L6-30R Floating ground when ground lug shows connection to earth ground.

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AAP

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Maintenance Technician
In the plant, 2 of our L6-30R receptacles on one panel show "NO GND" or "FLOATING GND". The receptacles all have good ground connection to the panel and to EARTH GND. The panel is supplied by a 480VAC 3Phase to a 240VAC 3Phase DELTA transformer with no center taps. All installed long before my time. Other than these 2 on this panel, all others in the plant show good grounds, not floating, no problems.
Should these be on a Wye transformer? Some equipment is 3 phase.
Or go thru a single phase 240 to 240/120 transformer?
Or is there something else that should be looked at?
Most equipment does run on these, but none that references GND.

Thanks,
Art
 
Can the outlets be wired to the panel that feeds your other L6-30 outlets?

If your source transformer is delta with no center tap, you have no normal way to ground it for handling fault currents back to the transformer to comply with it's design rating 1Φ 30A 3W Grounded.

Connecting it to a wye transformer would give you 208V 1Φ, mot the 240V it is listed for. Depends on what you power with it, but is not technically correct .

Powering it from a properly connected1Φ 240V transformer with a center tap output grounded, would give you the proper connections available

Can you supply them from one of the other panels that are properly grounded?.
 
AAP said:
In the plant, 2 of our L6-30R receptacles on one panel show "NO GND" or "FLOATING GND". The receptacles all have good ground connection to the panel and to EARTH GND. The panel is supplied by a 480VAC 3Phase to a 240VAC 3Phase DELTA transformer with no center taps. All installed long before my time. Other than these 2 on this panel, all others in the plant show good grounds, not floating, no problems.
Should these be on a Wye transformer? Some equipment is 3 phase.
Or go thru a single phase 240 to 240/120 transformer?
Or is there something else that should be looked at?
Most equipment does run on these, but none that references GND.

Thanks,
Art
Click to expand...
Can you post the transformer nameplate wiring diagram.

You describe a delta transformer without any center tap, so you probably have an ungrounded 240V 3-phase system. L-G voltages readings are rarely consistent, or meaningful, on ungrounded systems.

When troubleshooting it is good to know all of the possible voltages:
L1-L2
L2-L3
L3-L1
L1-N
L2-N
L3-N
L1-G
L2-G
L3-G
N-G
 
Rare to have a true no earth ground system today. That said if you have a true ungrounded system you should have a fault monitoring alarm system installed to warn of a L-G fault as there is no way to trip the breaker on a fault to G. At bare minimum it should have one of there installed if not already there.

Most today are either corner grounded delta (giving 240 or 480 L-L with 120 or 240 L-G on 2 of the phase and 0V on the one L-G) or mid phase (center) grounded delta giving a high leg Delta (120/240/208)
Are you sure (tested) that these panels are still ungrounded? Seen many time the system over time had been changed and old labels never removed or modified.
If the facility had any general use receptacles (120V 15-20A) you must have somewhere along the system created a grounded (neutral) system.
 
Agreeing with the above.

It sounds like the receptacles have a good connection to the supply EGC and to earth ground, but that the transformer source is not properly connected to ground. Because the source is not ground referenced, any test between the receptacle powered terminals and receptacle ground pin would show 'floating ground', even though the real problem is 'floating source'.

A good solution would be to replace the transformer with one that could have a grounded terminal. If your equipment only requires straight 240V, then either a secondary 240V delta with grounded center tap, or a secondary 240/139V wye would be fine.
 
AAP said:
Other than these 2 on this panel, all others in the plant show good grounds, not floating, no problems.
Click to expand...
If you are saying these 2 are the only ones from that particular panel/transformer but all others that are fine are from other sources, then your transformer likely isn't grounded and is essentially operating as an ungrounded system.
 
winnie said:
A good solution would be to replace the transformer with one that could have a grounded terminal.
Click to expand...
Why is any solution needed?
Maybe the system was installed with ground detection or it wasn't need at that time.
Maybe this is a labeling or training issue.
 
jim dungar said:
Why is any solution needed?
Maybe the system was installed with ground detection or it wasn't need at that time.
Maybe this is a labeling or training issue.
Click to expand...

Fair enough. Once the problem is understood, it may be that no change to equipment is needed.

The OP did ask:
AAP said:
[...]Should these be on a Wye transformer? Some equipment is 3 phase.
[...]
Most equipment does run on these, but none that references GND.
Click to expand...
So I would revise my answer to:

If a grounded supply is actually needed, for example if you have to run equipment that requires a proper ground reference, then a good solution would be to replace the transformer with one that could have a grounded terminal. If your equipment only requires straight 240V, then either a secondary 240V delta with grounded center tap, or a secondary 240/139V wye would be fine.

Corner grounding _may_ be a solution to provide a proper ground reference. Yet another option would be to leave the system ungrounded and provide ground fault detection.

-Jonathan
 
electrofelon said:
Oh that's more recent than I would have thought.
Click to expand...
It's actually even more recent than that. 2005 was when the systems allowed in 250.21 were required to have ground detection. Before that, some specific applications were required. A 1984 copy of Practical Electrical Wiring that I have mentions that ground detector lights should be installed on ungrounded systems.
 
CoolWill said:
It's actually even more recent than that. 2005 was when the systems allowed in 250.21 were required to have ground detection. Before that, some specific applications were required. A 1984 copy of Practical Electrical Wiring that I have mentions that ground detector lights should be installed on ungrounded systems.
Click to expand...
And just for completeness, note that medium voltage still has no ground fault detection requirement for ungrounded systems.
 
jim dungar said:
Can you post the transformer nameplate wiring diagram.

You describe a delta transformer without any center tap, so you probably have an ungrounded 240V 3-phase system. L-G voltages readings are rarely consistent, or meaningful, on ungrounded systems.

When troubleshooting it is good to know all of the possible voltages:
L1-L2
L2-L3
L3-L1
L1-N
L2-N
L3-N
L1-G
L2-G
L3-G
N-G
Click to expand...
Agreed.
All L#-L# are 240 plus/minus <5%
From Xfmr to panel only L1, L2, L3, and GND
any L# to GND changes based on what is connected or turned on.
No load all are in range of 105VAC to 138VAC
with loads random range is 11VAC to 198VAC

Thanks for your reply. I am just having problems convincing my coworkers and management that this is not correct.
 
MTW said:
Can the outlets be wired to the panel that feeds your other L6-30 outlets?

If your source transformer is delta with no center tap, you have no normal way to ground it for handling fault currents back to the transformer to comply with it's design rating 1Φ 30A 3W Grounded.

Connecting it to a wye transformer would give you 208V 1Φ, mot the 240V it is listed for. Depends on what you power with it, but is not technically correct .

Powering it from a properly connected1Φ 240V transformer with a center tap output grounded, would give you the proper connections available

Can you supply them from one of the other panels that are properly grounded?.
Click to expand...
Actually I had already wired an L6-30 directly to the panel and got identical results as other receptacles. Thanks for confirming my thoughts.
50 years of industrial maintenance, datacenter maintenance, military electronics repair, computer repair and more. But management chooses not to believe me because all but one piece of equipment operates on these outlets. So I am told I am not a licensed electrician, I must be wrong. Trying to explain that the one equipment utilizes 120VAC to GND for a safety circuit makes no sense to them.
 
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