Double primary single phase pole xfrmr confusion

[mjessb3]

I'm trying to understand my own condo's electrical service.

The pole transformer has 2 primary bushings. 2 distribution lines are running to it. One of the primary bushings is connected to the highest line on the pole via a fuse cutout. The other is connected to a lower line right above the pole transformer, what I would originally postulate is the neutral line, without a fuse cutout in between. Conflicting information tells me that single phase pole transformers with 2 primary bushing are on a delta distribution circuit, and those 2 primaries are each connected to hot leg lines, with no neutral returning to the substation.

How can I tell if a line is a 'hot' or a 'neutral' coming to a single phase, double primary bushing, pole transformer?

How can I determine if I'm on a delta distribution circuit? (I.e. no neutral returning to substation/use of delta hot legs for return paths)

Also, the *secondary* center tap also appears to be tapped directly to that same lower line, which also makes me think it is a neutral.

That being said, is it untrue that a single phase transformer with 2 primary bushings is always connected to 2 'hot' primary lines for the line-to-line derivative for 240V?

Additionally, there are 2 sets of 3 wires coming down from the secondary to feed the units. 2 hots and a neutral in each I would assume, because we have 120/240V service. But if the primaries are tapped to one leg of hot and one leg of neutral, how are we deriving the second hot wire in the secondary system? Considering the scenario that one primary bushing as well as the secondary center tap are connected to that same neutral line, how are we getting the phase-phase potential difference needed for a 240V supply?

And if the lower line on the pole is actually another hot leg and not a neutral, why would the secondary center tap be connected to it? And why would there not be a fuse in between that connection?

I've gone around and around trying to determine what's happening here. Insight will be much appreciated.

(Side note, the primary bushings predate 1979 by the looks of them)

 

[Hv&Lv]

It’s a conventional two bushing transformer on a single phase wye line. They are very versatile and can be used on wye or delta system.
one side of the H1 is primary, the H2 is grounded.

on a CSP XF there is only one bushing. The H2 is grounded internally.

 

[LarryFine]

Sounds like line-to-neutral to me, too.

 

[mjessb3]

It’s a conventional two bushing transformer on a single phase wye line. They are very versatile and can be used on wye or delta system.
one side of the H1 is primary, the H2 is grounded.

on a CSP XF there is only one bushing. The H2 is grounded internally.

Is it common to see 2 sets of service wires coming from a single one of these transformers? How many secondary services could you attach to one of these, say, 7200V single phase wye lines without disturbing power quality? I suppose these residential xfrmrs are designed for various amounts of services?

Also, if this type of xfrmr is used in a delta distribution circuit, would that mean you would ground one of the phases being used as H2? Or just ground your secondary neutral tap at the pole and have the primary side ungrounded?

 

[LarryFine]

Is it common to see 2 sets of service wires coming from a single one of these transformers? How many secondary services could you attach to one of these, say, 7200V single phase wye lines without disturbing power quality? I suppose these residential xfrmrs are designed for various amounts of services?

Also, if this type of xfrmr is used in a delta distribution circuit, would that mean you would ground one of the phases being used as H2? Or just ground your secondary neutral tap at the pole and have the primary side ungrounded?

He's talking about a transformer with two primary HV bushings, or terminals, instead of one. This type can be powered from either two lines or one line and the neutral, whereas single-bushing units can only be powered from line to neutral.

This has nothing to do with secondary connections.

 

[Hv&Lv]

Is it common to see 2 sets of service wires coming from a single one of these transformers? How many secondary services could you attach to one of these, say, 7200V single phase wye lines without disturbing power quality? I suppose these residential xfrmrs are designed for various amounts of services?

Also, if this type of xfrmr is used in a delta distribution circuit, would that mean you would ground one of the phases being used as H2? Or just ground your secondary neutral tap at the pole and have the primary side ungrounded?

(A) Can be from one to as many as can be squeezed on to the wires coming from the terminals.
I've seen a 25kVa with six services before.

(B) delta distribution would be H1 to phase “A”, the other to phase”B”. No grounding on the H side (Don’t get caught up in the abc, just know it’s two phases...)

secondary would be grounded.

 

[jim dungar]

How can I determine if I'm on a delta distribution circuit? (I.e. no neutral returning to substation/use of delta hot legs for return paths)

Can I ask why you are interested?

 

[mjessb3]

Just want to know distribution set-ups better; starting as a distribution technician soon and have always been very curious, seeking straight information from the experienced professionals

 

[electrofelon]

Also, if this type of xfrmr is used in a delta distribution circuit, would that mean you would ground one of the phases being used as H2? Or just ground your secondary neutral tap at the pole and have the primary side ungrounded?

You don't ground an ungrounded primary line ever. The grounded/not grounded decision has already been made, alas you do not get to be the decider.

Here, POCO uses two bushing transformers with selectable 4800 and 7620 voltages. Most of their distribution is either 4800 ungrounded Delta or 7.62/13.2kv mgn. As others have mentioned, they can use this same transformer on both systems, even though they don't need the second bushing for the MGN system.

A transformer creates a new electrical system that has no relationship to ground or the supplying system. The secondary can have a center tap, 2 3rd taps, or 37 taps and you can ground whichever one you want (or none).

 

[electrofelon]

Pad transformers can be confusing. Many have a "loop feed bushing" (an optional feed out)so at first glance it looks like it's intended for two ungrounded lines, but is actually for an MGN with one end of the primary coil connected to the tank. These will be labeled "H1a" and " H1b", and you can also tell from the data plate which will say something like 4160grdy/2400.

 

[don_resqcapt19]

Is it common to see 2 sets of service wires coming from a single one of these transformers? How many secondary services could you attach to one of these, say, 7200V single phase wye lines without disturbing power quality? I suppose these residential xfrmrs are designed for various amounts of services?...

The 50 kVA pole transformer that supplies my house, also supplies 9 other houses.