Different phase to ground voltage readings

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kmartinez5588

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Hello all,
I'm new to this forum and fairly new to the electrical field, so please excuse my ignorance. I work on an offshore oil and gas production platform and was recently troubleshooting a condensing unit for the living quarters freezer. The unit has 3 phase 208VAC contactor that wasn't closing. Found the relay that controlled the coils was burnt, so I replaced it, and now it works again. But while I was in the box, I noticed that each phase coming to the load side of thecontactor had a different voltage reading than the other, but read 208VAC phase to phase. My readings phase to ground were: L1-93V...L2-143V...L3-236V. Went to the main panel and found the neutral to ground reading at 120V. The X-MFR feeding this panel is a Delta/Wye. Primary is 3 wire 480V Delta and Secondary is 4 wire 120/208V Wye. The X-MFR case has an earth ground connection, but noticed the XO wire connected to the Secondary neutral was not bonded to enclosure. Could this be the problem for the unbalanced loads and the 120V reading on my neutral to ground? I apologize for the long initial post but, I was trying to give as many details as possible. Thanks!
 
What you have is a totally ungrounded system, which might at this time be accidentally grounded along a hot winding by an equipment fault or may just be randomly floating at an oddly familiar voltage.
NEC allows it in industrial environments as long as a ground detector is provided so that any fault can be corrected promptly before a second fault causes trouble.

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GoldDigger said:
What you have is a totally ungrounded system, which might at this time be accidentally grounded along a hot winding by an equipment fault or may just be randomly floating at an oddly familiar voltage.
NEC allows it in industrial environments as long as a ground detector is provided so that any fault can be corrected promptly before a second fault causes trouble.

Sent from my XT1585 using Tapatalk
Click to expand...

I agree, except for the allowed part in this case. This is a 208Y/120 secondary so it must be grounded.
 
kmartinez5588 said:
Hello all,
I'm new to this forum and fairly new to the electrical field, so please excuse my ignorance. I work on an offshore oil and gas production platform and was recently troubleshooting a condensing unit for the living quarters freezer. The unit has 3 phase 208VAC contactor that wasn't closing. Found the relay that controlled the coils was burnt, so I replaced it, and now it works again. But while I was in the box, I noticed that each phase coming to the load side of thecontactor had a different voltage reading than the other, but read 208VAC phase to phase. My readings phase to ground were: L1-93V...L2-143V...L3-236V. Went to the main panel and found the neutral to ground reading at 120V. The X-MFR feeding this panel is a Delta/Wye. Primary is 3 wire 480V Delta and Secondary is 4 wire 120/208V Wye. The X-MFR case has an earth ground connection, but noticed the XO wire connected to the Secondary neutral was not bonded to enclosure. Could this be the problem for the unbalanced loads and the 120V reading on my neutral to ground? I apologize for the long initial post but, I was trying to give as many details as possible. Thanks!
Click to expand...

Info needed on if you have an "earth" ground......since an offshore rig I do not know. Do such rigs have an anchor hanging in the water as a ground??? Only the readings from phase to center of Y aka neutral in this case have meaning. The phase to ground voltage is meaningless here. It is an offshore oil/gas rig. It is also an SDS. Were it in a plant on land then a Ufer ground would be tied to the neutral. In the case of an airplane the ground is the hull made of alloy. Anyone ever witness St Elmo's fire....I did once.
 
Thanks for the info guys. So should I isolate the X-fmr and bond the XO lead, along with the secondary neutral, to the enclosure? And yes I do have an earth ground. The production platform has 4 legs that drive into the sea floor, it doesn't float. We're only about 10 miles offshore in 60ft. of water.

Sent from my VS986 using Tapatalk
 
kmartinez5588 said:
Thanks for the info guys. So should I isolate the X-fmr and bond the XO lead, along with the secondary neutral, to the enclosure? And yes I do have an earth ground. The production platform has 4 legs that drive into the sea floor, it doesn't float. We're only about 10 miles offshore in 60ft. of water.

Sent from my VS986 using Tapatalk
Click to expand...

Explain what you mean by "isolate the trafo" here??? The sec neutral and X0 should be bonded to ground IMHO but more experience should weigh in. Is your case bonded to ground???
 
Besoeker said:
Just a comment in passing.
Where would you find a ground on an off shore platform?
Click to expand...

I think in a previous post he has earth ground via Ufer in the concrete pilon that goes down into the sea floor, it is not a floating platform.
 
kmartinez5588 said:
Hello all,
I'm new to this forum and fairly new to the electrical field, so please excuse my ignorance. I work on an offshore oil and gas production platform and was recently troubleshooting a condensing unit for the living quarters freezer. The unit has 3 phase 208VAC contactor that wasn't closing. Found the relay that controlled the coils was burnt, so I replaced it, and now it works again. But while I was in the box, I noticed that each phase coming to the load side of thecontactor had a different voltage reading than the other, but read 208VAC phase to phase. My readings phase to ground were: L1-93V...L2-143V...L3-236V. Went to the main panel and found the neutral to ground reading at 120V. The X-MFR feeding this panel is a Delta/Wye. Primary is 3 wire 480V Delta and Secondary is 4 wire 120/208V Wye. The X-MFR case has an earth ground connection, but noticed the XO wire connected to the Secondary neutral was not bonded to enclosure. Could this be the problem for the unbalanced loads and the 120V reading on my neutral to ground? I apologize for the long initial post but, I was trying to give as many details as possible. Thanks!
Click to expand...
What you measured was unbalanced voltages to what may or may not be a common ground. More telling would be the phase to neutral voltages. If these are unbalanced to any great degree you might expect to see problems elsewhere.
 
Besoeker said:
What you measured was unbalanced voltages to what may or may not be a common ground. More telling would be the phase to neutral voltages. If these are unbalanced to any great degree you might expect to see problems elsewhere.
Click to expand...

The measurements line to ground are enough to determine the NEC required bond is missing.

Please don't turn a simple thread into more than it is not.
 
iwire said:
The measurements line to ground are enough to determine the NEC required bond is missing.

Please don't turn a simple thread into more than it is not.
Click to expand...
What could be simpler than measuring phase to neutral to detemine voltage balance?
 
How well it is bonded to earth is a completely different issue then bonding the system grounded conductor to an equipment grounding conductor network and other equipotential bonding means. Having all the structural steel and other non current carrying metallic components at the same potential is the first thing of importance. Low impedance bonding to earth mostly is for lightning or for other transients originating from outside the system - on an offshore application like this chances are there is no outside source other then possibly lightning as the source probably originates at the offshore location.
 
Besoeker said:
What could be simpler than measuring phase to neutral to detemine voltage balance?
Click to expand...

Line to ground is always easier as far as using the meter lead and if I measure line to neutral I will not find a problem.

The way to locate this problem is by measuring from neutral to ground or line to ground. Either of those methods will show that the bonding jumper from neutral to ground is missing.
 
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