Why Grounded Delta?

[Hv&Lv]

I guess I am used to most of the linemen in this area that would likely have more theory knowledge than maybe some other areas. There is a community college near here that has a very good utility line program. They do learn a lot of electrical theory. Not many POCO employees around this area that have not attended this school, and if they haven't they are either older or not originally from the area.

A lot go to school here to. That doesn't mean they learn the material.

 

[kwired]

A corner grounded delta with a neutral?? I suspect it's a redundant grounding conductor.

I'm pretty sure the poco is supplying a redundant grounding leg to prevent the possibility of someone putting a fuse (and subsequently blowing it) on the grounded corner. If I remember right, the grounded corner should be B phase, should be identified white or grey, and should NOT be fused.

And out here in San Diego county we see this in rural areas to supply small three phase loads such as well pumps and irrigation pumps. They use two single phase transformers to create the open delta.

Corner ground is no different to connect than other systems. You have your ungrounded conductors that land on overcurrent devices and your grounded conductor that is bonded to the service enclosures and connected to a grounding electrode system. Any point beyond the service equipment you must separate grounded circuit conductors from equipment grounding conductors. Switches and circuit breakers are permitted to open the grounded conductor of any particular system, but must simultaneously open all conductors of the circuit. Fuses are not allowed in a grounded conductor on any system (in general anyway).

What you are referring to as a "redundant" ground is likely an "equipment grounding conductor", just like it is on any other system.

Add: A corner ground system has no "neutral" just a grounded conductor that happens to also be one of the three phase conductors. The other two phases are "ungrounded".

 

[sdbob]

Corner ground is no different to connect than other systems. You have your ungrounded conductors that land on overcurrent devices and your grounded conductor that is bonded to the service enclosures and connected to a grounding electrode system. Any point beyond the service equipment you must separate grounded circuit conductors from equipment grounding conductors. Switches and circuit breakers are permitted to open the grounded conductor of any particular system, but must simultaneously open all conductors of the circuit. Fuses are not allowed in a grounded conductor on any system (in general anyway).

What you are referring to as a "redundant" ground is likely an "equipment grounding conductor", just like it is on any other system.

Add: A corner ground system has no "neutral" just a grounded conductor that happens to also be one of the three phase conductors. The other two phases are "ungrounded".

You're correct, but this system, because of it's rarity and especially when it's delivered with that 4th wire, can still make a seasoned journeyman scratch his head. The 4th wire confused the hell out of me the first time I came across it. I agree it's an equipment grounding conductor, identified green and bonded at the service enclosure. It's ENTIRELY unnecessary, however, as it's nothing more than a redundant B Phase. It's no different than if the POCO supplied an EGC as a 5th wire in a WYE system and connected it to the neutral at their transformer. They don't, 'cause everyone knows to bond the neutral at the service. Apparently not so with the corner grounded system, so the 4th wire (the EGC/redundant B-phase grounded conductor) is provided as insurance.

B Phase is a grounded conductor, not a neutral, but still must be identified white or grey as per NEC 200.6 in the service enclosure and throughout the system. B-Phase at the weatherhead for a 480-volt corner grounded delta, at least according to SDG&E (our poco), must be identified RED. So white on one end and red on the other. Also, since B phase is a grounded conductor, it should be bonded to the GEC at the service enclosure, right? But not if the POCO supplies the 4th wire (redundant B phase), which they're not required to do. But it still can't be fused.

The diagram below shows, IMO, a better way for the utility to provide this system. Call a spade a spade, bond the grounded phase at the service enclosure and eliminate the 4th wire.

 

[kwired]

You're correct, but this system, because of it's rarity and especially when it's delivered with that 4th wire, can still make a seasoned journeyman scratch his head. The 4th wire confused the hell out of me the first time I came across it. I agree it's an equipment grounding conductor, identified green and bonded at the service enclosure. It's ENTIRELY unnecessary, however, as it's nothing more than a redundant B Phase. It's no different than if the POCO supplied an EGC as a 5th wire in a WYE system and connected it to the neutral at their transformer. They don't, 'cause everyone knows to bond the neutral at the service. Apparently not so with the corner grounded system, so the 4th wire (the EGC/redundant B-phase grounded conductor) is provided as insurance.

B Phase is a grounded conductor, not a neutral, but still must be identified white or grey as per NEC 200.6 in the service enclosure and throughout the system. B-Phase at the weatherhead for a 480-volt corner grounded delta, at least according to SDG&E (our poco), must be identified RED. So white on one end and red on the other. Also, since B phase is a grounded conductor, it should be bonded to the GEC at the service enclosure, right? But not if the POCO supplies the 4th wire (redundant B phase), which they're not required to do. But it still can't be fused.

The diagram below shows, IMO, a better way for the utility to provide this system. Call a spade a spade, bond the grounded phase at the service enclosure and eliminate the 4th wire.

When the service has a fourth wire you either have

A - a service that once had a neutral or is intended to possibly have one someday.

B - an installer that didn't know any better.

You did not change anything I said you just put it in different words.

The service side of a corner grounded delta only needs the two ungrounded conductors and the grounded conductor.

Anything beyond the service equipment needs the grounded conductor isolated from equipment grounding conductor just like any other system with a grounded neutral conductor.

Just how rare is corner ground system? It is kind of rare but there are some areas where you may find it quite often, especially where it supplies a single or very limited load. Backfed transformers with only three wire primary (now the secondary) you have little choice but to ground a phase or install ground detection and use as an ungrounded system.

 

[ptonsparky]

The POCO in this area does not supply the conductors from their meter pole for the wells. The installer does. Most of the wells we have worked on the last few days have been grounded deltas with 4 wire. All but one of those with the 4th wire paralleling the grounded conductor. The pump panels are all rated as suitable for use as service equipment as long as we install the nice little label that is sent to identify them as such.

I feel that the 4th wire should be used as an equipment ground even though it originates from the meter enclosure. That of course opens another can of worms, but I feel this removes any chance of becoming another path for current as you touch the PP enclosure.

The one pump that had a fourth wire properly installed from a service disconnect was one I put in several years ago. These pictures were taken there, although of different pump. The well driller had the transformers changed to 480v. He replaced my 240v PP with a 480v one. He just left the 240v Service Equipment installed as I had it. It is now locked off and I get the pleasure of making phone calls to the SED before I can start making changes. Fun.

 

[sdbob]

You did not change anything I said you just put it in different words.

Because you were correct sir.

I do respectfully disagree on the purpose of the 4th wire, speaking strictly on my experience with our POCO. It's absolutely a 2nd B phase here. Meant to be a EGC, to avoid to possibility that, due to the obvious confusion created by this system, the B phase conductor doesn't get bonded to the GEC as it should.

I'm sure a good look at the local POCO service manual would clear it up. Here in San Diego, it's a redundant grounded conductor.

 

[kwired]

Because you were correct sir.

I do respectfully disagree on the purpose of the 4th wire, speaking strictly on my experience with our POCO. It's absolutely a 2nd B phase here. Meant to be a EGC, to avoid to possibility that, due to the obvious confusion created by this system, the B phase conductor doesn't get bonded to the GEC as it should.

I'm sure a good look at the local POCO service manual would clear it up. Here in San Diego, it's a redundant grounded conductor.

Sorry but to properly use terminology as it is used in the NEC that fourth wire that is not intended to normally carry current but is intended to tie other non current carrying metal objects together is an "equipment grounding conductor" on any system, the grounded phase conductor is simply a "grounded conductor", and in a system with a grounded neutral that conductor is also a "grounded conductor".

Do not think you have something different - equipment grounding rules are same no matter what configuration the supply is. Even ungrounded systems must also have equipment grounding conductors to bond all non current carrying metal parts together. Only thing you have that is different involves the current carrying conductors, and one of them happens to be grounded.