Dual Stingers?????

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ionred

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OC, Ca.
Although this is my first post I have been lurking around this forum for about a year now and have found it to be very helpful, so let me begin by saying thank you to all posters and repliers. So on to my question. Today a friend of mine asked me to look at the power at his concrete plant to make sure he had the room and proper voltage for some equipment he wants to order. The panel is 3P 4W. Voltage reading came out as:

A to B = 240V
A to C = 240V
B to C = 240V

Then I got:

A to N = 240V
B to N = 0V
C to N = 240V

Power is fed from three pole mount Xfmrs on a piece of property that I couldn't access. Anybody here seen a configuration like this?
 
Sounds like a corner grounded, 3 wire, Delta system quite common in industrial establishments.

Welcome to the forum. :)
 
Rob
I would agree but what is the 4th wire for?
i thought he had a 4 wire delta with the center tap on the middle transformer grounded but that would not produce the voltages he is listing.
 
bob said:
Rob
I would agree but what is the 4th wire for?
i thought he had a 4 wire delta with the center tap on the middle transformer grounded but that would not produce the voltages he is listing.
Click to expand...

Good question, I don't know what the fourth conductor is for. I based my answer solely on the voltages he measured. Maybe the 4th conductor was an EGC?
 
you can also confirm that it is a grounded phase system by looking at the secondary configuration on the pole. None of the center taps on the transformers will have anything connected to them and one of the phase conductors will be connected to the primary neutral and a grounding electrode at some point if it is a grounded phase system.

There is no 120 volt taps in this system. You will need a separately derived system to get 120 volts.
 
Just to confirm, it is 4W. I would also agree with the corner grounded Delta except there is no voltage on B Phase to Ground. XFMR smoked?Im going to have to contact the POCO. I can only assume they configured this many years ago. The place is quite old with some really shoddy work. Just to give you an idea, 150' run, smurf tube tied to a fence, 3 red phase conductors and one ground disconnected at the starter, 3 green conductors to (1) 2P breaker and (1) 1 P breaker at the panel. No junction points. If I didn't know the guy I would run the other way. Thanks for the replies tho.
 
To clarify, that would be a high leg on A & C and a dead B. Now I certainly don't know everything and can't say I'm the best electrician around. All I can say is I've never seen or heard of such a configuration. BTW, thanks for the welcome. This forum is awesome.
 
bob said:
Rob
I would agree but what is the 4th wire for?
i thought he had a 4 wire delta with the center tap on the middle transformer grounded but that would not produce the voltages he is listing.
Click to expand...

This could be a relatively rare arrangement.
3 phase, corner grounded delta, but with a center tap on one transformer to give 120 volt service.
Some state that no such thing exists, but it does, though rare.
The center tap can not be grounded, since one corner of the delta is already grounded.
Such an arrangement would give 240 volts from any phase to any other, as measured.
The grounded corner to true earth would be virtualy zero volts.
The grounded corner of the delta to the center tap would give 120 volts.
Note that only one half of the center tapped winding can be used for 120 volt service.

Such services are rare, the main application is large pumping plants, 3 phase corner grounded delta is ideal for such purposes.
The center tap on one transformer gives limited 120 volt service for lighting and small appliances.

Great care must be taken in such cases to connect 120 volt loads only between the grounded corner and the center tap.
A 120 volt load connected between the center tap and the non grounded corner would still get 120 volts and would work, but be potentialy dangerous since 240 volts to ground would be present.

Another possibilty might be a 3 phase 3 wire corner grounded delta service, but with the EGC and neutral seperated at the pole.
What appear to be service conductors, would in fact be a 4 wire feeder, with the point of service being the pole, not the first disconnect within the building.
 
I have ran a 4 wire corner grounded system in the past at wells. The EG is bonded to the GC at the meter, or pole disconnect, then 4 wires on out to the load keeping the GC isolated at the pump panel, gutter or whatever I am feeding. We don't see to many grounded deltas anymore but this is my prefered method.
 
On a corner grounded delta there is no neutral.

I will bet the panel is rater at 3p 4w and there is no neutral on the bar.
One of the phases are grounded=no neutral.
 
ionred said:
Just to confirm, it is 4W. I would also agree with the corner grounded Delta except there is no voltage on B Phase to Ground. XFMR smoked?Im going to have to contact the POCO. I can only assume they configured this many years ago. The place is quite old with some really shoddy work. Just to give you an idea, 150' run, smurf tube tied to a fence, 3 red phase conductors and one ground disconnected at the starter, 3 green conductors to (1) 2P breaker and (1) 1 P breaker at the panel. No junction points. If I didn't know the guy I would run the other way. Thanks for the replies tho.
Click to expand...

ionred said:
To clarify, that would be a high leg on A & C and a dead B. Now I certainly don't know everything and can't say I'm the best electrician around. All I can say is I've never seen or heard of such a configuration. BTW, thanks for the welcome. This forum is awesome.
Click to expand...

No voltage to ground on B phase because it is grounded. That would be normal for any grounded conductor.

broadgage said:
This could be a relatively rare arrangement.
3 phase, corner grounded delta, but with a center tap on one transformer to give 120 volt service.
Some state that no such thing exists, but it does, though rare.
The center tap can not be grounded, since one corner of the delta is already grounded.
Such an arrangement would give 240 volts from any phase to any other, as measured.
The grounded corner to true earth would be virtualy zero volts.
The grounded corner of the delta to the center tap would give 120 volts.
Note that only one half of the center tapped winding can be used for 120 volt service.

Such services are rare, the main application is large pumping plants, 3 phase corner grounded delta is ideal for such purposes.
The center tap on one transformer gives limited 120 volt service for lighting and small appliances.

Great care must be taken in such cases to connect 120 volt loads only between the grounded corner and the center tap.
A 120 volt load connected between the center tap and the non grounded corner would still get 120 volts and would work, but be potentialy dangerous since 240 volts to ground would be present.

Another possibilty might be a 3 phase 3 wire corner grounded delta service, but with the EGC and neutral seperated at the pole.
What appear to be service conductors, would in fact be a 4 wire feeder, with the point of service being the pole, not the first disconnect within the building.
Click to expand...

Not saying you could not make that work but 250.26 will not allow it. If you are using a center tap of one of the windings to achieve 120 volts then the center tap must be the conductor that is grounded according to 250.26.

jim dungar said:
Give us all of the voltages.
L-L,
L-N,
L-G,
N-G
Click to expand...

All the voltages were stated in post #1 - he has a corner grounded system - apparently with an EGC ran with the service conductors.
 
kwired said:
All the voltages were stated in post #1 - he has a corner grounded system - apparently with an EGC ran with the service conductors.
Click to expand...
The OP is misleading and at this point you are guessing.

The voltages given were listed as L-L and L-N and the system was described as 4W. Since the OP the discussions have moved to corner grounded systems. Later the OP came back and again stated it was a 4W system, so I asked for all possible voltages as a means to confirm his system.

However, I too am inclined to believe the OP is interchanging the terms 'grounded conductor' and 'neutral' and the system is really a corner grounded delta.
 
Spoke to the POCO on Thursday. Haven't had time to post. Been swamped (bout 1time!). Anyhow, they're telling me its a 240V 3W. Says they have to ground B phase because its less than 300V. I'm failing to see the logic there but ok. I was told its a fairly common configuration although I've never run into it, neither has anyone that I've asked. There seems to be a bit of confusion to my original post so I will clarify.

3 phase conductors (to lugs)
1 grounded conductor (to N bus)
1 EGC (to G bus)

A to B = 240V. A to C = 240V. B to C = 240V

A to GC = 240V. B to GC = 0V. C to GC = 240V

A to EGC = 240V. B to EGC = 0V. C to EGC = 240V

There is no 120V period.

Now I'm no Lineman but I have run my share of Distribution but mostly underground 5 and 12KV, so I wouldn't be an authority on the subject of pole mount XFMR configurations. Guy says it will work for my customers application so I guess that's all I could hope for anyway. Thanks for the responses fellas!
 
ionred said:
Spoke to the POCO on Thursday. Haven't had time to post. Been swamped (bout 1time!). Anyhow, they're telling me its a 240V 3W. Says they have to ground B phase because its less than 300V. I'm failing to see the logic there but ok. I was told its a fairly common configuration although I've never run into it, neither has anyone that I've asked. There seems to be a bit of confusion to my original post so I will clarify.

3 phase conductors (to lugs)
1 grounded conductor (to N bus)
1 EGC (to G bus)

A to B = 240V. A to C = 240V. B to C = 240V

A to GC = 240V. B to GC = 0V. C to GC = 240V

A to EGC = 240V. B to EGC = 0V. C to EGC = 240V

There is no 120V period.

Now I'm no Lineman but I have run my share of Distribution but mostly underground 5 and 12KV, so I wouldn't be an authority on the subject of pole mount XFMR configurations. Guy says it will work for my customers application so I guess that's all I could hope for anyway. Thanks for the responses fellas!
Click to expand...

Are the five conductors you mention from the POCO transformer to the service equipment? If so and it is indeed a corner grounded delta system they only need the three phase conductors to the service disconnect. One of them happens to be grounded and is treated like the grounded service conductor on any other system. It is bonded to the service equipment enclosure, and the grounding electrode system. After it leaves the service equipment just like any other grounded system the current carrying grounded conductors are separated from equipment grounding conductors to prevent objectionable current flow through unintended conductive pathways.

If there is a fourth wire or even fifth as you mention there is likely no reason the POCO couldn't reconfigure the secondary connection and ground the mid point of one phase. Very good chance that the transformers are three terminal 120/240 transformers. Then you would have 120 volt available directly from the service and would not need additional transformers to get 120 volts.

I don't know why he said they have to ground B phase because it is less than 300 volt.
They can ground any of the phases and it works the same, all phases are same potential to each other, there is no potential to ground until a phase is grounded, then that phase becomes zero to ground.
 
corner grounded services are used around here for irrigation wells.the poco wants the middle leg(grounded phase)kept in the center and when ran to a pump panel they specify no fuse on center phase.
 
arits74 said:
corner grounded services are used around here for irrigation wells.the poco wants the middle leg(grounded phase)kept in the center and when ran to a pump panel they specify no fuse on center phase.
Click to expand...


240.22 will not permit a fuse to be installed in the grounded conductor even if POCO did not specify that. Circuit breaker is acceptable because it opens all conductors of the circuit when it operates.
 
kwired summed up my thoughts.
I am curious as to how many conductors you have from POCO transformers to the building.
These corner grounded (or ungrounded) systems were fairly common in this area, but the POCO run was always just 3 wires, with a 4th, equipment grounding conductor, added at the premises wiring system.
 
One thing that has always bothered me about grounded deltas is that once we make it a 4 wire, and the grounded phase is isolated from that point on, what is the harm in having a fuse in that phase? (other than NEC 240.22) The opening of that fuse will be no different to the load than the opening of any other.
 
ptonsparky said:
One thing that has always bothered me about grounded deltas is that once we make it a 4 wire, and the grounded phase is isolated from that point on, what is the harm in having a fuse in that phase? (other than NEC 240.22) The opening of that fuse will be no different to the load than the opening of any other.
Click to expand...

I kind of agree with you but, like any other grounded conductor you typically are not allowed to place switches or overcurrent devices in them either unless all poles are simultaneously opened. Any unintentional grounding will bypass the switch or overcurrent device.
 
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