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#198235 - 01/16/11 12:49 PM A real sticky wicket
Powercon Offline
New Member

Registered: 12/09/04
Posts: 6
Loc: Wi.
If anyone can help me explain this to my friend in an easier way I would be forever grateful.
This is his problem, but first a little info. He is a very knowledgeable gentleman and has engineered and installed many hydro installations in his life, both from the mechanical and electrical side of the desk. He is a hands on man and can still quote verse and verses on theory (I wish I had his knowledge on math and the sciences)and application. He is in his 90's now and kind of set in his ways, of course I'm not grin .
Now to his problem. His machine shop is a mess as far as the service is concerned, everybody and his brother has added devices through the years. One day he was having problem with a large milling machine, it would sometimes run for a few minutes and other times the contactor would energize and drop out immediately.
I checked all the common problems usually associated with these older machines and found one of the contactor coil leads grounded to the frame of the machine intentionally and asked him why this would have been done. His explanation was "a 480 volt coil with a ground".The only problem is the building is wired 480 Delta no taps for grounds and no other transformers (for this purpose). I tried to explain the situation but he was adamant that this should work with no circuit returns.
I explained that the coil should be wired with the now grounded conductor tied to one of the other two phases and he explained that it would burn out the coil. Again I explained that the coil was already being supplied by the 480 buss and this would have no affect. I also explained that with the 480 coil grounded to the machine this could be dangerous to an operator if the right (or wrong) problem occurred. Well he wouldn't listen so after he left I wired it correctly and the machine worked as it should.
Now this is one of the old WW2 machines without an LVT for control so the manual push buttons are across the line. I would like to rewire it but he will have none of that. All of the lighting are high bays at 480 with the offices and low voltage through a transformer.
I am at my wits end trying to explain the error in the way this machine was wired. He explains that we just need to "fix the ground problem". Now I am in agreement with fixing the ground problems but I am afraid he has other machines wired the same way and need to get through to him that the grounding system was never designed to be used for this purpose. Anybody have any suggestions for me, oh by the way violence isn't an option smirk . He's a nice person just a little hard to break old habits. Thanks for any suggestions, Bill

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#198236 - 01/16/11 01:21 PM Re: A real sticky wicket [Re: Powercon]
LarryC Offline
Member

Registered: 07/05/04
Posts: 775
Loc: Winchester, NH, US
Can you measure the voltage from each phase to "ground" and show him how it varies with what loads are turned on?

Perhaps you can find some machines where there is a significant voltage between the machine and building "ground" ?

How good is the building and service ground? Perhaps the grounding system needs to be restored to current standards.

480 delta service? Does he have functional ground fault indicators? (Lights connected between each phase and earth)

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#198237 - 01/16/11 02:01 PM Re: A real sticky wicket [Re: LarryC]
Tesla Offline
Member

Registered: 06/16/04
Posts: 1280
Loc: Sacramento, CA
Oral argument with a high-time veteran will not work.

Instead get xerox copies of the relevant wiring diagrams -- blown up of course.

Draw a one-line showing the SERVICE and detailing its bonding and GEC.

Show the derived 208Y120 panel...

And then detail the voltages with Tensors/Vectors...

-----

His mental block is two fold: you're too young to be lecturing HIM...

AND, his stuff 'works' ... so some other failure mode must be the cause.
_________________________
Tesla

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#198260 - 01/16/11 10:58 PM Re: A real sticky wicket [Re: Tesla]
twh Offline
Member

Registered: 03/11/04
Posts: 892
Loc: Regina, Sask.
He's probably toying with you. Even old people use new technology when they want. I bet he likes a new car and a modern ice box. Put your foot down and tell him it's the new rules and must be fixed. You can't drive a Model T at 70 mph and you can't endanger lives with old wiring. If he's smart like you say, he already knows this.

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#198265 - 01/17/11 01:43 AM Re: A real sticky wicket [Re: twh]
Trumpy Offline

Member

Registered: 07/05/02
Posts: 8530
Loc: SI,New Zealand
Your "friend" is operating an electrical death-trap, purely and simply.
No amount of dancing around that issue is going to solve it.
If an Inspector were to see what was going on there, chances are the installation would have the power cut to it until such time as it was safe.
This sort of voltage is not something to be messed with.
_________________________
Let's face it, these days if you're not young, you're old - Red Green grin

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#198364 - 01/19/11 02:11 PM Re: A real sticky wicket [Re: Trumpy]
Scott35 Offline

Broom Pusher and
Member

Registered: 10/19/00
Posts: 2724
Loc: Anaheim, CA. USA
The 480V 3 Phase System may be an Ungrounded Delta, or a Center-Tap Grounded Delta.
From the description of how the Contactor drops out, I am thinking the System is some type of Delta, which does not have a Grounded Conductor.

Warrant the services of a Qualified Electrician, and have that Person perform the following:

To verify if the System is an Ungrounded Delta, or a Corner Grounded Delta, take Voltage readings at either the Service Equipment, Panelboard, or Disconnect Switch.

Note: Disconnect the Contactor's Coil Leads prior to performing the Voltage Measurements.
Disconnect both Coil Leads - the Line Side Lead, and the Lead attached to the Metallic equipment.
If there are other connections of this type (L-G), disconnect them also (Both "Sides" of the Load).

Voltage Meters needed:

a: (1) Low Input Impedance Voltmeter: A "Wiggy", or Solenoid type Voltage Meter.

b: (1) High Input Impedance Voltmeter: A typical DVM (Digital Voltage Meter), such as the Standard Fluke Meter will work.

Voltage Measurements

Step#1:
Verify the Enclosure is solidly Grounded / Bonded to the Local Grounding Electrode System (GES). The Service Equipment is the best place to take measurements, as the Metallic Enclosures will (likely) be Bonded to the GES... unless someone has disconnected the Grounding Electrode Conductor (GEC) for some odd reason.

Step#2:
With a Grounded Enclosure verified, perform Three separate Line-To-Ground (L-G) voltage test with the Low Impedance Voltmeter (Low Z Voltmeter).

Test between:
  1. Line (Phase) A and Ground,
  2. Line (Phase) B and Ground,
  3. Line (Phase) C and Ground.


If the Low Z Voltmeter shows no Voltage on all three of these tests, the System most likely is an Ungrounded Delta.

If the Low Z Voltmeter shows apx. 480V on two of the three tests, and one test shows no Voltage, the System is a 480V Corner Grounded Delta.
The Line with Zero Volts to Ground is the System's Grounded Conductor - and should be Color Coded "White" or "Gray".

If the Low Z Voltmeter has a "Solid" Voltage reading on all three L-G tests, the System is a Center-Tapped 480V 3 Wire Delta.
One L-G Voltage reading will be considerably higher than the other two L-G Voltage readings.

Step#3:
With the High Impedance Voltmeter (Hi Z Voltmeter), perform the same Three L-G Voltage tests, as were performed with the Low Z Voltmeter:

Test between:
  1. Line (Phase) A and Ground,
  2. Line (Phase) B and Ground,
  3. Line (Phase) C and Ground.


If the System is a 480V 3 Phase 3 Wire UNGROUNDED DELTA:

a: The Hi Z Voltmeter will display a Voltage of apx. 150 to 400 Volts between L-G on all three test connections (Line A, B and C to Ground),
&
b: The Low Z Voltmeter will display Zero Volts (or close to Zero Volts) between L-G on all three test connections.

This type of System does not have a Grounded Conductor, but Metallic Enclosures WILL be Bonded to a Local GES.

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


If the System is a 480V 3 Phase 3 Wire CORNER-GROUNDED DELTA:

a: The Hi Z Voltmeter will display a Voltage of apx. 480 Volts between L-G on two test connections, with Zero (or close to Zero) Volts between the third L-G Test Connection;
&
b: The Low Z Voltmeter will display a Voltage of apx. 480 Volts between the same two L-G test connections as the Hi Z Meter displayed, with Zero (or close to Zero) Volts between the third L-G Test Connection.

This type of System DOES have a Grounded Conductor - the Line with Zero Volt reading to Ground is the Grounded Conductor.
This Phase Line, along with Metallic Enclosures WILL be Bonded to a Local GES.
Color Code for the Grounded Conductor is either White or Gray.
The Equipment Grounding Conductors may be Bare, Green, or simply the Raceway (Conduit).

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


If the System is a 480V 3 Phase 3 Wire CENTER-TAP GROUNDED DELTA:

a: The Hi Z Voltmeter will display a Voltage of apx. 240 Volts between L-G on two test connections, with Apx. 415 Volts between the third L-G Test Connection;
&
b: The Low Z Voltmeter will display a Voltage of apx. 240 Volts between the same two L-G test connections as the Hi Z Meter displayed, and "Slightly Lower Than 480 Volts" between the third L-G Test Connection.
The Pointer will be low in the "480V" Field, or high in the "240V" Field.

This type of System DOES NOT have a Grounded Conductor.
The Transformer has one Winding Grounded, via a tap at the Center of the Winding's length.
Metallic Enclosures WILL be Bonded to a Local GES, as will the Center Tap (most likely Grounded at the Transformer + Service Equipment, but sometimes the Grounding is at the Utility Transformer only).

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

NOTE: All the above tests will display 480V between any two Lines (Phases), using either the High Z Meter or the Low Z Meter...
i.e.:
  • Phase A and Phase B: 480V,
  • Phase A and Phase C: 480V,
  • Phase B and Phase C: 480V.


..................................................
..................................................

The reason I am leaning towards a System without a Grounded Conductor, is the way the Contactor drops out randomly.

The System might be an Ungrounded Delta, or a Center-Tapped Delta.

If the System is Ungrounded, Capacitive Coupling between the System and Grounded Enclosures will exist.
This is what gave the Hi Z Voltmeter a reading between each Line and Ground, while on the Low Z Voltmeter, there was no displayed Voltage reading.
The Higher the Impedance between L-G, the Higher the Voltage is between those two points.

As the Coupling effect changes (Impedance is reduced), so is the Voltage between the two points.
For a Contactor Coil, if it has latched in, it will attempt to hold until the Voltage across it becomes too low to develop Magnetic Flux, so it drops out.
If the Coupling Effect does not allow enough Current to flow, the Coil will not engage the Contactor.

Having more than one L-G Connections on the 480V side, would easily allow for a scenario like this.

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

If the System is a Corner-Grounded Delta:
A 480V rated Coil will function properly, but with one side directly Ground Bonded at the Machine, Currents will be flowing through the Grounded metallic Enclosures & Raceways between the machine and the Service Equipment.
This needs to be corrected.

If the Coil's rated Voltage is less than 480V (240V, 277V, 380V), eventually Smoke will ooze out from the Coil, and it will cease to work.

----------

If the System is a Center-Tap Grounded Delta:

A Coil rated for 440-480V is "Likely to Work" if connected between Line "B" and Ground, as this will result in +/- 416V across the Coil.
If / when the L-G Voltage drops below 360V, the Coil should drop out.

If the 440-480V rated Coil is connected between Line "A" and Ground, or Line "C" and Ground, the Coil "may" function "Randomly", according to the Voltage across the Coil.
Since the Voltage would be around 250V Nominal, the coil might latch in during a rise in Voltage (Voltage and Current Spike or Surges).
For large size Contactors, this would be highly unlikely, as the required Inrush Volt-Amps (VA) to pull-in and latch the Contactor are at least 10X the required VA for holding in a latched state.

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

Nevertheless, please have the system checked ASAP, by a qualified person.

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

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#199192 - 02/16/11 05:03 AM Re: A real sticky wicket [Re: Powercon]
Scott35 Offline

Broom Pusher and
Member

Registered: 10/19/00
Posts: 2724
Loc: Anaheim, CA. USA
--Bump / Question ---

Any update per this scenario???

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

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