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Discussion thread for this topic is located in the Electrical Theory and Applications area under the subject heading:

Discussion Thread For: 3 Phase 3 Wire Corner Grounded Delta

Subject: Corner Grounded Delta System And Usage Of Grounded Circuit Conductor.

The following Schematics and notations describe some basics regarding circuitry for a
3 Phase 3 Wire Delta Connected Grounded AC Power System.
In this example system, the Grounded Conductor is Line / Phase "A".
Any one of the Line Conductors derived from the Secondary side can be chosen for the Grounded Conductor (unless the AHJ has stated otherwise).

Ground ONLY ONE Circuit Conductor!!!

Grounding will be done just as with any other Grounded Circuit Conductor (i.e. Center Tapped "Neutral" Conductor on a 1 3 Wire system, or the Common Conductor on a 3 4 Wire Wye system).

The Grounded Conductor on this system is an active circuit conductor - like any other grounded conductor. Its only difference is the Potential (Voltage) To Ground.
Since it is bonded to a "Ground Reference" (in this case, Earth Ground is the bonding reference), it is at (or near) Ground potential during normal operation.

As with Grounded Neutrals, do not re-bond the grounded conductor on the load side of service equipment or Separately Derived System (Transformer or Generator).

Do not install any Single Pole OCPD in series with this conductor.
Switches, OCPDs and Mag Starters, which open all poles of a multi pole device at the same, time may include the Grounded Conductor.

Refer to NEC Articles 200, 210, 215, 220, 225, 230, 240 and 250 for full details.


3 3 Wire Corner Grounded Delta System - 240 VAC

[Linked Image]

Below is an overview of the complete system's equipment and circuit conductors:

[Linked Image]
Fig.1: System Overview

[Linked Image]
Fig.2: Closer view of Panelboards and connected loads

[Linked Image]
Fig.3: Close-up detail schematic of the Transformer and Main Service Equipment

Noted callout points for the image above:

(1): Secondary Windings of Utility Power Transformer. Windings connected in Closed Delta configuration.

(2): Utility Company's Grounding Termination for this Transformer, Grounding done to Terminal X1, which A is derived from.

(3): Service Entrance Conductors (Service Feeders). Line A = Gray, Line B = Red, Line C = Blue.

(4): Main Service Panel / Service Disconnecting Means. Note the fuses are used only on Lines B and Line C. The switch opens all conductors simultaneously.

(5): Grounding Electrode System for this service.

(6): Equipment Grounding Conductors. Note termination location for EGC and the Main bonding Jumper.

(7): Panelboard Feeders. For simplicity, we are using a single common Feeder. Panelboards are tapped from this common feeder.

[Linked Image]
Fig.4: Panelboard #1 and HID Ballasts Lighting Equipment connected loads.

Noted callout points for the image above:

(8): Panel #1's Feeder - tapped from common Feeder.

(9): Panel #1. Notice the bus kit used here. its a 1 Phase 3 Wire type Panelboard. Two Ungrounded Conductors, one Grounded Conductor, one EGC.

(10): Equipment Grounding Conductor to load equipment. notice that the EGC is bonded to the Panelboard's enclosure via a bonding jumper and the Grounded Circuit Conductor is NOT re-ground bonded here. The Bus Kit used for Grounded Conductors' terminations is Isolated from the Panelboard's grounded enclosure.

(11): Single Pole Circuit Breakers for Branch circuits. !!! IMPORTANT POINT !!! On This System, these frames (circuit breakers) will need a single pole AIC rating applied at 240 VAC. Breakers with either straight 120 VAC single pole rating, or "Slash" (120/240 VAC) rating cannot be used for this type of system, since the voltage to ground will be 240 VAC.

(12): Branch Circuits to loads. Note the "Common" Grounded Conductor used here, making this similar to a multiwire circuit.

(13) and (14): HID Ballast Kits (Linear Reactor types, with Power Factor Correction). Note the Screwshells on each Lamp Holder is connected to the grounded Circuit Conductor.
...p.s. these ballasts would be used with Mercury Vapor lamps, since most Mercury Vapor lamps take about 240 Volts to start.

[Linked Image]
Fig.5: Panelboard #2 and Motor Equipment load.

Noted callout points for the image above:

(15): Panel #2. This example uses a 3 Phase bus kit, and the Grounded Conductor (A) is included in this Bus Kit. Also, the circuit breaker for the Branch Circuit is a 3 Pole common trip.

(16): Equipment Grounding Conductor.

(17): Motor's Branch Circuit Conductors (3 Induction Motor). 3 3 Wire circuit to Motor (A, B and C).

(18): Combination Starter (Fusible EXO Disconnect and Magnetic Motor Starter).

(19): Solid Insert in fuse clip for Grounded Conductor.

(20): Contactors and Overload units of Mag. Starter.

(21): Motor. 240 VAC 3 3,450 RPM Squirrel Cage rotor type Induction Motor


I will create a discussion thread for this in the Electrical Theory and Applications area, then come back here and include a link to that thread.

Scott35 S.E.T. posted 03.08.2003 @ 15:36:00 PST

[This message has been edited by Scott35 (edited 03-08-2003).]

Last edited by Scott35; 05/13/10 09:31 PM. Reason: Fixed the "Phase" symbol issue

Scott " 35 " Thompson
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Discussion thread for this topic is located in the Electrical Theory and Applications area under the subject heading:

Discussion Thread For: 3Ø 3 Wire Corner Grounded Delta

Scott35 S.E.T.

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

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