Ungrounded Delta - additional notes

This message is regarding the most currently posted Transformer Connection Drawings I placed in the Tech. Reference

Section, in particular, the Ungrounded Delta drawings, located at:

3 Phase 3Wire Ungrounded Delta

Even though the Drawings describe Zero Volts from Phase "A", "B" or "C" to the Equipment Grounding Conductor (or Earth

Ground), this is only describing "Working Conditions", or "Normal Operating Conditions" as viewed by standard equipment

Simply stated, Voltages for normal operation and normal connections.

These systems will, however, allow external circuitry to be connected between any single Line ("Phase") and "Ground"

(typically the EGC, but may also include Earth Ground), which will allow Current to flow, and consequentially, a

difference in Potential to be established (A Voltage).

From here on, the following terms or abbreviations will apply:
* The term "Ground" will refer to the EGC (Equipment Grounding Conductor);

* The term "Line" will refer to either Phase "A", Phase "B" or Phase "C";

* The letter "Z" refers to Impedance (XL or XC + R);

* The terms "Circuit" and "Connection" refer to any closed loop between two points of Potential - in this case, L-G;

* The term "L-G" means "Line-To-Ground";

* The term "L-L" means "Line-To-Line";

* The term "L-L-L" means "Line-To-Line-To-Line" or 3 Phase;

* The term "Loading Effect" refers to how much Current a Meter will draw from the supply when connected;

* The term "Low Impedance Connection / Circuit / Meter" refers to a connected load item with an Impedance of 1K Ohms or

less - and includes Low Input Impedance devices;

* The term "High Impedance Connection / Circuit / Meter" refers to a connected load item with an Impedance of 100K Ohms or

more - and includes High Input Impedance devices;

* The abbreviation "Cap. Coupling" refers to Capacitive Coupling Effect,

* The term "Potential Difference" loosely translates into Impressed Voltage - and may be referred to as such in some

places, but is more relative to any points across any circuit, that when connected, a current will flow - and a

corresponding Voltage level may be figured between the two points of connection, or across the Impedance Load itself.


A: Normal Operations

Under normal operation, the loads are connected either L-L (for 1 Phase 2 Wire), or L-L-L (for 3 Phase 3 wire), and the

rated system Voltage is impressed across the loads - as measured between each given Line.

The Currents flowing through the connected loads are in respect to the connected load's Z.

Additionally, there will also be "Line Charging" Currents between the Conductors and Ground, and between each of the Line

Conductors. The value of the Charging depends on circuit length, proximity between points of potential, insulation

strength, and the RMS Voltage at any given point in time.
These Line Charging currents are found in ALL power systems; Grounded or Ungrounded.
This includes Cap. Coupling.


B: High Impedance L-G Connections

If a High Z connection is made L-G, the results will be as follows:

a: Current will be "Bled Off" the Line conductor, which was established from the normal Line Charging,
b: The level of Current (charges) drained will depend on the connected Z,
c: The higher the connected Z, the higher the Voltage across that Z will be,
d: With one L-G High Z Connection, the system has a more solid "Ground Reference" - depending on the level of Z.

If more than one L-G High Z connection is made - such as one from Phase "A" to ground, one from Phase "B" to Ground, and

one from Phase "C" to Ground, the current drawn across the Z will flow L-L-L and L-G.

See the Low Z Connections for what occurs with a Ground Fault.


C: Low Impedance L-G Connections

If a Low Z Circuit is placed across any L-G connection, the results will be as follows:

a: The Line Charges will drain considerably, depending on the level of Z connected,
b: The lower the connected Z, the lower the Voltage across that Z will be,
c: If one Line with Low Z is connected to Ground, the System becomes a Corner grounded Delta - and depending on the level

of Z, it may be "Impedance Grounded" (100-1K Ohm), or "Solidly Grounded" (25 Ohms or less),
d: The Phase which has the Low Z L-G connection on it is now the Grounded Conductor,
e: Any other L-G connections (opposing Phases) will result in a large Current flow; and if there is a "Solid Connection" -

such as a Wire "Shorting Out" against a metallic enclosure, the results will be same as a Ground fault on any other

Grounded System (Bolted Fault).


A High Z Meter - typical of the Digital Meters available, will show a high Voltage from L-G.
The Meter is responding to the level of Cap. Coupling + Line Charging being drained off through it.

Connecting a "Wiggy" (Low Z Solenoid type Volt Meter) from L-G will react as if the meter was connected to a charged

Capacitor; when first connected, there will be a very brief reaction - the solenoid will lightly click, and the Neon

indication Lamp(s) will glow for a very short period of time (less than 0.01 second - barely noticeable). Afterwards, there

will be no reading.
The Meter acts like a "Short Circuit" to the Cap. Coupling + Line Charging on the conductor at the point of connection,

which results in larger draining of charges, and as a result, very little potential difference is established between that

conductor and Ground at that point.

Connecting both a Low Z Meter and a High Z Meter across L-G at the same time will help determine if the system is

Ungrounded or Grounded.

Please direct any additional questions regarding the Ungrounded system to this thread.

Thanks for participating!


Scott " 35 " Thompson
Just Say NO To Green Eggs And Ham!