3-phase corner-grounded delta questions

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.

| 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?

Some of the power suppliers use corner ground 3ø 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

| 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.

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 "1ø - 3ø" 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 3ø motors. This applies to time-delay fuses and motor-overload relays in starters.

-s falke

| 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)?

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