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Re: Transformer Question [Re: poorboy] #193913 04/27/10 09:37 PM
Joined: Jun 2005
Posts: 124
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poorboy Offline OP
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I just found this thread and will be checking it out for a while: MIKE HOLT

Work Gear for Electricians and the Trades
Re: Transformer Question [Re: poorboy] #194139 05/12/10 09:41 PM
Joined: Jul 2002
Posts: 241
S
SJT Offline
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I would be curious to know the nameplate data on the table saw, to rate the circuit protection correctly. Any info. on amps?

Re: Transformer Question [Re: SJT] #194167 05/13/10 08:56 PM
Joined: Oct 2000
Posts: 2,720
Scott35 Offline
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*** THIS RESPONSE APPLIES TO BOTH "POORBOY" AND "WOLFGANG" ***

In regards to Grounded -vs- Ungrounded Separately Derived System ("SDS") referring to the Secondary, or Output of the Transformer...

Personally I would Ground the Secondary side (the Output, or the SDS), as leaving it Ungrounded places too much stress on the Conductors' and Motor Windings' Insulation, due to unstabilized Voltage-To-Ground per Capacitive Coupling effect.

Simply stated, even though the SDS is not Physically Ground Bonded, there still is a path from each Line Conductor to the Metallic Equipment + Earth Ground, via Capacitive couplings.
Think of this as thousands of small Capacitors connected between each Line (Phase) Conductor, plus between each Line and any Grounded Materials (Conduits, Enclosures, The Earth, etc.)

The issue here is that with high Impedance, the Voltage measured across the Capacitor is also high.
L-G Voltages of 1500 Volts are normal on an Ungrounded 480V Delta System.

These unstable Voltages tend to break down the integrity of Winding and Conductor Insulation, resulting in Arc leakage and eventually a Ground Fault issue.

Additionally, Filters, VFDs and other Electronic Circuitry are prone to premature failure due to the unstable L-G Voltage.

Lastly, NEC 250.21 requires Ground Fault Detection, which is something I would rather stay away from unless absolutely necessary!
(Read: Client will never look at the Indicator Lamps, or respond if an issue arises; also becomes something else to mess with...).

---- Grounded SDS ----

This refers to "Bonding" the Separately Derived System ("SDS"), to the local Grounding Electrode System ("GES").

Grounding the SDS (Output) may be done in several ways.

  • For a Wye Connected SDS, Bond the Star / Common Point to the GES,
  • For a Delta Connected SDS with a Center Tap, bond the Center Tap to the GES,
  • For a Delta Connected SDS without a Center Tap, bond the output designated as "Phase B" to the GES.


Per the OP's Delta "TEE" Connected SDS, there are two possible ways for bonding the SDS to the GES:

  1. "Corner Ground",
    or
  2. "Center Tap" Ground


Option #1 is the simplest, and creates a System Grounded Conductor.
Connect Line / Phase "B" to the GES at the Transformer.
Bond Line "B" to the Transformer's enclosure, and to any Equipment Grounding Conductors (EGCs) used on the SDS / Output side.
Color code for Line "B" is White (or Gray) - as it is the System's Grounded Conductor.
No Fuses in Line "B", however 3 Pole Circuit Breakers with common trip & handles may be used, since all three lines are opened simultaneously.

The "Cons" of the Corner-Grounded System include:

  • Full System Voltage to Ground,
  • Need devices rated for Full System Voltage (480V),
  • Tagging the Grounded Conductor at all accessible points,
  • Solid Links in 3 Pole Disconnects on Line B (replaces Fuse).


Option #2:

If it is possible to tap into the Jumper between the Center Tap of the "Main Winding", and the Winding End-Lead on the "Kicker Winding", this would result in a Grounded SDS which does not include a Grounded System Conductor.

Voltage to Ground would be reduced to 240V, and Slash Rated devices may be used, instead of Fully Rated Devices.
Slash Rated Devices = 277/480V
Fully Rated Devices = 480V

Bonding would be as normal:
* Bond Center Tap to the GES,
* Connect System Bonding Jumper (SBJ) from Center Tap to Transformer Enclosure,
* Terminate EGCs to Center Tap via terminations to SBJ.

No need to identify one Conductor with White / Gray.
No need to place solid links in Disconnect Switches - all 3 Poles may be Fused.

"Con":
* May not be able to connect to Center Tap without modifications.

Good luck everyone.

Scott


Scott " 35 " Thompson
Just Say NO To Green Eggs And Ham!
Re: Transformer Question [Re: Scott35] #194177 05/14/10 07:40 AM
Joined: Jun 2005
Posts: 124
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poorboy Offline OP
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Scott, thank you for the very detailed info. I need to ask a couple very specific questions as soon as I get a little time, so keep your eye on this thread.

The saw is running fine but the possibilities you raise about L-G voltages make me want to address the situation further if possible. At least I want to come to a good understanding of what I have done vs. what I should have done.

There are areas of this very diversified trade we work in that are not encountered even a very long career such as I have had. One can be an expert in several facets of the trade and still run into jobs which drop them back to novice (or less) status in an instant. Gotta know how (and where) to ask questions to get through many tasks.

Once again, Scott, you are a saint for taking the time to lay this info out for me...Ken


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