Different phase to ground voltage readings

[ptonsparky]

Here are my readings from the first main panel:
L1-L2 = 202v
L1-L3 = 202v
L2-L3 = 202v
L1-N = 117v
L2-N = 117v
L3-N = 117v
L1-GND = 151v
L2-GND = 250v
L3-GND = 96v
N-GND = 136v
The XFMR is grounded to earth, along with the bare ground wires from both the Primary and Secondary feeds. Does this sound like an ungrounded system? The xmfr is primarily used for lighting and feeds several subpanels from the main panel.

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Sounds like it is ungrounded to me. XFMR case to earth does not make it a grounded system. Typically XO needs to be earthed, or whatever serves as your "earth" and connected to the bare ground wires at a single point after which they are isolated from each other.

(I have not even seen an offshore platform let alone worked on one.)

 

[kwired]

Here are my readings from the first main panel:
L1-L2 = 202v
L1-L3 = 202v
L2-L3 = 202v
L1-N = 117v
L2-N = 117v
L3-N = 117v
L1-GND = 151v
L2-GND = 250v
L3-GND = 96v
N-GND = 136v
The XFMR is grounded to earth, along with the bare ground wires from both the Primary and Secondary feeds. Does this sound like an ungrounded system? The xmfr is primarily used for lighting and feeds several subpanels from the main panel.

Sent from my VS986 using Tapatalk

It is an ungrounded system, until you bond the neutral to ground. I doubt it was intentionally ungrounded or was misunderstood by whoever initially installed it.

 

[Besoeker]

Here are my readings from the first main panel:
L1-L2 = 202v
L1-L3 = 202v
L2-L3 = 202v
L1-N = 117v
L2-N = 117v
L3-N = 117v
L1-GND = 151v
L2-GND = 250v
L3-GND = 96v
N-GND = 136v


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Yes, much more credible than what you put in post #23:

Phase to neutral readings are: L1-203V, L2-98V, L3-203V

Can you see now why those readings were considered suspect and not measurements of what they were purported to be?

The XFMR is grounded to earth, along with the bare ground wires from both the Primary and Secondary feeds. Does this sound like an ungrounded system?

If you measure 136V N-GND, then there no other explanation I can think of.

 

[iwire]

It is an ungrounded system, until you bond the neutral to ground. I doubt it was intentionally ungrounded or was misunderstood by whoever initially installed it.

If you measure 136V N-GND, then there no other explanation I can think of.

Great, it only took about 40 posts for you guys to reach the same conclusion GoldDigger did in the first response to this thread.


If he is under the NEC he has an incorrectly installed grounded system. His source is a wye and the NEC requires it to be a grounded system so a bonding jumper must be installed. Once that bonding jumper is in place and the power restored it would not surprise me if he finds a ground fault on a branch circuit somewhere.

 

[Besoeker]

Great, it only took about 40 posts for you guys to reach the same conclusion GoldDigger did in the first response to this thread.

Except that the OP provided some duff information at the very get go. And in several posts subsequently.

A question for you.
Is the NEC totally applicable to an off shore rig?
How would you ground it?
Driving in ground rods would not seem to be an entirely practical proposition............)

 

[GoldDigger]

Except that the OP provided some duff information at the very get go. And in several posts subsequently.

A question for you.
Is the NEC totally applicable to an off shore rig?
How would you ground it?
Driving in ground rods would not seem to be an entirely practical proposition............)

The salt water provides a very good linkage to the underlying "earth". Even better than the same volume of soil.
"Drive" a rod into the water. Or just use the legs of the tower (building steel!)

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[iwire]

A question for you
Is the NEC totally applicable to an off shore rig?

I have no idea, that was why I prefaced many of my comments with 'if under the NEC' etc.

How would you ground it?

You keep going back to the connection to dirt / earth that is not the issue here.

But if I was charged with providing a grounding electrode system I would see if the AHJ requiring NEC compliance would go with this.

250.52(A)(2) Metal Frame of the Building or Structure. The metal
frame of the building or structure that is connected to the
earth by one or more of the following methods:

(1) At least one structural metal member that is in direct
contact with the earth for 3.0 m (10 ft) or more, with or
without concrete encasement.

(2) Hold-down bolts securing the structural steel column
that are connected to a concrete-encased electrode that
complies with 250.52(A)(3) and is located in the support
footing or foundation. The hold-down bolts shall
be connected to the concrete-encased electrode by
welding, exothermic welding, the usual steel tie wires,
or other approved means.

The OP said the unit is not floating so I am guessing that means some sort of structure is driven into the seafloor.

 

[kmartinez5588]

Thanks for all the help gentlemen. I apologize for giving the wrong info. I'll talk with our facility engineer and get this issue resolved.

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[Besoeker]

The salt water provides a very good linkage to the underlying "earth". Even better than the same volume of soil.
"Drive" a rod into the water. Or just use the legs of the tower (building steel!)

I think it was mentioned that it has concrete legs that go to the seabed. And in 60 ft of seawater before you get to it. Then you have the height of the platform above sea level. The one's I've been on in the North Sea were huge.

I don't know how practical it would be to conduct to the underlying earth. I'm inclined to think not easy.
Someone here mentioned equipotential bonding. In effect, the hull of the rig and all non-live metal parts would be at the same potential and could be connected to the transfomer neutral. That would make it like a ground would't it?

 

[GoldDigger]

I think it was mentioned that it has concrete legs that go to the seabed. And in 60 ft of seawater before you get to it. Then you have the height of the platform above sea level. The one's I've been on in the North Sea were huge.

I don't know how practical it would be to conduct to the underlying earth. I'm inclined to think not easy.
Someone here mentioned equipotential bonding. In effect, the hull of the rig and all non-live metal parts would be at the same potential and could be connected to the transfomer neutral. That would make it like a ground would't it?

What I was saying is that regardless of whether or not the legs ever reach the sea floor, they will still provide an excellent ground. The effective resistance from one point to another in an infinite mass of water is zero, just as for dirt. And the water couples perfectly (for all practical purposes) to the underlying dirt.
Old low frequency marine radio service transmitter antennas were often built over water or marsh because it made a near perfect ground plane for the antenna tower(s).

And yes, the rig metal would serve perfectly as the equipotential reference, just as for a ship, car, or truck.
That is what is important for safety and fault clearing.

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[Besoeker]

What I was saying is that regardless of whether or not the legs ever reach the sea floor, they will still provide an excellent ground. The effective resistance from one point to another in an infinite mass of water is zero,

Don't think that's correct. You may have an infinite number of parallel paths but they vary in length from the shortest to infiniity.

Here's a link:

https://physicstasks.eu/1040/measuring-of-the-electrical-conductivity-of-sea-water

Mods, this is related to grounding issue of the rig from the OP.

 

[GoldDigger]

Don't think that's correct. You may have an infinite number of parallel paths but they vary in length from the shortest to infiniity.

Here's a link:

https://physicstasks.eu/1040/measuring-of-the-electrical-conductivity-of-sea-water

Mods, this is related to grounding issue of the rig from the OP.

If you read that argument carefully you see that either the argument only applies to a finite sized body of water or else the same argument applies equally well to dirt. But we accept that the resistance of earth is zero and we fold all of the other effects into the electrode to earth resistance.
Same for water (salt or fresh).
For salt water the voltage zones around the electrode are just a lot smaller, even compared to dirt.

Actually, the linked calculation ( non-https since there is a certificate problem at the site) looks at the resistance between two very finite electrodes in seawater and calculates the effective resistance without trying t separate it out into electrode resistances.
When we say that the resistance of earth is zero we really mean that the resistance between any two physical ground electrodes approaches a constant value as the separation increases. We then associate that limiting resistance with the sum of two interface resistances.

I think you are deliberately misreading my use of the word point in my original statement.

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[jim dungar]

Mods, this is related to grounding issue of the rig from the OP.

No it is not related to the OP. The original question clearly asked about the XO connection in relation to the transformer enclosure.

Per the NEC, Grounding of the X0 bushing requires a connection to the reference plane (creating a grounded conductor) not to the planet Earth (creating a grounding conductor).
Only in the recent editions of the NEC do we see a move toward using the term bonding.

 

[iwire]

Bennie would have loved this thread.

He may have asked a few questions, like how do you ground equipment on the ice at the poles? If I recall he was there doing such things.

Or how do you ground the electrical system on an airplane? His point being that a connection to dirt is not always needed to make electrical systems safe.

 

[Besoeker]

If you read that argument carefully you see that either the argument only applies to a finite sized body of water

"High quality deionized water has a conductivity of about 5.5 μS/m, typical drinking water in the range of 5-50 mS/m, while sea water about 5 S/m (i.e., sea water's conductivity is one million times higher than that of deionized water)."
Note the per metre term in there. An infinite body would have infinite didtances so the conductivity would zero there.

But we accept that the resistance of earth is zero

I wouldn't. Actually, don't you have maximum values for ground rods?

Nice chatting to you but I'm off to bed now.

 

[GoldDigger]

"High quality deionized water has a conductivity of about 5.5 μS/m, typical drinking water in the range of 5-50 mS/m, while sea water about 5 S/m (i.e., sea water's conductivity is one million times higher than that of deionized water)."
Note the per metre term in there. An infinite body would have infinite didtances so the conductivity would zero there.


I wouldn't. Actually, don't you have maximum values for ground rods?

Nice chatting to you but I'm off to bed now.

You are confusing the geometric dimensions of the unit with its physical measurement.
If you take a unit cube of material there will be a resistance between two opposite faces, of unit area. The resistance goes up as the plates separate and goes down as the area increases. The result is that the bulk resistance property (i.e. the resistivity) has the dimensions of ohm-meters.
And conductivity has the dimension of Siemens/meter.
As you increase the size of the unit cube the conductance goes UP linearly.

As you approach a perfect electrode (plate) in size the resistance goes to zero independent of the distance.
Your argument that conductance goes to zero as distance increases is analogous as saying that if you travel at one meter per second the distance goes to zero as time increases. :angel:

See you in the morning.

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[iwire]

I wouldn't. Actually, don't you have maximum values for ground rods?

Ground rods yes.

Other electrodes no.

Good night.

 

[kwired]

don't you have maximum values for ground rods?

Ground rods yes.

Other electrodes no.

Good night.

Actually the ground rods is more of a "sort of". If first ground rod is over 25 ohms all you have to do is install a second one and it doesn't matter what the resistance is after that - per NEC, job specs can be different.

Earth is considered zero resistance, problem with grounding electrodes isn't the resistance of the electrode itself but that it is difficult to make a low impedance connection to earth with most individual electrodes (especially only an 8 or 10 foot piece of rod or pipe) unless they are fairly large.

 

[iwire]

Actually the ground rods is more of a "sort of". If first ground rod is over 25 ohms all you have to do is install a second one and it doesn't matter what the resistance is after that - per NEC, job specs can be different.

Earth is considered zero resistance, problem with grounding electrodes isn't the resistance of the electrode itself but that it is difficult to make a low impedance connection to earth with most individual electrodes (especially only an 8 or 10 foot piece of rod or pipe) unless they are fairly large.

So actually what I said was correct.

 

[kwired]

So actually what I said was correct.

sort of, is best I can do here.