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Old July 12th, 2012, 09:14 AM   #1
tspisak
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Odd three phase power question - one leg dead

I have a customer that has an existing automation that has been running for years that was designed to operate on 480vac three phase power. On a service call we discovered that they appear to have 480vac single phase (507v between two legs, and 507v from either of those legs to the third wire - the third wire appears to be zero volts). There are only three wires coming into the disconnect (two blacks and a white).

I don't know why this system (it has a fanuc robot in the cell) hasn't just faulted out for loss of one phase. This is an OLD facility that has been around since electricity was invented.

Has anyone heard of this configuration for three phase power? I'm suggesting that they get it fixed but they say that its been this way for years so they aren't changing it w/o a good reason.
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Old July 12th, 2012, 09:18 AM   #2
bernie_carlton
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It is a "Corner Grounded Delta" system
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Old July 12th, 2012, 09:33 AM   #3
DamianInRochester
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That still doesn't explain the high voltage reading. How well do you trust your meter? Are they capable of adjusting the taps on their transformer down to reduce the voltage.

Running equipment designed for 480 on 507 should definitely have a negative impact on lifetime.
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Old July 12th, 2012, 09:55 AM   #4
bernie_carlton
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NFPA79 says that machinery should be designed to operate +- 10% of rated voltage. 507 is withing 10% of 480.
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Old July 12th, 2012, 10:04 AM   #5
DamianInRochester
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True, but it still has a negative impact on the life of the equipment. NFPA is concerned about safety.
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Old July 12th, 2012, 11:57 AM   #6
Mickey
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A key phrase...

Quote:
but they say that its been this way for years so they aren't changing it w/o a good reason.
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Old July 12th, 2012, 01:12 PM   #7
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Tspisak, This is a problem of which your customer should be aware for the old corner-grounded system:
Quote:
• Most electrical distribution equipment manufactured in North America is not rated for use on this system.
The grounded leg is required to be identified, and they apparently used a white wire for that, but I wonder if that is enough? Obviously it was not enough to tell YOU about the system power. This is dangerous for outside contractors. It would be safer to create a big label to stick to the outside of the main disconnect.

If this is in a plant in an industrial park, it is common for the voltage to go to 500 volts or more during slack periods and for night shifts. The reason is that the voltage has been stepped up at the main utility substation transformer to handle the periods of heavy load, and usually there are some utility power-factor correction capacitors at the substation that increase the voltage during low-current periods. The only way for the utility company to offset the voltage problem caused by PF correction capacitors is to install expensive variable-controlled capacitors, where some get switched off during periods of low current.

Last edited by Lancie1; July 12th, 2012 at 01:22 PM.
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Old July 12th, 2012, 03:21 PM   #8
joseph_e2
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Note also that the grounded leg is not supposed to be fused. It also shouldn't be switched unless all 3 legs are switched. One way would be to use a 3-phase fused disconnect with a slug in the grounded leg. A 3-phase circuit breaker is OK, as long as it's properly rated.
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Old July 12th, 2012, 06:45 PM   #9
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Quote:
One way would be to use a 3-phase fused disconnect with a slug in the grounded leg.
I don't think that would be a smart idea in this case. Tspisak has already said that it is a 3-phase disconnect, which will disconnect all 3 phases. If you bypass the grounded leg switch, and then later someone accidently cuts or ignorantly removes the ground from the transformer, suddenly there is live unswitched 480 volts on the slugged leg. It shouldn't do any damage - unless there is also a short to another phase.

Last edited by Lancie1; July 12th, 2012 at 07:04 PM.
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Old July 12th, 2012, 07:20 PM   #10
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Some related information, starting on pg. 126.


http://www.mjobee.com/projects&news/delta%20xfmr1.pdf
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Old July 12th, 2012, 08:05 PM   #11
joseph_e2
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Quote:
Originally Posted by Lancie1 View Post
I don't think that would be a smart idea in this case. Tspisak has already said that it is a 3-phase disconnect, which will disconnect all 3 phases. If you bypass the grounded leg switch, and then later someone accidently cuts or ignorantly removes the ground from the transformer, suddenly there is live unswitched 480 volts on the slugged leg. It shouldn't do any damage - unless there is also a short to another phase.
I meant a slug in place of the fuse. Sorry for the confusion.
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Old July 14th, 2012, 07:35 PM   #12
DickDV
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This power configuration has some serious issues related to drives and safety.

Briefly, don't ever consider running a DC drive on this stuff. You will need a delta/wye isolation transformer with grounded center point ahead of the DC drive so the SCR's can switch switch the armature uniformly to ground. Running directly on the corner-grounded power would produce large charge/discharge currents to ground in the armature resulting in lots of commutator arcing and short brush life.

While you can run a PWM type AC drive on corner-grounded power, you must disconnect the input MOV's to ground and turn off the output fault and short circuit protection. It's a high price to pay since the drive is essentially unprotected on input and output. A delta/wye transformer is again recommended.

Finally, there are serious hazards that are unique to floating or corner grounded power networks. A grounded balanced wye network enforces 277VAC to ground on each phase which is low enough to not ionize most dusts. With a floating or unbalanced system, you can have as much as 500VAC to ground which often ionizes common dusts. The result is that you do your normal lockout/tagout procedure on a machine and, due to ionizing dust around the disconnect contacts on the two high voltage phases, the load side conductors can float up to full line voltage due to the leakage around the disconnect thru the dust. Contact with the locked out equipment conductors puts you in contact with 480VAC. I almost lost my life due to this on a rubber extruder and have campaigned against this kind of power distribution ever since. For those of you that have to work with the floating or unbalanced networks, I recommend a four-legged ground strap. Connect the first leg to ground first, then the remaining three to the three disconnected (load side) conductors. Then you are sure they are grounded and stay grounded. This is standard procedure with electric utilities on their floating high lines. It becomes necessary for safety at 480V too if the network floats or is corner grounded.

One other safety consideration on floating systems. You probably have a CAT III 1000V meter in your tool kit. So you would think that it is adequate for safe contact with the power line. But, if it is a floating delta, you only know what the phase-to=phase voltages are. What is it to ground? Zero, 500VDC, 2000VAC, or maybe there is an insulation defect in the distribution transformer and the secondary delta network if floating at 4160VAC to ground. Maybe that meter isn't as safe as you thought it was! See my point?

Last edited by DickDV; July 14th, 2012 at 07:42 PM.
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Old July 14th, 2012, 11:50 PM   #13
HJTRBO
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Well written DickDV. This thread certainly sparked an interest for me to learn more about the different configurations you guys have over there in the states. Amazing are the differences.
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