beaker rating that gets overlooked for corner-grounded, | midtap-grounded, high-resistance-grounded or ungrounded | 240 and 480V systems is that molded-case breakers' *single-pole* interrupting | ratings must be applied, which for up to 1200A frame size is 8.66 or 12.1kA.
Wouldn't the impedance of the system also dictate the rating at possibly a higher interrupting rating? If you do have 1200A service, I'm sure that transformer can deliver quite a lot of fault current. Are you sure breakers as low as 8.66 or 12.1 kA are going to handle that?
| UL White Book: "Two-pole circuit breakers which are suitable for controlling | three-phase, corner grounded delta circuits are marked "1? - 3?" to indicate | their suitability. Three-pole circuit breakers are suitable for use only on | three-phase systems unless marked to indicate otherwise." | | 3-pole overcurrent protection is required {i,e,. in the neutral conductor, | too} would be for running overload protection of 3? motors. This applies to | time-delay fuses and motor-overload relays in starters.
And if you use fuses, you could end up with the neutral wire opening alone?
OK, suppose you are wiring up a 240 volt corner grounded delta system for a motor and will be using 3-pole protection. Are you going to use a panel with 2 hot bars and neutral, or 3 hot bars and neutral? If the latter, which of the bars are not going to be used (assuming safety ground is separate)?
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wrote:
| Some of the power suppliers use corner ground 3? 480 for irrigation well | hookups in southeast Nebraska. | We run 4 wires to the wells or pivots. The grounded phase is treated | just like the ungrounded conductors except we use a slug in the grounded | phase instead of fuses.
And the 4th wire is the "green" safety ground?
Having a slug in the fuse holder of the grounded corner phase sounds like it's equivalent to a single phase grounded center tap, which is also not interruptable.
| I've never checked to see if some sort of 480 volt breaker would work. | We were told breakers aren't as reliable as fuses in outdoor applications. | Pump panels all seem to be set up for fuses instead of breakers.
I'd prefer breakers where viable just to kill both poles at the same time in the event an interruption is needed. But out in the web environment, I suppose breakers could start to corrode unless they are well sealed.
At least if your neutral opens for some reason, there won't be any higher voltages presented, as would be in the case of an open neutral on a single phase center tapped system like 240/120.
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On 2/21/04 11:53 PM, in article snipped-for-privacy@enews1.newsguy.com,

Yes. We bond it to the grounded conductor at the service disconnect switch.

Pivot motors all have some sort of individual overload protection. It's usually just a thermal device in the motor. The irrigation pump panels have either the old style motor starters with heaters or an electronic overload sensor for the well motor. They open all three phases.
Dean
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Looking at my 7th edition Soars Grounding book figure 3-11 shows corner grounded delta hooked to a 2 pole panel with the grounded leg connected to the neutral bar, run in white wire.
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| Looking at my 7th edition Soars Grounding book figure 3-11 shows corner | grounded delta hooked to a 2 pole panel with the grounded leg connected to the | neutral bar, run in white wire.
But does it show what specific ratings that equipment must have, considering the fact that hot to neutral is 240 volts?
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