TRANSFORMERS - A solid connection from primary to secondary ?

[electrofelon]

I recently participated in a discussion on another site where the design engineer was going to connect the neutral from the primary feeder to the XO on the secondary of a delta/wye transformer to avoid any requirements to provide a grounding electrode system for the transformer secondary. With the feeder neutral connected to the secondary XO, the transformer is not a separately derived system and the requirements in 250.30 do not apply.

Right, and assuming the secondary is a grounded system, the XO will be connected to the primary neutral thru grounding and bonding connections anyway, so intentionally making the connection doest seem to do anything electrically other than result in having to run a primary neutral instead of a GEC to the transformer.

 

[don_resqcapt19]

Right, and assuming the secondary is a grounded system, the XO will be connected to the primary neutral thru grounding and bonding connections anyway, so intentionally making the connection doest seem to do anything electrically other than result in having to run a primary neutral instead of a GEC to the transformer.

But much easier, in many cases, to pull that extra conductor in with the feeder than a separate path to a grounding electrode. Also, there is no load on that primary neutral, so it would only have to be sized like an EGC based on the feeder OCPD.

 

[electrofelon]

But much easier, in many cases, to pull that extra conductor in with the feeder than a separate path to a grounding electrode. Also, there is no load on that primary neutral, so it would only have to be sized like an EGC based on the feeder OCPD.

So why doesn't everyone just do that?

 

[winnie]

That is an interesting take.

We've discussed the primary EGC serving as the GEC. But the the requirements for a GEC are pretty strict.

You are saying that a primary neutral is sufficient? How do you calculate its size? Is it a sufficient fault current path?

Jon

 

[don_resqcapt19]

So why doesn't everyone just do that?

There was an interesting discussion on the code on this, but it appears the code would permit it. At first a number of people in the discussion said that you would be rebonding the primary neutral, but you aren't and the bonding requirements of 250.104(D) don't apply because it is not a SDS.

 

[don_resqcapt19]

That is an interesting take.

We've discussed the primary EGC serving as the GEC. But the the requirements for a GEC are pretty strict.

You are saying that a primary neutral is sufficient? How do you calculate its size? Is it a sufficient fault current path?

Jon

Not saying it is sufficient, as there is no system bonding jumper, just not seeing anything in the code that would prohibit this or require that primary neutral to be sized any larger than the EGC for the primary feeder. The fault clearing path would be a long one with this set up...and would depend on both the primary neutral and primary EGC.

With the EGC serving as the secondary GEC, there would still be a system bonding jumper and the only fault current it would ever be called on to carry is from the a fault on the primary feeder or primary windings. In this case, there is no reason for a large GEC for the secondary. I have a PI in to permit the primary EGC to be the secondary GEC under some specific very restrictive conditions.

 

[gar]

230523-2022 EDT

Consider a pole mounted transformer 20 ft above earth with an impedance of 20 ohms from ground rod to remote earth. Then assume a neutral conductor to to main panel ground bar as about 1 ohm. Main panel ground bar has the same impedance to earth as the ground rod at the pole.

A lightning bolt strikes the pole neutral with 1000 A of current. The voltage rise is about 500 * 20 = 10,000 V above ground. It would be twice this with no pole ground rod. These are wild estimates, but somewhat illustrate what happens.

Now add a copper plate at the input to the home. Place shunt capacitors from each incoming line to earth, and some series inductance in each incoming line. All this is grounded before entering the home. Then earth ground the home some distance from where this copper plate is grounded.

The result of all this keeps the rise of voltage inside the the home much lower.

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[mtnelect]

230523-2022 EDT

Consider a pole mounted transformer 20 ft above earth with an impedance of 20 ohms from ground rod to remote earth. Then assume a neutral conductor to to main panel ground bar as about 1 ohm. Main panel ground bar has the same impedance to earth as the ground rod at the pole.

A lightning bolt strikes the pole neutral with 1000 A of current. The voltage rise is about 500 * 20 = 10,000 V above ground. It would be twice this with no pole ground rod. These are wild estimates, but somewhat illustrate what happens.

Now add a copper plate at the input to the home. Place shunt capacitors from each incoming line to earth, and some series inductance in each incoming line. All this is grounded before entering the home. Then earth ground the home some distance from where this copper plate is grounded.

The result of all this keeps the rise of voltage inside the the home much lower.

.



the

How about all the voltage gradients OUTSIDE the home like around a swimming pool.
So now the NEC wants the homeowner to pay for a copper grid mesh installed around the pool.
How about the utility companies paying for the shortcut they created, and the lives that have been lost because of this.

 

[don_resqcapt19]

How about all the voltage gradients OUTSIDE the home like around a swimming pool.
So now the NEC wants the homeowner to pay for a copper grid mesh installed around the pool.
How about the utility companies paying for the shortcut they created, and the lives that have been lost because of this.

In most pools around here there is steel mesh in the perimeter surface and all the contractor has to do is to bond to the steel. Under the current code, unless you have adopted TIA 23-9, you are still permitted to use a single copper bond wire for the perimeter surface if there is no conductive reinforcing in the concrete.

 

[Joethemechanic]

Just construct the pools out of copper, or better yet silver. Easy peazy problem solved

 

[mtnelect]

I have a swimming pool that was constructed in 1990. I watched as the rebar frame was installed and the bonding wire attached to the equipment and fixtures.

Last year I was in the process of getting ready for the installation of fiber optic cable. The demarcation box is close to the ground and below the electrical service., so I decided to sit on the concrete that surrounds the pool to be more comfortable. In the process of checking the wiring, I accidentally touched the ground wire in the box, and became a human "Wiggy". It was not the full 120 volts, only tested at 26 volts.

So, later that week when I had time, I double checked all the pool and home wiring. And found no apparent problems. At that point I decided to turn all the power off and retest, it would fluctuate between 26 & 37 volts.

That's when I went of the internet and found the article on multi grounding.

So, I was wondering what's the big deal, it's only around 30 volts A.C. No, turn all the power off when swimming.

 

[don_resqcapt19]

I have a swimming pool that was constructed in 1990. I watched as the rebar frame was installed and the bonding wire attached to the equipment and fixtures.

Last year I was in the process of getting ready for the installation of fiber optic cable. The demarcation box is close to the ground and below the electrical service., so I decided to sit on the concrete that surrounds the pool to be more comfortable. In the process of checking the wiring, I accidentally touched the ground wire in the box, and became a human "Wiggy". It was not the full 120 volts, only tested at 26 volts.

So, later that week when I had time, I double checked all the pool and home wiring. And found no apparent problems. At that point I decided to turn all the power off and retest, it would fluctuate between 26 & 37 volts.

That's when I went of the internet and found the article on multi grounding.

So, I was wondering what's the big deal, it's only around 30 volts A.C. No, turn all the power off when swimming.

Not really caused by the mutli neutral grounding, but caused by excessive voltage drop on the primary neutral. This could be eliminated by the utilities connecting all of their primaries line to line, but many are connected line to neutral and that results in neutral to earth voltage on everything connected to the electrical grounding system.

What else were you touching besides the concrete around the pool? There needs to be two points of contact, and the very reason for the pool bonding is to raise the voltage of everything that can be touched from in or close to the pool to the same level. When it is all energized to the same level there is no shock hazard.

 

[mtnelect]

Not really caused by the mutli neutral grounding, but caused by excessive voltage drop on the primary neutral. This could be eliminated by the utilities connecting all of their primaries line to line, but many are connected line to neutral and that results in neutral to earth voltage on everything connected to the electrical grounding system.

What else were you touching besides the concrete around the pool? There needs to be two points of contact, and the very reason for the pool bonding is to raise the voltage of everything that can be touched from in or close to the pool to the same level. When it is all energized to the same level there is no shock hazard.

Inside the demarcation box is a grounding post that I touched accidentally when I got the buzz.

 

[Joethemechanic]

Just be glad that we have a MGN system and not a SWER system

 

[electrofelon]

Just be glad that we have a MGN system and not a SWER system

Also you'll be glad it's an MGN when you have to run an UG single phase primary. I just did one that was about 2000 feet and unfortunately a delta system, so take $3.75 for your 15kv primary, and I don't have to mention how much conduit is these days..... Then multiply by 2 for your two runs. You'll probably never talk crap about an MGN system after you do that.

 

[Joethemechanic]

I thought California mostly used 2 bushing transformers phase to phase. I swear I just read that somewhere

 

[winnie]

Not really caused by the mutli neutral grounding, but caused by excessive voltage drop on the primary neutral. This could be eliminated by the utilities connecting all of their primaries line to line, but many are connected line to neutral and that results in neutral to earth voltage on everything connected to the electrical grounding system.

While I agree that connecting the transformers L-L would eliminate this source of the elevated neutral voltage, I'd say that the multi grounding is what connects the elevated neutral voltage to the soil.

Jon

 

[winnie]

So, later that week when I had time, I double checked all the pool and home wiring. And found no apparent problems. At that point I decided to turn all the power off and retest, it would fluctuate between 26 & 37 volts.

That's when I went of the internet and found the article on multi grounding.

So, I was wondering what's the big deal, it's only around 30 volts A.C. No, turn all the power off when swimming.

Keep in mind that there are other potential causes of the voltage you measured.

You have a symptom, and did an internet search to find a potential cause. Stray current from primary distribution could be one cause.

The phone company has it's own wires, and their ground is clearly not bonded to your electrical system ground. That is a serious problem that should be fixed. The elevated voltage might be coming from the poco lines.

Take a look at this recent thread. We all jumped on the stray voltage explanation, but the OP traced the problem to a neighbor's well....

Utility Creating High Service Ground vs. Earth Ground potential

I have recently had issues with my utility creating a high ground potential on my neutral and thus ground circuits. Service is fed from an overhead 2-phase 15 kva transformer directly to the main which is properly bonded and grounded. There is a sustained 20-25 vac potential between any grounds...

forums.mikeholt.com

 

[mtnelect]

Keep in mind that there are other potential causes of the voltage you measured.

You have a symptom, and did an internet search to find a potential cause. Stray current from primary distribution could be one cause.

The phone company has it's own wires, and their ground is clearly not bonded to your electrical system ground. That is a serious problem that should be fixed. The elevated voltage might be coming from the poco lines.

Take a look at this recent thread. We all jumped on the stray voltage explanation, but the OP traced the problem to a neighbor's well....

Utility Creating High Service Ground vs. Earth Ground potential

I have recently had issues with my utility creating a high ground potential on my neutral and thus ground circuits. Service is fed from an overhead 2-phase 15 kva transformer directly to the main which is properly bonded and grounded. There is a sustained 20-25 vac potential between any grounds...

forums.mikeholt.com

Thank you for your time to reply. I live on an eluvial fan from the mountains above, and it's very rocky environment. So, ground rod don't work to well.

These pictures are 4160 volts on the primary with a 75K transformer, and multiple taps from the secondary feeding my neighbors.

 

[winnie]

That looks like a two bushing transformer fed from two primary phases, not fed phase to neutral.

Now it is possible that one of the insulated wires on the cross bar is actually a neutral, but I think your local transformer is eliminated as the source of the elevated ground voltage you measured.

Jon