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#15568 - 10/21/02 08:36 PM Odd voltages
amp-man Offline
Member

Registered: 06/23/01
Posts: 144
Loc: Sacto, California US of A
Hi y'all,

Help me out here--I'm missing something and just can't see to clear.

I was working in an area used now for retail, previously was a cannery (built in the late '40s). Supplied with 480/277 with stepdown transformers where 240/120 is needed.

Tenant wanted a few 120V receptacles in the center of the space. On a support column, I found a blanked-off single gang handy box at the end of a run of heavywall. Opened it up, see two wires, rag and tar insulation, one w/white paint.

Using a solenoid voltage tester (Knopp), I find 120V from white wire to conduit ground (not unusual in this facility, a lot of the identified conductors are hot--so much for "qualified" maintenance men), and 120V between the two conductors. So far so good. Then I check from black to ground and get 240V.

My first thought is that the conduit is at 120V to ground. I check with a Greelee
volt-tic, find no voltage. Then I check with a CTC/Fluke 9970 voltage detector (used by telco linemen to make sure they don't have forgien voltage on terminal boxes). No voltage.

I re-capped the conductors and reinstalled the blank plate. I can't figure out why the 240V to ground. It's not phantom voltage. What am I missing?

I am going to be back at the facility late this week for some phone and data wiring. Any additional (quick and easy) test I could do to pin this down? I was thinking of running a lead to a known good ground and testing the conduit-to-ground voltage with the Knopp.

Any suggestions appreciated.

Cliff

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#15569 - 10/22/02 04:28 AM Re: Odd voltages
sparkync Offline
Member

Registered: 10/17/02
Posts: 811
Loc: NC
Cliff, just a few quick suggestions, Maybe trace the conduit if possible to a junction box, maybe they tied in to a 277 volt lighting circuit?? Maybe the circuit lost it's neutral and acting crazy?? ?????????

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#15570 - 10/22/02 04:40 AM Re: Odd voltages
Redsy Offline
Member

Registered: 03/28/01
Posts: 2138
Loc: Bucks County PA
Are the step-down xfmrs 3 phase? If someone intentionally grounded a phase conductor instead of the center-tapped neutral, it seems like it would result in these readings.

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#15571 - 10/22/02 05:26 AM Re: Odd voltages
Scott35 Offline

Broom Pusher and
Member

Registered: 10/19/00
Posts: 2724
Loc: Anaheim, CA. USA
I am thinking the same way as Redsy about the Grounded Conductor mismatch.

Seeing that the stepdown transformer is rated for 120/240 volts (unless this is a typo or just over simplified) would make it either a 1 phase 3 wire secondary, or a 3 phase 4 wire delta secondary.
Either case, the center tap point "X0" should be the Grounded Conductor's termination point - along with all the secondary grounding.
This will limit the voltage to ground to +/- 120 VAC (if the coil with the center tapped point is wound for 240 volts).

If instead, an end of the coil is grounded - such as either "X1" or "X2", this would result in voltage readings as follows:

L1-L2: 240 VAC,
L1-N: 120 VAC,
L2-N: 120 VAC,
N-G: 120 VAC,
L1-G: 240 VAC,
L2-G: 0 VAC

"L1" = Line 1, connected to "X1",
"L2" = line 2, connected to "X2",
"N" = Common "Neutral" conductor, connected to "X0",
"G" = Equipment Grounding Conductor / Grounded Equipment.

In the example above, it would describe how a system would have voltage readings if the "X2" termination and conductor was the grounded conductor.

Your situation may be as simple as this, or something completely different. Best if you can verify all the data relating the effected system, then post results here.

Find out what the connections at the transformer are, type of transformer, if any fuses are blown or faulty breakers, any running motors during voltage tests, and what the voltages read using a DVM.

Scott. s.e.t.
_________________________
Scott " 35 " Thompson
Just Say NO To Green Eggs And Ham!

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#15572 - 10/22/02 05:57 AM Re: Odd voltages
EVAD7 Offline
Member

Registered: 03/11/02
Posts: 37
Loc: MN
check voltage again, if you get 208v to ground you have two legs of 240v 3ph

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#15573 - 10/22/02 03:45 PM Re: Odd voltages
jes Offline
Member

Registered: 07/12/02
Posts: 103
Loc: CT
Having worked in some old mills here in New England your situation is not unfamiliar. If you cannot trace the circuit back to the supply and determine what it is and how it's protected then close it up and run your own. You might have anything from a control circuit to a something fed from some distribution long abandoned but still powered, possibly with the wrong voltage. Eyeball it or fuhgetaboutit!!

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#15574 - 10/22/02 10:16 PM Re: Odd voltages
amp-man Offline
Member

Registered: 06/23/01
Posts: 144
Loc: Sacto, California US of A
Thanks to all.

Evad, yes I did check with a DMM. It was 240V.

I will be there later this week and will have the chance Saturday to spend a little time tracing it out. Scott35 and Redsy, I'll report back on the transformer setup.

This whole factory complex is slated to be razed in about a year, so there's no reason to fix it--I just want to understand it!

BTW, one part of this complex has some cold storage warehouses that are still active. I'll see if I can get some shots of the motors running the ammonia compressors--beautiful late 1940s art deco GE units, running on 2kV. Also the old PBX switch--old 1A2 system in a cabinet 7 ft high, 12 feet long, three feet deep; dozens of step-by-step relays and miles of neatly dressed wires!

Best reagrds,

Cliff

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#15575 - 10/23/02 05:14 AM Re: Odd voltages
EVAD7 Offline
Member

Registered: 03/11/02
Posts: 37
Loc: MN
The system may also be "floating", no grounded conductor from the power company or the transformer. On a DMM you will get wacky readings.

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