ECN Forum Forums Code Related Discussion NEC & other Code issues Open Delta

There is an industrial building with a 480V open delta service. Is it common to run 3 phase, without an Equipment ground? I had an electrical engineer tell me the equipment ground is not needed because if a leg hits ground, it would not trip the breaker until a second phase also grounds out. The situation is a cable tray with just the 3 phase conductors in it, and then conduit will be ran to a 200 amp disconnect.
The NEC wants us to run equipment grounds in all systems, right?
Is it a poor design, not to have an Equipment ground?
Thanks - SJT

The EE needs to be reminded that he's not an electrician, and that the NEC is not in the college curriculum.

For nearly any circuit (power cords to double-insulated tools being the only exception I can think of), we need a ground or fault path. Things still need to be bonded together.

For a branch circuit, the conduit can be the fault path. For a feeder or service, you need a wire.

Otherwise, you have the possibility of one piece of equipment becoming energized by 'phase A' and another being energized by 'phase B." Without bonding, this double fault will exist untol the two pieces are connected - maybe by you. Who want's to be nicnamed "Fuseboy?"

BTW, you have an 'ungrounded Delta.' An 'open Delta' is a three-phase system with only two transformers. Yes, it is possible to have an ungrounded, open Delta system.

Are you talking about "open" as in 2 transformers or "ungrounded" delta.

Even in ungrounded delta you need the EGC connecting to ground and bonding all non-current carrying conductive materials.
250.4(B)(1)&(2)

This is an ungrounded Delta. The source (Distribution Panel) is approx. 200' away. From that, they ran 3 - 4/0's ungrounded conductors, 480v and no equipment ground in a tray that is bonded to Bldg. steel.
These were ran to a small Bldg. that will be removed. They want to splice the conductors in the cable tray and continue them to another small Bldg. In the Bldg. there will be a trough and 2 disconnects that will be mounted.
This would be considered a feeder to the Bldg. and not a service. There is a grounding electrode at the Bldg. steel.
Would it be to code, if I were to run a #4 from the new disconnects to the Bldg. steel, to create an equipment ground? Or must I run a #4 back into the tray and back to the source with the feeders, (200'away)which they should have done from the get-go.
SJT

Could it be a Corner grounded delta?

I suppose as long as you are connected to the grounding electrode (building steel) you have complied with 250.4 as long as the building steel is continuously bonded together.
I would say you size that with 250.122

Just for fun... here are some of the many, many, many Flavors for Delta Configurations!!!
(only 3 Phase 3 Wire Deltas shown here)


Figure #1-1: 3 Phase 3 Wire Open Delta "VEE" - Center Tap Grounded.
........................................................................................................................


Figure #1-2: 3 Phase 3 Wire Open Delta "VEE" - Corner Grounded (Phase "B").
........................................................................................................................


Figure #1-3: 3 Phase 3 Wire Open Delta "VEE" - Ungrounded.
........................................................................................................................
__________________________________________________________________________________



Figure #2-1: 3 Phase 3 Wire Closed Delta - Center Tap Grounded.
........................................................................................................................


Figure #2-2: 3 Phase 3 Wire Closed Delta - Corner Grounded (Phase "B").
........................................................................................................................


Figure #2-3: 3 Phase 3 Wire Closed Delta - Ungrounded.
........................................................................................................................

_END OF DRAWINGS_

________________________________________________________________________________________________________________________

Each of these systems needs the Metallic Equipment bonded to a Grounding Electrode System - including the Ungrounded version.

The EE was mistaken in the description:



First thing to do with a statement like this;
Remove any references to
"Not Needing an Equipment Grounding Conductor"
being
"OK"


What occurs with an L-G Fault on an Ungrounded System is:

1: With properly Bonded Metallic Raceways / Equipment:, the first L-G Fault results in the System becoming a Corner Grounded Delta,

2: System will function properly; there will be no indication of any Circuitry issues experienced by the connected Loads (Lighting and Motors will function normally),

3: Ground Monitoring Lamp Array will indicate the L-G Fault. The Lamp on the Faulted Conductor will now be dark, and the remaining Two Lamps will be bright (previously, all 3 Lamps had dim light output),

4: If / When one of the remaining Two Phase Lines becomes Ground Faulted (2nd L-G Fault), an Overcurrent will occur - hopefully causing the Overcurrent Protection Device (OCPD) for that Branch Circuit or Feeder to trip.

5: Ground Fault issue is now a Production issue also!

------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------

Grounded and Ungrounded Deltas may be either Closed (3 Transformers), or Open (2 Transformers).

Open Deltas may be:

"Vee" typical - especially for 45 KVA and larger capacity, most common Open Delta Arrangement found on Utility Poles,

or;

"Tee" typical 3 Phase 3 Wire dry type Transformer for 50 KVA and less capacities; very common for 3 Phase to 2 Phase Conversions.

So, to sum this up:
Both Grounded and Ungrounded Delta Systems require the Metallic Enclosures / Raceways to be Bonded to a local Grounding Electrode System (GES).

To understand why the Ungrounded Delta needs Bonded Enclosures and Raceways, try this simple experiment...

1: Take your High Z Input Volt Meter (AKA "Fluke DVM"), and your Low input Z Volt Meter (AKA "Wiggy" / Solenoid type Volt Meter) with you when returning to this Project.

2: Go to an accessible point on the Ungrounded System - as far from the Service as possible; something like a Disconnect Switch that may be Live tested safely.

3: Place the High input Z Meter on the Highest AC Voltage Scale.

4: Measure the Voltage between One of the Phase Lines and the Metallic Enclosures / Conduits (Raceways). Make sure the Metallic Equipment is Bonded to Earth Ground in some way (hopefully via an Equipment Grounding Conductor).

5: Measure the L-G Voltage between the remaining Two Phase Lines and Eq. Ground.

6: Measure the L-L Voltage between any Two Phase Lines.

7: Now measure L-G Voltage with the Low input Z Volt Meter.

8: Come Home, log in to ECN, pull up this thread, and tell us what the readings were!

p.s. "Z" = Impedance i.e.: Low input Impedance, High input Impedance.


-- Scott

El-Bump'o

Any updates per this Topic???

-- Scott