Utility Overhead Single Phase Transformer Grounding 2

[saladhawks]

I am curious as to utility grounding practices related to center-tapped single-phase overhead transformers. The utility I work for has traditionally not grounded either the overhead transformer tank or secondary center tap bushing (X2) that is used to provide 120/240V, 1-phase, service drops.

I can somewhat understand not grounding the actual overhead transformer tank, but not grounding the secondary center tap bushing (X2) concerns me. I realize that all customers are required to ground the service entrance neutral at their individual panel, but that still leaves the service conductors between the overhead transformer and customer service panel vulnerable to high impedance ground faults. What am I missing here?

 

[dpc]

The center tap needs to be grounded at the pole. I'm really surprised a US utility would not be doing this. I've never seen an installation that didn't at least attempt to have a local ground. The NEC assumes the neutral to be a grounded conductor. Without this ground, if the customer loses their main bonding jumper or grounding electrode conductor, they can have 240 V on their 120 V devices and voltage from neutral to ground.

 

[wareagle]

The utility I used to be with had a #6 cu that ran from the transformer neutral down the pole to a ground rod at the base of the pole. The #6 was bonded the the system neutral conductor.

 

[waross]

Hello saladhawks;
Is your distribution primary a delta system? If the primary is wye is there a system neutral?
respectfully

 

[saladhawks]

Waross,

There are 2 different primary overhead systems available:

4.16 kV, 3-Phase, 4-Wire Wye (multigrounded system)

17.2 kV, 3-Phase, 3-Wire Wye (unigrounded at source)

The issue at hand are some new 17.2 kV Overhead 1-Phase Transformers (2 Primary Bushings) where the crews are not planning to either ground the transformer tank or the center-tap (X2) of the 120/240V secondary winding. I can understand not grounding the transformer tank, but I believe it is incorrect to not ground the X2 center-tap.

 

[jghrist]

NESC Rule 092 B. requires the secondary neutral of single-phase three-wire systems to be grounded at the transformer and at the line side of service equipment.

NESC Rule 123 A. requires transformer cases to be grounded or physically isolated.

 

[dpc]

Thanks, jghrist. I was pretty sure it was an NESC requirement, but I was too lazy to look it up.

 

[saladhawks]

jghrist - I do not believe your NESC references are applicable to my specific Overhead Distribution Transformer installation as follow:

NESC Rule 092 B is included in Section 9 that covers the methods of protective grounding. The actual rules requiring grounding are found in other parts of the code.

NESC Rule 123 A is included in Part 1 that covers Electric Supply Stations and Equipment.

My take on the requirements for an Overhead Distribution Transformer installation based on NESC is as follows:

NESC Rule 215 B requires the secondary neutral of single-phase three-wire systems to be grounded at the transformer.

NESC Rule 215 C EXCEPTION 1 allows the local utility to decide if the overhead transformer tank should be grounded if located more than 8 feet above readily accessible surfaces. The utility I work for has a uniform practice not to ground overhead transformer tanks on poles, so that tradition continues with all new installations and is allowed by the NESC.

Thanks for taking the time to respond to my original post.

 

[dpc]

But you also said the neutral was not being grounded at the pole. Is there an exception for that, or have you decided to ground the neutral now?

Why would 092B would not be applicable? In fact, Rule 215B you reference just refers back to Section 9.

So if you have to run a ground wire up to ground the neutral, what is the rationale for not grounding the tank?

 

[saladhawks]

My understanding is that the rationale for not grounding the overhead transformer tank is for the safety of the field crew performing energized work in an insulated bucket truck. Many field crews assume that the only thing that can get them in "trouble" is contacting an energized conductor + somthing else (such as another phase or neutral wire) since they are in an insulated bucket truck with no reference to ground. Grounding the transformer could in theory be that something else if they happen to brush up against the overhead transformer tank when they are doing energized work. Not being in the field crew myself, I guess it is simply one less thing to have to worry about...........

 

[pooslinger]

If the center tap of the transformer is connected to "earth ground", then all of your outlet power is in reference to "earth ground. If you sink a grounding rod outside of your house and connect the neutral to your grounding rod your will not pass your electrical inspection. Ground and neutral must be isolated from each other in a residential breaker box in my area. Does anybody know why? I do.

 

[jghrist]

You're right about the code references. I was too hasty in my review.

If the secondary neutral is grounded, then the transformer tank will be essentially at ground potential. Even if the grounding strap from the X2 bushing is removed, if the neutral ground is connected to the pole near the transformer and the transformer tank is bolted to the pole, I wouldn't count on whatever wood insulation there is between the bolt and the neutral for safety of the linemen. I think it would be safer if the crews in a bucket truck considered the tank to be grounded. If a lineman ever climbs the pole, the possibility of inadvertent contact with the transformer tank make grounding the tank preferable. In case there is an internal short to the tank, if it is grounded, then a fuse will blow.

Pooslinger, the neutral must be connected to a ground rod or other grounding electrode at the service. The neutral cannot be connected other places because that will lead to neutral current flowing through the ground and will mess up ground fault interrupters.

 

[stevenal]

I agree that case grounding is preferable for safety reasons. Also, with an internal primary connection to the grounded tank, one primary bushing and a possible failure point is eliminated.

Beware of exception 1. Mergers, acquisitions, condemnations, etc. make a "uniform practice over a
well-defined area" hard to find and even harder to prove to the regulating authority.