UnGrOuNdEd three phase delta voltages???

[powerplay]

I was looking at an old 100amp 240V meter on an 100A meter base feeding an disconnect switch, but discovered there were 3 fuses in the disconnect and no neutral....turns out that in the old days they used 2 pole transformers in a "Scott Delta" setup no longer available, to save money and achieve 3 phase power which was metered by a single phase meter with one leg run straight through to the disconnect.

I have taken readings to the bonded enclosure, and realize there is no real ground reference, but would like to understand how I got the readings I did...I have suggested installing a ground fault detection device, but have been informed that an old installation like this is an old "existing non-conforming" setup and does not need to be enforced.

Phase A - 93 volts
Phase B - 159
Phase C - 250 volts

...A and B connected to the old meter base probably causing those voltages ??


What maintenance usually is involved with this old system to ensure safety? ...I'm thinking of installing the Ground Fault Detection anyways. The panel feeds rooftop A/C and heating loads. The building also has grandfathered an second 120/240 volt service in a different room inside the building.

I don't usually work with a Delta system so I'm grabbing my old school books to learn more, but would appreciate feedback/info about this old system to further understand what to expect......any advice/great reference material ???

Thanks... "Back to School Sparky"

 

[nakulak]

if its ungrounded then the only readings that matter are phase to phase, the readings you have are floating readings and irrelevent

 

[jim dungar]

Actually the old 2-phase to 3-phrase transformer is a "Scott-Tee".

One 3-phase connection using 2 transformers is simply called an "open delta". An open delta can also be connected with one center tapped winding to provide a 240/120 4 wire output. A third 3-phase connection is called a "T", this 2 transformer connection has an output the acts just like a wye.

You need to tell us all 6 voltages and if you are using a high impedance meter (i.e an electronic one).
A-B
B-C
C-A
A-G
B-G
C-G

In a true ungrounded delta system, a voltage to ground can actually be measured. This voltage exists because of coupling capacitance. The voltage you measure will often vary with the loading of the system (on a perfectly balanced 240V delta you might measure 138V to ground).

In a delta system with one phase solidly grounded, you would measure 0V to ground on that phase and 240V on the other phases.

The voltages you gave are somewhere in between these possible values, which may indicate a "poor" phase to ground connection.

 

[powerplay]

I am indeed using an high impedance electronic mulitimeter, and the voltages I was reading are as follows:

A-B 250v
B-C 250V
C-A 250V
A-G 93V
B-G 159V
C-G 231v

...interesting about what one can or cannot read to ground with this system...probably is a good idea to put a ground fault indicator on this old existing system?? The "Scott-Tee" if that is what it is, is located on a pole is about 20 feet from the building by the road, while the system ground is the water line which exits the basement within 70 feet from the hydro pole where the transformers are mounted. The clamp on readings from the electronic multimeter is negligible, as the heating is supplied from the forced air furnace, and the A/C isn't on yet.

 

[jim dungar]

A Scott-Tee is for converting 2-phase to three phase, these are very rare. A Tee connected unit requires transformers with three bushings, these are usually used for 208V secondaries.

You most likely have an "open-delta" system.

Do your L-G readings change as loads come on and off line?

 

[LarryFine]

I am indeed using an high impedance electronic mulitimeter, and the voltages I was reading are as follows:

A-B 250v
B-C 250V
C-A 250V
A-G 93V
B-G 159V
C-G 231v

Could you possibly repeat the test with a solenoid tester? If not, how about some type of resistive 240v load in parallel with the voltmeter?

 

[powerplay]

I have used my Ideal solenoid tester (wiggy) and roughly seen 250 volts between the three phases, and an lit up indication of "AC" between each of the phases to ground, and no voltage.

 

[broadgage]

This would appear to be a standard ungrounded delta system, without any intentional connection to ground.
Such systems may be either "open delta", or "closed delta".

A closed delta system uses three transformers,or a three phase transformer.

An open delta system uses only two single phase transformers to provide three phase service. This saves the expense of a third transformer.
An open delta 2 transformer bank provides only 57% of the capacity of a closed delta 3 transformer bank (and not 66.6% as one might expect)
Sometimes a third transformer is added later if required by increased load.

The voltages to ground from each phase will vary according to the amount of capacitance and leakage from each phase to ground.

If a ground fault occurs from one phase, then the installation will still work, and in the short term may continue in service.
In order to detect any such ground fault, it is usual to install a ground fault indicator. This consists of three 240/277 volt incandescent lamps, one between each phase and ground.
Under normal conditions all lamps will be lit dimly on about 140 volts. If a ground fault occurs then the lamp on the faulted phase will go out and the other lamps will light brightly.
Ground faults should be rectified promptly, since a second fault will open the fuses/circuit breakers.

 

[scott thompson]

As mentioned by others, the L-G Voltage you are reading (across the Hi input Z Meter) is due to Capacitive Coupling effect.
The higher the Capacitive value, the higher the Voltage readings will be.

Even though the System is not _Physically_ connected to Earth Ground, there is a Capacitive Circuit between the location of the Grounding Electrode System for the Building / Structure, and the Utility Transformer supplying Power to that Building's Service.

Ungrounded Deltas come in many flavors - 240V 3? 3 Wire, 480V 3? 3 Wire, 600V 3? 3 Wire; Open Delta "Vee" and "Tee" connections (use two single phase Transformers), and the Closed Delta configs (use three single phase Transformers).

The "Scott Tee" setups may be used for 3 Phase Primary to 2 Phase 4 Wire Secondary setups, or they may be used for 3 phase in, 3 phase out.

Commonly used Dry Type Transformers, with Secondary Systems designated as 208Y/120V 3? 4 Wire; & rated upto 15 KVA, employ the Open Delta "Tee" configuration, even though they are designated as 208Y/120 (Wye Secondary)

I have attached several Transformer Schematics for review.
Additional Schematics may be found at ECN's Technical reference area:
Menu For technical Reference Section

Images below:

Schematic #1: Open Delta Tee; 3 Phase to 2 Phase 4 Wire.

3 Phase Delta Tee Primary, 2 Phase 4 Wire Secondary.

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Schematic #2: Open Delta Tee to Open Delta Vee.

Open Delta Tee Primary (2 or 3 Phase),
2 Phase 3 Wire Secondary,
or,
3 Phase 3 Wire Open Delta "VEE" Secondary.

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...................................................................................................

Schematic #3: Closed Delta Pri, x Taylor Sec.

3 Phase Closed Delta Primary,
2 Phase 4 Wire "Taylor" Connected Secondary.

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Schematic #4: Open Delta "Vee" and "Tee" connections.

Left side drawing is Open Delta "Vee" setup.
Right side drawing is Open Delta "Tee" setup.

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Schematic #5: 240V Ungrounded Closed Delta.

Drawing shows Schematic view of the 240 Volt 3 Phase 3 Wire Ungrounded Delta (closed), with Voltage at given points.

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Schematic #6: Layout view of 240V Closed Delta.

Layout view of the 240V 3 Phase 3 Wire Ungrounded Closed Delta, with Voltage at given points.

.................................................................................

Scott

 

[jim dungar]

The "Scott Tee" setups may be used for 3 Phase Primary to 2 Phase 4 Wire Secondary setups, or they may be used for 3 phase in, 3 phase out.

Not quite.

A 3-phase to 2-phase Scott connected transformer uses two transformers; one unit (the main winding) must have a tap at 50% and the other one (the teaser) has a tap at 86.6%. One end of the teaser is connected to the 50% tap of the main. The input is connected to the two ends of the main winding and to the 86.6% tap of the teaser.

A 3-phase to 3-phase Tee transformer also uses two transformers except the teaser output winding needs a tap at 66.67% if a neutral point is required for a 4-wire output. Again one end of the teaser is connected to the 50% tap of the main. However, in this case the input is connected to the two ends of the main winding and to the 100% tap of the teaser.

 

[powerplay]

Fantastic....thanks for the feedback everyone!!

 

[powerplay]

thanks for the excellent feedback!

 

[LarryFine]

Yer welcome, already! Sheesh! :roll: D)